‘Mwm .a l . -- . ’3 .J ..\J 'f‘ij“ .31..) ‘1 f' Y; \‘-:\'? 7 o—I’dof‘.-\ '3 Mic}. igrn“ mate i {ya' EJRNW?3IY l c..- ,. rum” {a a... This is to certify that the thesis entitled THE UTILIZATION OF A TROPICAL SEED RESOURCE BY THE VARIEGATED SQUIRREL AND THE COTTON STAINER BUG presented by Elizabeth Wisk Hutchison has been accepted towards fulfillment of the requirements for .MasiaanJegree in imlngy— Major p essor Date August Ll, 1980 0-7639 1 WM lllllllllllllllllllll 3123 #L OVERDUE FINES: 25¢ per day per item RETURNING LIBRARY MATERIALS: Place in book return to remove charge from circulation records THE UTILIZATION OF A TROPICAL SEED RESOURCE BY THE VARIEGATED SQUIRREL AND THE COTTON STAINER BUG By Elizabeth Wisk Hutchison A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Zoology 1980 ABSTRACT THE UTILIZATION OF A TROPICAL SEED RESOURCE BY THE VARIEGATED SQUIRREL AND THE COTTON STAINER BUG By Elizabeth Wisk Hutchison The high species diversity in the tropics often results in complex interactions between predators and resources. Up to eleven animal species may utilize the seed crops of the deciduous tree, Sterculia apetala. Two species, the variegated squirrel (Sciurus variegatoides) and the cotton stainer bug (Dysdercus bimaculatus) play important roles in the tree's ecology. The squirrels harvest the majority of each tree's crop. Their patterns of feeding resemble those predicted by optimal foraging theory. The bugs must scramble for the seeds dropped by the squirrels. The bugs respond to food limitation by developing to a smaller size, by migration or by cannibalism. To reproduce successfully, seeds must escape the injurious insects. The squirrels, as dispersers, preferentially visit large tree crops. The squirrels may create selection pressure for trees to concentrate squirrels' activities in one canopy simultaneously, resulting in the population's asynchronous fruiting and in individual trees producing crops every other year. ACKNOWLEDGEMENTS Many people have advised and supported me throughout this work. I eSpecially value the enthusiasm, encourage- ment and insights Don Hall has contributed to this "out- of-line" work. Earl Werner has tempered my eagerness with thoughtful perceptions and Patricia Werner made sure that I did not place the plant secondary to the other species. On my earlier proposal, Dick Root, Steve and Sue Chapman. Hugh Dingle and Janice Derr offered many excellent suggestions. Correspondence with Janice Derr especially developed my understanding of Dysdercus. The Costa Rican staff of the Organization for Tropical Studies and the Ministerio de Agricultural y Ganaderia ensured my survival while at the field station. In particular, I would like to thank Guillermo Canessa. Julio Sanchez and Eduardo Lopez. During the research. Juan Miranda. Ana Rita Vasquez. Steve Trombulak, Mike Scott, Juan Rodriquez, Carlos and Nina helped me find and/or map trees. An OTS 80.1 field problem group spent the day with me examining the squirrels' behavior. Sherry Kinsman and Steve Trombulak kept me thinking creatively during the long field hours. ii Funds for the research came primarily from the NSF grant to Duke University for the Organization for Tropical Studies (#SER76-17508). The College of Natural Sciences at Michigan State University awarded me a travel grant which covered part of those expenses. A small amount of equipment was paid for by the current NSF grant to E.E. Werner and D.J. Hall (DEB 78-24271-01). The Kellogg Biological Station lent me a microscope during the research.months. Throughout my graduate career, the ecology group at Michigan State University has advanced my ecological thinking. In particular, I thank Chris Carmichael, Martha Potvin, Judith Soule, Ed Turanchik, and Nancy Winters who have seen me through this time of self- appraisal. In Costa Rica, Juan Miranda and Ana Rita Vasquez were the first of many to give me their friendship when I was so in need of it. Finally, loving thanks go to my parents and family who still seem to be trying to understand why I ever went to the tropics. iii TABLE OF CONTENTS List of Tables List of Figures Introduction Description of Study Area Characteristics of the Resource, Sterculia apetala The Utilization of Sterculia apetala by the Variegated Squirrel ' Introduction Methods Results I. Description of Foraging Behavior II. Selection of Pods III. Rate of Harvesting per Tree IV. Effects on the Total Tree Crops Discussion Responses of Cotton Stainer Populations Developing at Patchy Resources Introduction Methods Results I. Changes in Behavior and Body Size within Populations II. Changes in Body Size Across the Season Discussion Conclusion iv vi viii 9 15 15 20 20 22 24 28 33 33 #2 42 54 61 69 TABLE OF CONTENTS (cont'd.) Appendix A. Characteristics of the Tree Population, Sterculia apetala Appendix B. Size Measurements of Dysdercus bimaculatus Appendix C. Squirrel Foraging Data from Observations at Trees 3. 17. 15 and 8 List of References Page 72 77 81 86 Table Table Table Table Table Table Table Table Table Table Table Table Table Table 2. 3. 6. A1. A2. A3. B1. B2. B3. B4. B5. LIST OF TABLES Average daily rates of squirrel predation at four trees. Characteristics of trees 2, 8 and 15. Mean pronotum widths of insects at tree 2 compared with the Student- Newman-Keuls test. Mean pronotum widths of insects at tree 15 compared with the Student- Newman-Keuls test. Mean pronotum widths of insects at tree 8 compared with the Student— Newman-Keuls test. Significant changes in insect body size across the dry season. Crop size, circumference at breast height and percentages of crops aborted. dehisced or eaten. Distances between patches and between trees within patches in meters. Date of initial fruit dehiscence at thirty trees. Size measurements of female adults. Size measurements of male adults. Size measurments of female fifth instars. Size measurements of male fifth instars. Size measurements of fourth instars. vi Page 24 38 46 51 58 58 72 74 76 77 77 78 78 79 LIST OF TABLES (cont'd.) Page Table B6. Size measurements of third instars. 79 Table B7. Size measurements of second instars. 80 Table C1. Squirrel foraging data from observations at trees 3. 17. 15 and 8. 82 vii Figure Figure Figure Figure Figure Figure Figure Figure Figure 50 6. 7. 8. 9. LIST OF FIGURES The relationship between crop size and CBH in Sterculia apetala (r=O.45). The relationship between crop size and time of initial fruit dehiscence in Sterculia apetala (r=-O.72). The successive reduction in the size of pods eaten by squirrels over a two- week period at two trees (Tree 3: r=o.45, Tree 8: r=0059)o The percentages of pods harvested by the squirrels during fruit availability at four trees. The relationship between the size of the crop and the percentage of the crop harvested by the squirrels. The weekly abundance of seeds available to the insect population at tree 2. The weekly change in the number of bugs feeding per fresh seed at tree 2. The reduction in the mean width of the insects' pronotums at tree 2. The weekly abundance of seeds available to the insect population at tree 15. Figure 10. The weekly change in the number of bugs feeding per fresh seed at tree 15. Figure 11. The reduction in the mean width of the insects' pronotums at tree 15. viii Page 13 14 23 25 27 44 45 48 50 50 53 Figure Figure Figure Figure Figure 12. 13. 14. 150 16. LIST OF FIGURES (cont'd.) The weekly abundance of seeds available to the insect population at tree 8. The weekly change in the number of bugs feeding per fresh seed at tree 8. The reduction in the mean width of the insects' pronotums at tree 8. The reduction in mean body size of Dysdercus bimaculatus with the progression of the dry season. The reduction in mean monthly relative humidity during the dry season. Data from Frankie et al, 1974. ix Page 56 56 57 6O 65 INTRODUCTION Convinced from previous experience that temperate biologists can be lured into working in the tropics, the Organization for Tropical Studies sponsors courses in Costa Rica. During the winter of 1979. I participated in one of these intensive courses. I returned to the United States under the spell of this biologically diverse area. My fascination partially arose from a perplexing set of observations on the cotton stainer bug, Dysdercus bimaculatus. While at the tropical dry forest in Guanacaste, I examined a population of this insect con- suming the fallen seeds of Sterculia apetala, a deciduous tree. Immatures of all instars were feeding in groups of more than 400 bugs; such aggregations covered all of the seeds on the ground. A sizeable number of bugs were wan- dering about the leaf litter and some of them were attempt- ing to feed on conspecifics. These observations led me to suspect that food stress was causing intense intra- specific competition. Two other biologists have previously studied this plant-insect system in the same general area of Costa Rica. Janzen(1972) noticed that all fallen seeds were fed on by the bugs. After discovering that these fallen, bug-eaten 1 2 seeds were inviable, he proposed that mammals feeding in the canopies were essential to the trees' reproductive success. More pertinent to my interests, Derr(1977) concentrated on the population movements of Q. bimaculatus with respect to the fruiting of the host trees and to environmental moisture stress. She found that the life history of the insect reflects characteristics of the host plants whose fruiting spans the dry season. Through laboratory exper- iments she learned that the growth and survival of the immature insects, developing on seeds of Sterculia, were adversely affected by a decrease in the moisture present in the growth chambers. She interpreted these results in terms of the decreasing relative humidity during the dry season. In both of these studies trees of Sterculia were found to be widely dispersed. Distances between trees were stated as being greater than 200 meters. On this basis, the two authors concluded that the immature insect popula- tions at different trees were isolated from each other and from other sources of food. Using this basic information, I proposed to study the competitive interactions between instars in these isolated populations. Because the number of seeds is finite at each tree, I expected populations of immatures to be stranded once the seed crops were depleted. After 3 considering morphological, physiological and behavioral aspects of each size-class, I made predictions concerning the relative success of instars under intense intraspecific competition. I began collecting data in November, 1979, which, hopefully, would support these predictions. The proposal was abandoned after four weeks of field work for two major reasons. Populations of Q. bimaculatus were not isolated due to the distances of Sterculia trees. Trees of Sterculia were often found within thirty meters of a conspecific. Once seeds at a particular tree were depleted, immatures could be seen walking as far as 250 meters away from that host plant--a distance greater than those found between trees in the studies of Janzen(1972) and Derr(1977). Intense intraspecific competition seemed a less important factor in the bug's ecology than the in- sectfs ability to escape a tree of dwindling resources. The second reason I relinquished my original goals was due to a misconception that the resource would be easily quantified. Instead, I discovered that the seeds available to the bugs fluctuated in quality and abundance from one minute to the next. Such dynamics were mostly created by the variegated squirrel, Sciurus variegatoides, which fed on the canopy seeds. Because the squirrels depleted the crops so extensively, they greatly increased the probability that the insects did not complete their development on one tree crop alone. 4 With these changes in my understanding of the system, I began to gather information on the overall utilization of Sterculia seeds. Resource utilization has often been a major consideration in animal ecology (Schoener, 1974). A greater ability to find and use food efficiently is thought to be one of the reasons for differential success under competitive regimes. I expected several factors to affect the utilization of the rich food patches represented by Sterculia: the distribution, abundance and seasonality of the food, the availability of other food sources, the mobility of the predators, and the interactions of the foraging competitors. Of the eleven potentially competing species known to feed on this resource (Janzen, 1972: Derr, 1977; F. Chavez, pers. comm.), only the variegated squirrel and the cotton stainer bug affect large percentages of the fruit crops. As expected of two animals with such disparate biologies, the importance of Sterculia as a resource was quantita- tively different. The observed patterns of their seed utilization conformed to two different aspects of theor- etical ecology and are treated separately in this thesis. My observation of the squirrels using Sterculia reflected patterns often predicted by optimal foraging theory. Studies with other species of squirrels, the pine squirrels and the gray squirrels, have shown that these animals feed on their resources in a similar 5 manner (Smith, 1970 3 Lewis, 1980). Although I did not test predictions arising from a foraging model, my obser- vations suggest that the variegated squirrel samples the resource, maximizes its energy return for the amount of time invested and switches to other resources as they become available. On the other hand, Sterculia is the only resource available to the cotton stainer bugs for the first four months of the dry season. Reproduction by these bugs is limited to the dry season months. The adult bugs which have survived the wet season in reproductive diapause depend on this host plant to produce the first of the next generation's insects. The earliest fruiting Steggulia are colonized by these adults. As new individuals join the adult insect population, they colonize the next fruiting trees. Since the life history of a species often reflects characteristics of its resource (Stearns, 1976), the patchy nature of the trees was expected to affect the ecology of the insect. The growth of the bug populations, initiated by fe- males laying eggs under trees, is based on the number of seeds the squirrels happen to drop without eating. If the squirrels deplete the resource before all immature bugs reach the adult stage, a population of bugs may be without food at that tree. However, the young bugs can respond by developing to a smaller adult size, by walking 6 to another tree with seeds, or by switching to cannibal- izing conspecifics. Any combination of these three re- sponses provides a possible escape from a once plentiful but subsequently depleted patch. As Janzen(1972) first suggested, the insects and the squirrels directly affect the tree's biology. Since the bugs injure all seeds that fall beneath the parent tree, no successful Sterculia offspring are produced in that area. The squirrels, by carrying away seeds from the vicinity of the parent tree, serve as dispersers. The populations of the insect and the squirrels are indirectly linked through their use of this common resource. The extensive reduction of seeds by the squirrels probably affects the number of adult Dysdercus produced during the dry season. DESCRIPTION OF STUDY AREA The research was conducted at the Refugio Rafael Rodriquez Lucas Caballeros (formerly named Palo Verde), located thirty-six kilometers northwest of Canas in Guanacaste, Costa Rica. The 4700 hectare area consists of a combination of dry, deciduous forest, riparian forest, derived savanna, seasonal swamp, pastureland, and second growth vegetation. Holdridge et al(1971) have described the area and its vegetation. For the most part, the forests are continuous across the steep hills whereas the disturbed areas are found in the lowlands. The majority of the research was concen- trated on the hills surrounding the Organization for Tropical Studies' field station. Fences have restricted cattle from grazing in this area. The work began at the end of November, 1979. and continued through the beginning of March, 1980. Sampling schedules were on a weekly and daily basis, except when interrupted in the fourth weeks of December and February because of necessary trips for visa extensions. These four months when research was conducted comprise the first two-thirds of the six month dry season. Temperatures 8 remain at a relatively unfluctuating monthly mean of twenty—eight degrees Centigrade throughout the dry season but mean monthly rainfall drops from 375 mm. in October to 86 mm. in November; in March, less than 5 mm. of rain falls (Frankie, Baker and Opler, 1974). The decreasing amount of rainfall and the strong north winds result in -a steady diminishing of the relative humidity. The strong seasonality is reflected in the deciduous nature of the trees and in their tendency to produce mature fruit during this dry period (Frankie et al, 1974). CHARACTERISTICS OF THE RESOURCE, STERCULIA APETALA Sterculia apetala (Malvales: Sterculiaceae), commonly known as the Panama tree, is a versatile member of the tropical deciduous forest. Possessing the fast growth characteristics of a colonizing plant, the tree also attains heights greater than thirty meters and maintains itself as an integral part of the canopy forest. Trees in various stages of development may be found in recently disturbed sites, secondary growth areas and primary forest (Janzen, 1972). Sterculia produces mature fruit during the first part of the dry season. The trees in a local area dehisce fruits asynchronously. In other words, the earliest fruiting trees begin dehiscing at the end of November while the latest trees have mature fruit at the end of March (Frankie et al, 1974; Derr, 1977). Each tree possesses mature fruit for approximately four weeks (Derr, 1977). Larger trees tend to have dehiscent fruits for longer periods of time. The fruit is a cluster of five carpels (called pods in this thesis), connected at the stem ends and located towards the ends of branches. The pods vary in size both within and between trees; for two trees the lengths of the 10 pods were 77.9 I 4.3 mm. and 72.7 I 6.0 mm. (mean : stan- dard error; n=39, 52). The fruits require an entire year to mature after pollination of the flowers. Each carpel dehisces the following dry season along one suture, exposing two to eight large brown seeds that are 25 mm. by 10 mm. in size. After the pods have reached their maximum size but before they have opened, the seeds develop through three distinct stages: the least mature with a liquid endosperm and a wet weight of 1.2 grams, an; intermediate stage of a solid endosperm and a wet weight of 2.2 grams, and the mature seed with a dessication- resistant coat and a wet weight of 2.4 grams (Derr, 1977). After the seeds have fallen from the dehisced pod, the pod itself dries and falls to the ground. In order to characterize this plant as an animal resource, I chose to study seventy trees within a three kilometer radius of the field station. Twenty-two trees in two other distant areas were also studied for a total of ninety-two trees. For each tree, trunk circumference at breast height(CBH) was measured and the total number of mature pods in the canopy was counted. At thirty of the trees the maturation and dehiscence of the fruit crops were tracked at weekly intervals by counting the number of the pods opened in the canopy with binoculars and by recording the types of seeds beneath the trees. Crop sizes varied fr6m zero to 2972 pods per tree. 11 The average crop was 254.8 1 530.7 pods (mean : standard error) with a median of 43 pods. Thirty-three percent of the trees had no pods. Fifty-four percent of these fruit- less trees had a CBH of less than one meter in circumfer- ence. Of the trees that had pods, the average crop size. was 389.9 3 611.4 pods (mean : standard error). Of those trees with fruits (n=63), the size of the crop was positively correlated with the circumference of the tree (Figure 1: r=0.45) and negatively correlated with the time of fruit dehiscence (Figure 2: r=-O.72). Thus, larger, older trees tended to have bigger fruit crops with seeds that matured earlier in the dry season. In addition to collecting these data on the fruit crops, I also mapped the distribution of the trees. The trees were commonly distributed in widely dispersed patches. Twelve patches of trees were mapped relative to each other (trunk to trunk) with a compass and measured tape. Distances between patches were estimated by paces that had been standardized to a known distance. A tree more than seventy-five meters from a conspecific was considered to be a new patch. All of the mapped patches contained more than three trees. The largest patch contained ten. The average distance between patches was approximately one hundred and fifty meters; within patches trees were often within fifty meters of each other. The distance between trees within 12 patches was 30.5 i 15 meters (mean I s.e.). The size distribution and asynchronous fruiting within each patch seemed to represent the entire tree population, i.e., at the "Marsh" patch the CBH ranged from 0.95 meters to 3.22 meters and at least one tree was fruiting from January 5, 1980, to March 8, 1980. Therefore, at an individual level, Sterculia is a more dynamic resource than at the patch level. Each tree serves as a temporarily abundant source of food which changes in value as it is depleted and others mature. At the patch level, within a relatively small area, at least one tree is often present as a source of food. 13 2600 2400 2200 2000 1800 1600 1400 ' 1200 Number of Pods/ Tree 1000 o 800 _ 400 . 200 o .9 ° 6 .0 o ' Circumference of Tree (m.) Figure 1. The relationship between crop size and CBH in Sterculia apetala (r=0.45). 14 3000 . 2300 2600 2400 2200- 2000- 1800' 1600 Number of Pods/ Tree tum 1000 . 330-1 600- 400- . 0 2 12 22 1 11 21 31 10 20 1 11 Dec. Jan. Feb. Mar. Date of Initial Fruit Dehiscence Figure 2. The relationship between crop size and time of initial fruit dehiscence in Sterculia apetala r=‘0 072 e THE UTILIZATION OF STERCULIA APETALA BY THE VARIEGATED SQUIRREL Introduction Animals' resources are often concentrated in localized areas, commonly referred to as patches. The use of such patches may affect the animal's foraging pattern, habitat selection, dispersal, and social organization (for a review, see Wiens, 1976). Patch utilization has been increasingly incorporated into optimal foraging models and greatly increases the reality represented by them (MacArthur and Pianka. 1966: Charnov, 1976). Empirical studies have supported the theoretical predictions that some animals do sample, learn, and forage in areas where they will obtain the highest rate of energy return (Smith and Sweatman, 1974; Werner, Mittelbach and Hall, MS). W. Glanz and G.G. Musser have observed the Costa Rican variegated squirrel, Sciurus variegatoides, congregating at sources of abundant food. These observations suggest that this animals may be a patch forager. Studies of other species of Sciuridae have shown that squirrels are able to use their food resources in such a way that they 15 16 maximize their food intake per time investment. Tamisciu- ‘pgs spp., feeding on cones in pine trees, chose trees with the largest number of mature seeds per cone (Smith, 1970). Lewis(1980) actually tested an optimal foraging model with a population of the gray squirrel, Sciurus carolinensis, and found that the animals foraged most heavily in areas of more plentiful nuts. The foraging behavior of the variegated squirrel was examined to see whether it reflected the patchy character- istics of one of the squirrel's resources. The resource, seed crops of Sterculia apetala, is one of eleven plant species known to be included in this animal's diet of fruits and flowers (Glanz, MS). During.the first part of the dry season, this tree is the only species available to the squirrels (Frankie et al, 1974). Later in the dry season, other species are included in the squirrel's diet. At an individual level, a tree with numerous seeds is a rich patch whose value decreases as the seeds are depleted. The squirrels were observed feeding in the canopies of Sterculia. From other research on foraging animals, I expected several patterns to become evident if the squirrels were feeding under a time or energy constraint. Some animals track the availability of patches by sampling (Smith and Sweatman, 1974). The ephemeral nature of the mature crops of Sterculia might require sampling by the squirrels before the animals begin foraging in earnest. Second, when patches of food vary in quality or quantity, 17 some animals concentrate their foraging in the most valua- ble patches (Smith and Sweatman, 1974; Werner and Hall, 1977). The variability in the crop sizes suggested that the squirrels should forage most intensively on crops with more seeds, assuming that the patchy nature of the trees does not affect their foraging. Third, given that the components of a foraging bout--searching, handling and feeding--require different time and energy allocations (Charnov, 1976), the squirrels' feeding activity within a tree should also result in a maximization of their energy return. Finally, as other resources become avail- able, animals have been known to change their foraging patterns (Werner, Mittelbach and Hall, MS). Foraging on Sterculia might become less intense towards the end of the dry season. These expectations were investigated using the population of squirrels at the refuge. Methods The squirrel's foraging patterns were observed at four Sterculia trees (#‘s 3, 8, 15, and 17) for a total of 66 hours. Observations were made from 6 a.m. to 11:30 a.m. This is the time of day when the animals are most active (W. Glanz, G.G. Musser, pers. comm.). Since the squirrels were quite variable in size and markings, I could distin- guish individuals simultaneously feeding in the canopy Eighty—seven foraging bouts were observed. A foraging 18 bout began either when the squirrel entered the tree, or dropped a pod, and ended when he dropped the pod that he had selected during that bout. Events within each foraging bout were timed and recorded as to searching, handling and feeding. Searching time began with the initiation of the bout and ended when the squirrel removed a pod from the branch. Handling consisted of the squirrel carrying the pod to a horizontal branch and chewing through the pod wall. Feeding time encompassed the period when the animals actually ate the enclosed seeds. Any type of interference from con-specifics or potential predators lengthened these activities. Such interrupted bouts (23% of observed) are not included in the statistical analyses of average search- ing, handling and feeding times. When I noticed that the squirrels were searching with- in the crops, I collected data to see whether the animals were distinguishing between different types of pods. Since the pods were similar in color and texture, I chose the largest dimension of the pod as a criterion for establish- ing the squirrel's preference. The squirrels usually chewed through the pod walls to feed on the enclosed im- mature seeds, dropping the empty, chewed pod to the ground. At two of the trees (#‘s 3 and 8), eaten pods that had been dropped by the squirrels were collected every three days over a two-week period. Their lengths were measured and the seed scars counted to learn the number of seeds they had contained. Means and standard errors were calculated 19 for each sampling date. To obtain an estimate of the total number of pods harvested per tree, pods were collected beneath the forty- six trees that had dropped all of their pods--the loss of pods from the canopies being due to the dried pods falling off the branches or to the squirrels' harvesting activity. I then determined whether the pods had aborted. dehisced or been opened by animals. Besides the squirrels, the white- faced monkeys, Cebus capucinus, also chewed through the pod walls on rare occasions. However, their method of opening the pods left a different mark. Monkey-chewed pods could be distinguished from those harvested by the squirrels. The numbers may be underestimated because squirrels do occasionally carry pods away from the trees: however, during the 87 observed bouts, only one pod was carried from .the tree's canopy. This fruit fell within the area of pod collection. Therefore, categorizing the number and type of pods beneath the trees gives a fair estimate of the history of foraging at each tree. In addition, the pods at four of the trees (#‘s 2, 3, 8 and 15) were counted and categorized bi-weekly or every other day to determine the rate at which the pods were being harvested. 20 Results I. Description of Foraging Behavior The squirrel, upon entering a tree with solid, im- mature seeds, would move from one cluster of pods to ano- ther, apparently sniffing the pod wall but possibly nib- bling on it as well. The amount of searching varied greatly both within and between squirrels but ranged from 0.05 minutes (no clusters checked) to 3.5 minutes (11 clusters checked). Over all 32 searching bouts in which the number of clusters checked was counted, the correlation between the length of time spent searching to the number of pods checked was not significant at the 0.05 level (r=0.35). However, searching did decrease in trees with larger crop sizes (r=-0.42, significant at the 0.01 level). After finding a suitable pod, the squirrel, in all but one percent of the bouts, snipped off the pod with its incisors and carried the pod in its mouth to a horizontal branch. Holding the pod length-wise between its front paws, the squirrel would pull away strips of the fibrous pod until it had made a hole averaging 25.4 t 7 mm. wide and 40.1 t 5.7 mm. long (mean : s.e., n=15). Making such a large hole to gain access to the seeds required the most time within each bout; the mean handling time was 3.63 I 1.95 minutes per pod (mean : s.e., n=36). After the hole was made, the squirrel would eat the seeds one at a time. 21 Negligible amounts of time (less than two seconds) were spent removing seeds from the opened pod. I included this time in the estimates of feeding time. The actual eating of a seed took 1.4 t 0.7 minutes (mean : s.e., n=117). No correlation between handling time and number of seeds per pod was found (r=0.16, n=31). Therefore, the handling time per pod was constant and the time spent feeding per pod depended on the total number of seeds the pod contained. Approximately one percent of the time, the squirrels did not remove the pod from the branch before chewing through the wall. During the timed observations one squir- rel consecutively ate three pods without removing them. His handling time on these averaged 6.7 t 3.2 minutes (mean t s.e.). a greater time investment per pod than if he had taken them off the branch. This individual may have been just as slow if he had removed the pods. The squirrels did drop many pods (29% at one particu- lar tree) without completely chewing through them or eating all of the enclosed seeds. This occurred when a large bird flew overhead and the animal flattened against the branch (1% of timed observations), when another squir— rel was chasing it about in the canopy (11.5 %), or when it was turning the pod around in its paws (4.6 %). The large size of the pods probably contributed to this clum- siness. Squirrels were never seen foraging on the dropped pods beneath the tree. 22 Interference from con-specifics seemed to occur at higher frequencies in trees with larger fruit crops where more squirrels congregated (Tree 15: 53% of bouts inter-- rupted by conspecifics, Tree 8: only 16% of bouts inter- rupted). Trees with crops of greater than 1500 pods often had four or more squirrels feeding in them, whereas trees of less than 1500 pods seldom attracted more than two squirrels at a time. Interactions between squirrels occasionally led to a squirrel being chased from then tree's canopy with a pod in its mouth (once during the 87 timed bouts). The chased squirrel usually fed on the pod in a neighboring tree. II. Selection of Pods Since the squirrels within each canopy were actively checking pods in the tree and since all pods were available to the squirrels within a relatively short period, I thought that the animals might be feeding on a particular type of pod. Lengths of the pods, opened and dropped over a two-week period, show a significant decrease in the average size of the pod taken (Figure 3). Comparing the mean lengths of the pods harvested at the beginning of this period to those eaten at the end indicated that they changed significantly (Tree 3: ts=4.2, Tree 8: ts=6.8; probability less than 0.01). The correlations between the lengths of the pods and the numbers of seeds they contained 82 E E 3 FIUUICCIIIICIIIIIII 32 Length of Pods 1mm.) 0‘ to 54 .8 D O D O U l D D 0 O O O ‘I'IIICII 23 0"- Tree 3 Au" Tree 8 III-II-II’CIIIIIII" > III-CIOII-UIIIICICIIII' . II I ‘0. I . . ~I : = ...O. : I ...0. : E .90.. = E ....... E 'I 6‘ ' l ........ g I ...O ' : .......‘ - : 1... . I ‘I ' : E ‘ "m i I = : 1‘ I I I 3 : : i : : I I I I : : : : = ' : J : I ‘ I I I E j, 5 = A I I I I : 3 i 3 3 I I I I : E I I I I I ‘5 4 5 6 7 8 9 10 1 1 1 2 1 3 1 4 15 Day of Pod Collection Figure 3. The successive reduction in the size of pods eaten by s uirrels over a two-week period at two trees Tree 3:r=0.45, Tree 8:r=0.59). 24 are significantly positive at the 0.01 level for both trees (Tree 3: r=0.45, n=65; Tree 8: r=0.59. n=91). III. Rate of Harvesting per Tree Each tree bears mature fruit for approximately four weeks, the length of time depending on the size of the crop (Derr, 1977). Figure 4 shows the rates at which four of these crops were harvested by the squirrels. In each case a slow initial rate is followed by a much accel- erated pace of pod depletion (Table 1). Crop size does not appear to have any major effect on this pattern. However, the very large crop of Tree 15 had a much higher daily rate of predation than the other three trees, pro- bably due to more squirrels feeding in this canopy. Table 1. Average daily rates of squirrel predation at four Tree 15 i + s.e. # of Pods Harvested per Day over trees. # of _ Pods Period: Day 0-20 677 4.82 1 3-13 897 3-73 1 BILI’Z 991 1.47 I 1.74 2972 18.8 1 12.12 Day 21-40 13-7 : 8.20 32.19 1 19-59 16.6 + 26.46 58.36 t ’ 76-97 Day 41-60 3.49 1 6-33 5-37 + 2-73 20.49 I 4.62 56.0 + 33-73 Beyond Day 61 20.0 + 13.15 25 .mmmpp mach pm SPHHHQMHHN>N pfisum wafinsa mamhpflsam can an ampmo>nmg maom mo mommvzmouom one .: mpswflm 3:50:05 :2“. co .60 R 8 on ow an cm 2 . w. 2 u e a. m. m ‘ 0 Mn 0 o~.w a a. o no) 0 on 0..» we D: oe s .H m on m 5 a 00 P u. 0 AK w n. .m om nu 00 II I I I < .oo. 26 The curves shown in Figure 4 resemble the Type II functional response described by Holling (1959). The shape of this response by an individual predator is thought to be due to the development of a search image during the initially slow rate of predation, familiarity with the prey leading to an accelerated rate, and depletion of the prey resulting in the final slow rate. It is interesting that predation on the crops by groups of squirrels apparently showed this same response. IV. Effects on the Total Tree Crops The amount of harvesting on different sized crops was extremely variable (Figure 5: r=0.13, n.s.). However, 74% of the trees had more than 60% of their pods harvested. 0f the twelve which had less than 60% predation, all had relatively small fruit crops of less than 400 pods. Two aspects of these twelve trees were examined to see whether they would explain the low percentage of predation. No connection was found between the distance to the nearest fruiting conspecific and the amount of harvesting on the trees. However, the time of fruit maturation was a possi- ble factor. The smaller fruit crops do tend to bear mature fruit later in the dry season. 0f the thirty trees whose fruiting and flowering were recorded throughout the season, eleven had crops of over 600 pods and nineteen had crops less than this quantity. Ninety-one percent of the trees 27 .maoppwsvm can an ampmm>nwa mono map mo mmmpamopom map and mono map Mo muflm one somzvmp mflnmcowpmaoh one .m ohsmfim 00:.\npoa *0 53:52 00mm 8cm OOON . . 2 009 com 00v 0 . o— Q I I M .n m I D I 6 0 On a O I.) I I 0V m. D: I . S H m 00 A . . . m m. ON / I. I I ‘I J I I I II ow ”a I I II 0 O. O u 00 O O O O. I 8 28 with smaller crops bore mature fruit after the first of February, whereas the crops of only 45% of the larger trees matured fruits after this date. This seasonal trend is discussed in terms of other resources in the squirrel's diet. Discussion The utilization of a particular food resource depends on a number of diverse factors. The resource is often distributed in patches with the abundance of each patch being depleted over time. The availability and value of the resource are likely to change seasonally as other resources become available. The extent of pressure exerted by the animals on the resource may vary with the size of their population, their mobility, and their specialization in feeding. The patterns shown by the resource, Sterculia, indi- cate that such factors should be important in the ecology of the variegated squirrel. The trees vary greatly in the size of their fruit crops, in their distribution and in their timing of fruit maturity. Studies of other squirrel species have shown that these animals do forage in a manner which maximizes their energy return (Smith, 1970: Lewis, 1980). While my study was not a test of the var- iegated squirrel as an optimal forager. the patterns shown by their foraging on Sterculia resemble those predicted 29 from optimal foraging models. Evidence supporting this conclusion comes from the squirrels' activity in individual trees, their selection of particular pods, their rates of harvesting individual crops, and their predation on differ- ent sized crops. Within an individual tree's canopy, the squirrels do search among the available pods. When I examined the pods, I found that the squirrels were first feeding on the larger pods which contained more seeds. Since the handling of the pods requires the greatest time investment per pod, since handling time per pod is constant, and since searching time is relatively less costly, such selectivity increases the probability of obtaining a greater food reward per foraging bout. In a slightly different system, Smith(1970) found that pine squirrels select to forage in trees with more seeds per cone. An alternative explanation to this selection is that, within the short time when pods are maturing, the larger pods could be the first to mature within each crop. With- out knowing the exact characteristics of the fruit crops before the squirrels begin harvesting them, this second hypothesis is impoSsible to negate. However, it is thought that the fruits within each tree mature seeds fairly synchronously with both the solid immature and mature seeds being present once the pods begin to dehisce (Derr, pers. eomm.). In addition, the range of pod lengths eaten at 30 each tree is constant throughout the weeks of squirrel predation. A second line of supporting evidence for the squirrels minimizing their costs while searching for and handling the resource is the pattern shown in their rates of harvesting individual crops. The initial rate was quite slow and resembles a pattern which would result if the squirrels were sampling the maturity of the pods. The intermediate rate was much accelerated, data also implying that a major- ity of the fruit crop was at a suitable stage during one relatively short period. The resemblance of the curves to the Type II functional response suggests that the animals were developing a search image while depleting the crops. However, at trees fruiting later in the season when the squirrels should have already learned to feed on Sterculia, the same changes in rates were found. The squirrels initiated feeding on the crops when the seeds were at the immature-solid stage. These seeds had already gained a large percentage of their potential wet weight and, presumably, their caloric value. At this stage the seeds were also relatively inaccessible to the other canopy feeders. The monkeys (Q. capucinus) have difficulty opening the pods (Janzen, 1972) and chewed through only 0.3% of all pods classified. They visited the trees only after the majority of pods at those trees had already dehisced. Therefore, the squirrels exploited and depleted 31 this resource before it was available to their potential competitors but after it had gained the majority of its food value. 0f final consideration is the percentage of harvesting on different sized fruit crops. Not only did trees with more abundant seeds all have a high percentage of preda- tion, but the majority of individual crops in the local tree population was extensively harvested. Such high le- vels of predation suggest that Sterculia is an important resource to the squirrels during this time. A small number of trees did have a low percentage of their pods harvested. Such variability in the amount of predation could result from different numbers of squirrels in the localities of the trees, differential palatability of the seeds between trees or differences in the timing of fruiting in relation to other available resources. The last of these possibilities was checked in this study. Of the fruits and flowers known to be included in this squirrel's diet (Glanz, MS), only one out of ten species was available at the time when the earliest Panama trees were fruiting. By February, four of the other species became available simultaneously with Sterculia (Frankie et al, 1974). The less intensive foraging on some of the later fruiting trees could be due to the squirrels switching to other food sources. The variegated squirrel has been implicated as a 32 disperser of Sterculia seeds (Janzen, 1972). In my study, the squirrels harvested the majority of the pods while the seeds were still at the solid-immature stage and carried them away from the parent plant at an extremely low rate. Both of these considerations suggest that the squirrels are not very reliable as seed dispersers. However, within the very large crops, a sizeable number of pods reached seed maturity even with 60% predation. Since search time is lower in these trees, squirrels should prefer to feed at them. These abundant seed sources did seem to attract relatively more squirrels per tree, thus creating more intraspecific confrontations and increasing the probabil- ity that a squirrel carrying pods would be chased from the area. Therefore, the variegated squirrel acts as a more reliable seed disperser when it preferentially visits the richer, larger trees. RESPONSES OF COTTON STAINER POPULATIONS DEVELOPING AT PATCHY RESOURCES Introduction The cotton stainer bug (Dysdercus spp., Heteroptera, Pyrrhocoridae) is a wide—spread tropical genus; the species vary little in their biology and morphology (Pearson, 1958; Van Doesburg, 1969). In order to complete a life cycle, most species must feed on host plants in the order Malvales (Pearson, 1958; Van Doesburg, 1969). Consequently, mono- cultures of cotton often support economically-damaging populations of this bug. The few reported vertebrate predators are considered insignificant in the bug‘s ecology, but may have once exerted a selective pressure that has resulted in the immatures being aposematically colored (Pearson, 1958; Janzen, 1972). Invertebrate pred- ators and parasites have been found more frequently with the bug populations but are also not thought to affect the insect's population growth to any great extent (Pearson, 1958). The insects, typical of the Heteroptera, undergo incomplete metamorphosis to reach the adult stage. After hatching, they develop through five size-classes, instars, 33 34 in approximately thirty days (Pearson, 1958). Growth through these stages is distinctly allometric with a decreasing growth rate in the older instars (Blackith et al, 1963). The first instars do not feed, but remain aggregated at the site of oviposition and molt directly to the second size-class (Pearson, 1958). Wing buds first appear in the third instar, are larger in the fourth, and extend the length of the first three abdominal segments in the fifth (Fuseini and Kumar, 1975). Sexual size dimor- phism is conspicuous with the adult females being larger than the males; its onset occurs late in larval develop- ment (Blackith et al, 1963). Fifth instars show some size differentiation of the sexes; less of a difference is found in the fourths. A special aspect of the Dysdercus species is their subsocial behavior of aggregating. The bugs tend to aggregate while they are feeding and molting (Pearson, 1958: Melber, 1979a). Pearson(1958) was the first to suggest that the occurence of aggregations under these situations facilitates the bugs' development by pooling their saliva while feeding and by reducing evaporation from their body surfaces. The bugs feed by inserting the stylets of their mouth parts into seeds via enzymes (Saxena, 1963). These stylets then secrete other enzymes within the seeds which break down the endosperm. The resultant liquid is imbibed. In 35 laboratory studies of bugs feeding on a cotton seed, the feeding activity and the volume of food ingested were both greater per bug in a group of two to four bugs than for a solitary individual (Bongers and Eggerman, 1971). Other attempts to document benefits of group feeding have clarified the consequences of this behavior. All work has been done in the laboratory. Youdeowei(1966) suggested that larvae are born with an innate tendency to aggregate, vision being a prime cause for many insects to collect on one seed when other seeds are available. When starved or dessicated, the older instars (third through adults) prefer to aggregate in moist air when given a range of humidities (Youdeowei, 1967). As temperature increases above thirty degrees Centigrade, the activity of the bugs also increases and aggregations scatter (Youdeowei, 1968; Melber, 1979a). Life history characteristics are affected by.this tendency to associate (Hodjat, 1969; Melber, 1979b). There are positive group effects for all parameters concerned with the rate of development: the duration of embryonic and larval development, egg size, and the maturation of ovar- ies. However, larger groups, twenty-four bugs per confined area, showed negative effects in parameters associated with reproduction: the percent of eggs hatching, the survival of larvae and adults, the number of eggs laid, and delays in oviposition. The densities of feeding insects at which possible advantages due to aggregating have been found in the 36 laboratory are extremely low when compared to the densities found in the field (Derr. 1977). The disadvantages due to crowding might often outweigh the advantages resulting from aggregating. Laboratory tests of crowding and its subsequent food limitation have shown that mortality was increased even before individuals reached the adult stage (Dingle, 1966: Hodjat, 1969). Females were affected to a greater extent than males. Adults of both sexes attained a significantly smaller body size when food-limited (Hodjat, 1969) and cannibalism seemed to occur more frequently (Youdeowei, 1967a). Finally, fewer adult fe- males broke reproductive diapause by laying their eggs (Derr, Alden and Dingle, MS). These laboratory results of changes in life history characteristics, behavior and adult size suggested several predictions of what should occur in field populations. These predictions were examined with the cotton stainer most common in the Costa Rican dry forest, Dysdercus bimaculatus. In this region four host plants are available to the insect populations: Sterculia apetala, Bgmbacqpsis quinatum, Pseudobombax septinatum and ggibg pentandra. For the first four months of the dry season, only Sterculia is present as a resource. Population growth at seed crops of Sterculia show three distinct stages (Derr, 1977). After adults colonize the tree, food is abundant and initial immature densities 37 are low. Adults produced under these conditions were expected to show the benefits of aggregating by maturing at a larger size. In the second stage, immature densities are high and food is rapidly being depleted. Immatures were still expected to reach the adult stage but at a re- duced size due to the effects of crowding. In the third stage, immatures are still present but virtually all food is gone. In order to escape this situation, predictions were that immatures would seek alternate food sources, possibly including cannibalism. One potentially over-riding factor in the ecology of Q. bimaculatus is environmental moisture availability. During the six months of the dry season, rainfall drops considerably and the relative humidity decreases to an annual low. Through laboratory and field work, Derr(1977) has shown that the bugs are limited in their food utiliza- tion by the availability of water. Because increasing population size and food depletion covaries with decreasing relative humidity, the effects of these two in limiting body size can not be distinguished in a field study. The data presented in this thesis suggest that both play a role in affecting the bug's life history. Methods When seeds first become available beneath a tree, adult insects fly to the tree, feed and produce immatures. 38 The populations at three trees--identified at #2, 8 and 15--were monitored in their utilization of the seed crops. These trees were located in continuous upland forest and had little vegetation beneath them. The sizes of the fruit crops, the percentages of the crops harvested by squirrels, the total numbers of seeds available to the insects and the dates when seeds were present beneath the canopies are shown in Table 2. Table 2. Characteristics of trees 2, 8 and 15. Tree Total # % of Pods Total # of Period of of Pods Eaten by Seeds Avail. Seed Availability Squirrels to Insects 2 991 86 .1 1315 11/25/79-2/6/80 8 677 76.6 237 1/2780-3/3/80 15 2972 95.4 3037 12/3 79 -2/6/80 The abundance of food at each tree was determined by recording all seeds on the ground. Every two days all freshly fallen seeds were marked with a number written on a rectangle of plastic flagging and secured beside the seed with a toothpick. The state of previously fallen, marked seeds and the numbers of bugs feeding on them were noted on subsequent visits. Seeds on the ground under trees 2 and 8 were frequent- ly eaten by agoutis, Qgsyprocta punctata. At tree 8 all seeds disappeared within 48 hours after they were marked: at tree 2 only 5% of the seeds remained under the tree for 39 longer than a week. Therefore, seed abundance at these two trees is probably underestimated. No seeds were removed from beneath tree 15. A small percentage of fruits did expose their seeds in the canopy. However, these seeds were soon eaten by the squirrels, Sciurus variegatqiggg, the white-faced monkeys, Cebus capucinus, and the small parrots, Amazona albifrons, before the bugs could reach them. To discover the insects' utilization patterns on these ground seeds, five fresh seeds were placed in shade around each tree, two meters from its trunk and equidistant from each other. At trees 2 and 15 seeds were placed at weekly intervals: at tree 8 they were placed every other day. These seeds were obtained from entire pods dropped by squirrels at other trees. Bugs were allowed to colonize the seeds beginning at 9 aam. After one and a half hours, each seed with its feeding bugs was scooped into a half pint jar and covered with a lid. The immatures were allowed to escape at a slow rate by off-setting the lid slightly. The bugs being released were counted and categorized as to stage of devel- opment. These numbers indicate the weekly change in the densities of bugs on fresh seeds. After several weeks of populations growth at particu- lar trees, immatures were seen walking away from the host plant. Bugs were found up to 200 meters away from the tree 40 whereas on other dates they had remained concentrated beneath it. Since all three trees had at least one fruit- ing conspecific within thirty-five meters of them, the bugs had a high probability of finding a new source of food. In order to quantify the distances the immatures travelled and to determine at what levels of food stress or crowding the migrations occurred, the following manipu- lations were made. At tree 15, two days after the.major migration was observed, two fresh seeds were placed every 25 meters in transects running east and west for a distance of 500 meters. After one and a half hours, the numbers of bugs which had colonized these seeds were counted. At tree 8, three piles of two fresh seeds were placed 100 meters from the trunk. These seeds were placed to the north, south and east every four days at 9 a.m. The seeds were again left for an hour and a half. Bugs which had colonized these seeds were subsequently counted, their stages recorded and their pronotums measured. Size changes in all stages of this insect are based on measuring the widest part of the pronotum. Measurement is easily and accurately made because this body part is so flat. For the adults, the lengths of the wings were also measured. This measurement is the one most often reported in the literature. The lengths of the adults‘ fore-wings were positively correlated to the widths of their pronotums (r=0.92). 41 I expected the adult body size attained by individuals to decrease as food became limiting. To monitor this change, colonizing adults were collected at the beginning of the fruiting period of each tree. Their pronotums and wings were measured with a pair of calipers. Sexes were distinguished by differences in the terminal abdominal segments. The bugs were then marked on the doral side of the thorax with a spot of colored paint particular to that tree and released. The number of adults measured and marked at different trees varied since it depended on how many adults could be found within a time-limited search. Recently molted adults can be distinguished from colonizing adults. For a short time after they molt, their colors are extremely pale. These new adults were collected and preserved when the first group of fifth instars meta- morphosed and at weekly intervals thereafter. Their pro- notums and wing-lengths were measured using an ocular micrometer. These adults were difficult to find, probably because the color change in the exoskeleton is rapid. Once a population of immatures was present, weekly samples were collected to see whether the nymphs also showed any size change. Individuals feeding on five dif- ferent seeds, covered by aggregations of bugs, were shaken into a jar of 70% alcohol. When possible, the pronotums of fifty individuals of each instar (2nd through 5th) were measured using an ocular micrometer. Occasionally, fewer than fifty individuals of the older instars had been 42 collected. In the fifth instar, sexual size dimorphism was evident, resulting in two distinct size classes for this instar. In the other three instars, no such distinc- tion was found and the sexes were not differentiated. No attempt was made to collect and measure first instars since immatures in this stage remain at the oviposition site and do not feed. Means and standard errors of pronotum width were calculated for the five size-classes on each collecting date at the three trees. Differences in variance were insignificant by the Fm statistic: means ax were compared with the Student-Newman-Keuls test. Results I. Changes in Behavior and Body Size within Populations As shown in Table 2, each tree possessed a different set of resource characteristics. In this first section, the development of the insect populations at the three trees will be treated separately. Tree 2 was the first of the three trees to have seeds dropped beneath it (Figure 6). These few seeds attracted a sizeable population of adults, probably because it was one of the first trees with mature seeds. More than two weeks then passed before more seeds became available at tree 2. During this time, adults frequently cannibalized 43 each other. When seeds were next available (December 14th), a population of immatures was already present and the bug density on the seeds was quite high (Figure 7). After this date, the squirrels fed in the canopy regularly and seeds continued to be available to the bugs. The average density of bugs per seed decreased, possibly indicating that the total insect population was not increasing. As the crop was depleted, fewer seeds fell to the ground and the number of bugs per seed again increased. Crowding on the seeds became more severe. Many immatures were seen climbing across the aggregations, unable to find a space to feed. At this point, in the eighth week of population growth, two different behaviors became noticeable. Immatures were found wandering farther from the base of the tree and were climbing up the trunk into the canopy. Second, cannibal- ism was noted among the immatures. The older instars would approach a feeding aggregation, grab an individual with all six legs, and attempt to pierce it with its mouthparts. When all seeds had been depleted, a large population of immatures was still present beneath the tree. The adults produced at this tree (#2) were signifi- cantly smaller than the adults that had first colonized the tree (Figure 8, Table 3). This was true for both the males and the females. Figure 8 shows the mean pronotum widths for each size-class plotted across time. 44 .N coup pm Seascasmom PoomcH may ow oapwaflm>m macaw mo moamccsnm %meoz one .0 onsmflm do“. .coq .000 $02 a v. N .m vn N. o_ n mm _N 3 A — vm 0 g o o 8 )gaeMm,pun019 Si: spaeg usaid 10 °°N 45 .N can» ya comm amomm pom mcflaoom mmsn mo nonssc map CH mwamco magmas one .5 opswflm do“. 62 .000 .>oZ .w v. A On em N. o. n mm a v. n _ vm oo— O O peag/Sfing ’0 'ON )1 oon .oov 46 Table 3. Mean pronotum widths of insects at tree 2 compared with the Student-Newman-Keuls test. Stage Range of Sample Obs. LSR Level of Dates Compared Range Significance Adult 2 11/30/79 to 8 0.431 0.16 0.01 1 0 Adult or 11/30/79 to 0.455 0.24 0.01 1/ 5/80 Fifth 3 12/28/79 to 0.15 0.66 n.s. 2 4 80 Fifth on 12/28/79 to 0.09 0.23 n.s. 2/ 4/80 Fourth 12/28/79 to 0.11 0.20 n.s. 2/12/80 Third 12/28/79 to 0.02 0.18 n.s. 2/12/80 Second 12/28/79 to 0.01 0.14 ms. 2/12/80 Within the new generation of bugs there was no significant size change at different dates for any of the immatures or adults. In comparison to tree 2, tree 15 supported a rather different insect population. This tree had almost three times as many seeds which were continuously available to the developing insects (Figure 9). The density of bugs per seed remained low although the total number of bugs feeding beneath the tree was much higher than the total at tree 2 (Figure 10). In the seventh week of population growth there was a temporary decline in the number of seeds on the ground and crowding on the seeds increased. At this tree cannibalism and migration were not ob- served until the ninth week. Of the seeds placed along 47 Figure 8. The reduction in the mean width of the insects' pronotums at tree 2. 48 . 4.4 . 4.0 O 3.6 O 3.2 2.- W O 2.4 M O E 20 E . E 3 O 5 I i i 1.6 \ u- . ° I '5 3112 3 ———‘¥ . 'x M 08 .1r_l._!L 1,,lllir_n_____ 3 0 9 1 9 2 8 8 1 8 2 8 l 7 Nov. Dec. Jan Feb. Figure 8. M Adult 9 Adult 0" V? VI 49 Figure 9. The weekly abundance of seeds available to the insect population at tree 15. Figure 10. The weekly change in the number of bugs feeding per fresh seed at tree 15. 50 m m m 6 4 2 0303\9590 :0 Boom 5.0.“. *0 .02 14 24 3] 27 10 l7 l3 3 0 Nov. Feb. Jan. Dec. Figure 9. 300 0 0 2 poom\mmam *0 .02 0 m M 14 24 31 17 10 20 27 13 30 Nov. Dec. Feb. Jan. Figure 10. 51 transect lines after the first wave of migration was seen, none were colonized beyond 100 meters. The largest number of bugs was found at 50 meters. Ninety-three percent of them were fourth and fifth instars even though only 60% of the insect population at the host tree was in these two stages. When seeds were entirely depleted, a large population of immatures again remained beneath the tree. In terms of their body size, both male and female adults again showed a significant decrease when compared to their parents (Table 4, Figure 11). Sizes of the second through fifth stages showed no significant dif- ferences. However, the range of means between the initial and final sampling dates decreased with decreasing instars. Table 4. Mean pronotum widths of insects at tree 15 compared with the Student-Newman-Keuls test. Stage Range of Sample Obs. LSR Level of Dates Compared Range Significance Adult 2 12/16/79 to 0.733 0.27 0.01 1/30/80 Adult a 12/16/79/to/ 0.851 0.18 0.01 1 30 80 Fifth 9 12/28/79 to 0.30 0.43 n.s. 2/12/80 Fifth.d' 12/28/79/to/8 0.281 0.47 n.s. 2 12 0 Fourth 12/20/79 to 0.25 0.49 n.s. 2/12/80 Third 12/20/79 to 0.07 0.18 n.s. 2/12/80 Second 12/20/79 to 0.00 0.15 n.s. 2/12/80 52 Figure 11. The reduction in the mean width of the insects' pronotums at tree 15. 4A 4.0 3.6 3.2 2-8 2.4 N o 'o 71 Width of Pronotum (mm) 5 53 03 _JL_.'_JL_I__‘p——4h—‘—-I- 16 Dec. Figure 11. 26 Size Class Adult 2 Adult 6' 54 Unlike trees 2 and 15, tree 8 fruited during the latter part of the dry season. In addition, its seed crop was extremely small. However, like tree 2, seed availa- bility lapsed initially (Figure 12). Again, a large popu- lation of immatures built up without a resource base. The density of bugs on seeds was extremely high (Figure 13). Cannibalism by both the adults and the immatures was observed throughout population growth at tree 8. Immatures first colonized the seeds placed 100 meters away from the trunk just a week and a half after the population of insects began developing. The bugs colonizing these seeds were all in the fourth and fifth instars and were not significantly different in size from the population as a whole. The average body size of the colonizing adults and the next generation of adults again showed a significant difference (Table 5, Figure 14). The male fifth instars also showed a significant decrease in mean pronotum width. The other size-classes exhibited no significant change in body size. II. Changes in Body Size Across the Season The individual populations indicated that a decrease in the insects' body size occurred with resource depletion. When the size data from the different trees' populations were combined and plotted across the season (Figure 15). 55 Figure 12. The weekly abundance of seeds available to the insect population at tree 8. Figure 13. The weekly change in the number of bugs feeding per fresh seed at tree 8. 56 m 0 I 0302:9590 co boom 32:". *0 .02 18 25 11 28 21 14 Mar. Feb. Figure 12. 300 .m. poow\nmam *0 62 m 10 18 25 11 Mar. Feb. Figure 13. 57 4.0 C 3.. i_z_9_s Adult 9 12 \ . I o 28 Adult 0' I 2A I 9 \19 o E‘zo O Si Va” E 2 O C 3 L6 1:. “o- W '5 IV 3112 .3 Ix .-——.——.-——I_ III 08 O---‘—-I—-C-- ll 28 ' ‘1 17 27 Jan. Feb. Figure 14. The reduction in the mean width of the insects' pronotums at tree 8. 58 Table 5. Mean pronotum widths of insects at tree 8 Stage Adult-9 Adult 3 Fourth Third Second compared with the Student-Newman-Keuls test. Range of Sample Dates Compared 1/28/80 to 2/29/80 1/28/80 to 2/29/80 2/ 4/80 to 2/18/80 2/ 4/80 to 2/18/80 2/10/80 to 2/27/80 2/10/80 to / /8 2/27/80 2 10 O to 2/27/80 Obs. Range 0.437 0.307 0.240 0.20 0.50 0.00 0.00 LSR 0.32 0.22 0.27 0.20 0.22 0.11 0.09 Level of Significance 0.01 0.01 n.s. 0.05 the decrease in size was negatively correlated with time for the older size-classes (Table 6). When the range of means were compared, again, only the adults showed any significant decline in size. Table 6. Significant changes in insect body size across Stage the dry season. Coefficient of Correl. betw.sMean Pronotum Width and Date (r) Le of Si vel gn. Obs. Range of Mean Pro. Width over Samples Adult Adult Fifth Fifth Fourth Third Second -0096 -0096 -0 I88 -0.90 -0089 -O.64 +0.46 0.01 0.01 0.01 o .01 0.01 0.01 0.05 092‘ O O H mu, PM H.141 t0 «Pl-4A0 -\)~\).| A ‘ OOOOOHH LSR 0.38 0.22 0.54 0-55 0.51 0.20 0.16 Level of Sign. 0.01 0.01 n.s. n.s. n.s. n.s. n.s. 59 Figure 15. The reduction in mean body size of Dysdercus bimaculatus with the progression of the dry season. 60 .Tree 2 ITree 8 ATree 15 4A1 40 u 16' Size Class - Adult 2 53.2 2.8 AdU” I u 2.4- .. V 3 E 2.0 .5, v d‘ E 2 O c 2 L6 m H- O .1: IV 3212 3 pr Ill Q3 11 3 1 3 2 3 2 12 22 1 l 1 21 2 Dec. Jan. Feb. Mar. Figure 15. 61 Discussion A reduction in adult body size has far-reaching consequences for Dysdercus bimaculatus. The adults pro- duced by the end of the dry season must over-winter the wet season in reproductive diapause. Since larger bodies gen- erally have greater fat reserves (Derr et al, MS), a smaller individual might not survive the food stress associated with the wet season. In laboratory studies of a close relative of Dysdercus, Dingle et al(in press) demonstrated that during periods of food deprivation larger individuals suffered lower mortality than smaller ones. Population growth may also be affected by body size. An individual which colonizes temporary resources possesses a competitive advantage if it gives rise to relatively more offspring (Lewontin, 1965). In the laboratory, larger, heavier Q. bimaculatus produced greater numbers of eggs (Derr et al, MS) and so had a higher reproductive poten- tial. There was no significant change in the size of the eggs produced. However, if the individuals are constrained by the requirements of being larger, there are disadvantages. They require more energy for growth and their generation time is longer. With a resource such as Sterculia, these handicaps may become critical. Derr(1977) has suggested 62 that adult females "choose" to lay their eggs at trees with seed crops sufficiently large to support at least one generation of bugs. However, squirrels harvest a large percentage of the crops in an unpredictable manner after the females lay their eggs. For example, a tree studied by Derr(1977) had a crop size similar to that of tree 15 but with only 50% squirrel predation. A.population of insects was supported at this tree for twenty weeks and adults were produced for three-quarters of this time. At tree 15 in my study, the resource was available for only half as long a period due to the depletion by the squirrels and adult insects were produced for only one third as many weeks. Both of these two trees had extremely large seed crops. However, the majority of trees have crops that are magnitudes smaller in size. Even with no squirrel predation, seeds are available at these trees for the development of only one insect generation (Derr, pers. comm.). A decrease in this resource places the bug populations under a time and food stress. The insects respond to such stress in three ways. First of all, as resources are depleted, their average body size declines. The advantages of being larger are lost but the bugs do survive to the reproductive stage. This size change is only seen in the adults. There are possible explanations for this pattern. Immatures molt to the second stage without feeding or leaving the 63 relatively moist habitat where they were laid. There should be no stresses in molting to the second instar. The second, third, and fourth instars remain in these stages for only two to four days. Their change in body morphology from one instar to the next is slight and probably requires a relatively small amount of food. However, to reach the adult stage, immatures must develop through a more extensive morphological change. The molt to the adult stage requires complete maturation of the wings and the reproductive organs. The extensive histogenesis which takes place at the end of the fifth stage consumes a large percentage of stored fats (Rockstein, 1964). The immatures also remain in this instar for a relatively longer period of time, suggesting that fat storage occurs during this stage. If the amount of stored fat is low, the bugs might be limited in the size at which they mature. When resources were extremely low, the immatures responded through migration and cannibalism. In popula- tions studied by Derr(1977), the bugs did not have the first option. Her trees were widely spaced and immature populations were isolated from new trees. However, in the patches at my study site, the bugs leaving a depleted crop had a distinct possibility of finding one that was beginning to produce mature seeds. Additional support for the earlier suggestion that the fourth and fifth stages are those most stressed is that these two instars were the ones 64 found migrating away from their original host plants. It is also likely that with their larger size these bugs have greater mobility and so can afford this avenue of escape. Cannibalism has often been observed in natural populations. Fox(1975) suggested that it is a sensitive method of preserving the competitive abilities of the successful individuals while maintaining the reproductive output of the survivors. For 2. bimaculatus, cannibalism appears to be a response to extreme food deprivation and crowding. Fourths and fifths initiated most of the attacks although the younger instars were quick to take advantage of a recently assassinated conspecific. The change in body size must also be considered with respect to the environmental factor of moisture availa- bility. Work by Derr(1977) indicated that reduced moisture results in food limitation even when seeds are abundant. Laboratory experiments conducted with the insects feeding at either 95% or 55% humidity demonstrated that indivi- duals had decreased survival and growth rates at the lower humidity. She suggests that at lower humidities the pro— duction of the bugs' saliva, essential to their consump- tion of the seeds, is drastically reduced. In my study, the average body size continually decreased with the drop in relative humidity. Figure 16, drawn from data published in Frankie et al(1974), shows that the relative humidity had already diminished when 65 the first adults matured in December. The smallest amount of moisture is available in February when the smallest adults matured. Derr(1977) found that the survivorship of immatures reared at 95% relative humidity was greater than when reared at 55% humidity. Unfortunately, both of these test humidities are outside the average range of humidities in which the natural insect populations develop. Q 0 My Relative Humidity \: o 60'- 3; Mont 0 N D J F M A M J Month Figure 16. The reduction in mean monthly relative humidity during the dry season. Data from Frankie et al, 1974. Both the physical factor of moisture and the biologi- cal ones of food limitation and crowding probably affect the populations. Even with abundant seeds and low density aggregations, the first offspring produced at tree 15 were smaller than the adults that had begun the population. 66 This decrease suggests that, at approximately 75% humidity, the drying air was already stressful. However, the first offspring that developed at tree 15 were significantly larger than those first produced at tree 2. These two populations underwent the same climatic effects in similar habitats but had widely varying amounts of seeds. There- fore, it appears that food limitation also affected the attained adult body size. The finding of significantly smaller individuals towards the end of the dry season produces a puzzle. At the beginning of the dry season, large adults which puta- tively represent the previous year's final adults colonized the trees. There are at least three possible solutions to this riddle. Before the end of the dry season, generations of bugs develop at the two other host species, Pseudobombax septinatum and Qgibg pentandra (Derr, 1977). These trees may provide a more accessible, richer source of energy which permits the bugs to develop to a larger size. This suggestion seems unlikely, however, because these seeds are magnitudes smaller in size and are scattered through dispersal by wind (Derr, 1977). The second possibility deals with the fact that some trees of Sterculia are located in riparian habitats. These trees do not lose their leaves and relative humidity probably remains high around them. Insects developing 67 in these areas might not be stressed by dry air. On the other hand, these trees typically fruit in late March (Janzen, 1972). The very large adult populations that build during the dry season (Derr, pers. comm.) would be present to colonize these trees. Food limitation due to crowding might still result in smaller adults. The third possibility, and the most probable, is due to the cultivation of cotton in October and November, just before the natural hosts first fruit (Derr, 1977). These fields of cotton would undoubtedly provide an excellent, abundant resource. In addition, moisture stress at this time is low; relative humidity is at the annual high of 80%. Such conditions would be optimal for the production of large, unstressed adults which then colonize the first Sterculia. Support for this solution comes from work by Withycombe(1924). He found that adults of Q. howardi that developed in cotton fields were larger than those that migrated from secondary hosts in the mountain forests. The on-going irrigation program in Guanacaste will permit the cultivation of cotton during most of the dry season. This practice might affect the ecology of D. bimaculatus. As speculation, adult populations produced during the dry season would be much larger initially, crowding at the natural hosts would become more intense sooner and the maturing adults would be small and numerous. However, in terms of the annual cycle of the 68 insect population, there is no reason to believe that the density-independent factors which operate during the wet season would be affected. Therefore, the responses of Dysdercus bimaculatus while developing under food and moisture stress, elucidated in this study, should remain important in the bug's life history. CONCLUSION As stated in the preface, the two sections of this thesis are disjunct. I have separately discussed and drawn my conclusions for each one. However, a few final remarks on the ecology of this animal-plant system are appropriate. One animal which has not yet been associated with Sterculia in the literature is the agouti, Qgsyprocta punctata. At all but one of the trees I studied, I observed these rodents feeding on the seeds beneath the canopy or found evidence that they had visited the tree. Typical of the scatter—hoarding behavior of this animal (Smythe, 1970), twice I saw two different agoutis burying seeds approximately thirty meters away from the parent tree. This distance is equivalent to that found between trees within patches in my study. Seeds within this distance, dropped by the squirrels, would be exposed on the ground and vulnerable to injury by the wandering immature cotton stainers. Therefore, I propose that the accumulation of more trees in a patch is a result of the agoutis burying whereas the creation of new patches is effected by the more mobile squirrels. 69 70 The reproductive success of a tree may be proportional to the size of its crop. The trees with larger crops attracted more squirrels and thus had an increased proba- bility that pods would be carried away from the injurious insects. The asynchronous fruiting of the local tree population might also be a factor which concentrated the squirrels at one particular tree. The very large trees also had a greater number of seeds falling to the ground because more pods dehisced before the squirrels depleted the crop. This increased the probability that agoutis would store some of these seeds by burying them. There- fore, there may be some selective pressure for Sterculia to produce large crops. Janzen(1972) noted that these trees often fruit every other year, possible support for my suggestion that a large crop is better than two small ones. The disparity in the distribution patterns of Sterculia--widely dispersed in the areas studied by Derr(1977) and Janzen(1972) versus patchy at my study site--is probably due to human interference. In the areas studied by the other biologists, squirrel predation on the trees' crops was low ("less than thirty percent" (Janzen, 1972), averaging 0.05% in the crops studied by Derr(1977)). and the agoutis were not implicated in their findings. Their study areas are located much closer to human establishments and are more disturbed by cattle 71 operations. Hunting of the mammals in these two areas evidently decimates the squirrel and agouti populations, thus reducing the establishment of new trees. APPENDICES APPENDIX A Characteristics of the Tree Population, Sterculia apetala Table A1. Crop size, circumference at breast height and percentages of crops aborted, dehisced or eaten.1 Tree Crop Size CBH % % % (# of Pods) (meters) Aborted Dehisced Eaten 1 157 3.22 4.5 69.4 26 1a 75 1-73 1b 736 1.76 1.5 33.2 65.3 1C 131 1036 1d 0 1.85 1e 0 0.95 If 43 1.54 O 60.5 39.5 1g 43 1-79 2-3 2-3 95-3 1h 19 1.68 11 19 1.50 0 63.2 36.8 1j 410 1.78 3.4 79.6 16.8 1k _59 1.23 0 8.5 91.5 11 9 1.80 0 55.5 44,4 2 991 2.54 0 13.8 86.1 2a 92 2.52 0.02 23.0 76.9 2b 6 3.22 0 0 100.0 2c 28 2.94 2d 0 3.10 3 897 1.58 1.1 18.7 80.2 4 1159 1-77 0-5 21-5 77-9 5 211 1.69 0 10.4 86.9 6 0 0.72 6a 0 0.55 8 677 1.78 O 23.3 76.6 8a 58 1.12 O 36.2 63.8 8b 287 3.00 1.1 83.6 15.3 8bb 0 3.20 1Trees with fruit crops but without data on the percentages of the crops had not yet produced mature fruit by March 8, 1980. 72 73 Table A1 (cont'd). Tree Crop Size CBH % % % (# of Pods) (meters) Aborted Dehisced Eaten 8s “,0 0.40 80 22 1.70 0 27.3 72.7 8d 17 1.20 0 47.1 52.8 8P 16 2.28 100.0 (in past.) 9 17 1.46 93. 0 1.36 9b 236 1.75 9c 201 1.76 0 15.7 84.3 9d 223 1.46 40.4 13.9 82.1 9e 257 1.48 0 6.2 91.3 9f 0 0.72 98 931 2.71 0 3-9 96-3 9h 92 1.48 0 22.8 77.2 15 2972 3.30 4.5 4.5 95.4 15a 100 1.50 0.03 0.03 97.0 15b 1116 2.63 13.6 13.6 84.0 150 0 2.07 16 343 1.84 0.05 2.6 96.8 16a 0 1.44 16b 0 0.84 17 2658 3.65 0 15.7 84.3 18 1503 2.70 8.4 8.7 82.8 C1 0 2.00 Claa 191 1.67 3.7 14.6 81.7 0100 O 0.69 02 0 2.30 C3 184 1.90 1.6 1.08 97.3 C4 43 1.00 05 213 1.70 2.8 6.6 90.6 C6 1920 2.00 C7 131 2.50 0 8.39 91.6 C8 0 3.10 G8cc 92 1.48 O 22.8 77.2 C1a 239 1.70 1.7 7.1 49.4 C1b 207 2.88 1.9 6.8 91.3 C10 18 1.81 O 22.8 77.8 C1d 0 0.81 C1e O 1.82 le 256 2.13 01g 0 1.27 Clgg O 0.66 C1h 0 1.74 C11 0 0.95 C13 70 2.30 s 72 2.55 2.8 20.8 76.4 Sa 1826 2.70 O 36.4 63.6 Sb 344 2.00 74 Table A1 (cont'd.). Tree Crop Size CBH % % % (# of Pods) (meters) Aborted Dehisced Eaten SC 0 1.37 Sd 169 1.37 O 68 31.9 Se 124 3.00 O 59.7 40.3 Sf 175 1.50 Sh 496 2.23 1.2 71 27.6 Si 73 1-33 Stl 139 1.18 0 2.2 97.8 St2 263 1.07 0.8 1.1 98.1 St3 31 1.33 0 3.2 96.8 M 334 1.98 0.6 9.9 90.1 W1 0 0.70 82 O 2.96 W2a 0 0.11 W3 46 1.94 0 5.6 93.5 W3a 0 0.55 WS 0 1.40 W6 0 1.85 Table A2. Distances between patches and between trees within patches in meters. Patch Name Distance to Tree Distance to Distance to Nearest Nearest Nearest Patch Conspecific Conspecific w/Fruit Marsh 80 1 17 17 1a 17 17 1b 38 38 lo 26 31 1d 25 25 1e 58 58 1f 27 27 1g 25 27 1h 42 42 11 42 42 Grove 400 2 13 13 2a 29 29 2b 13 13 Cerro de 99 3 18 18 Sopapillo 4 18 18 5 38 38 6 18 18 Table A2 (cont'd.). Patch Name Distance to Nearest Patch Guayacon 76 Guayacon 76 Look-out 1 Station 76 Guayacon 95 Look-out 2 Stream 400 Hacienda 99 Cueva del 350 Tigre Steve's 110 75 Tree Distance to Nearest Conspecific 30 37 29 30 47 6O 60 Distance to Nearest Conspecific w/Fruit 30 37 29 30 47 60 6O Table A2 (cont'd.). Patch Name Distance to 76 Tree Distance to Nearest Patch Rosario 220 W1 W2 W2a W3 W3a W5 Nearest Conspecific 75 26 26 24 20 20 Distance to Nearest Conspecific w/Fruit Table A3. Date of initial fruit dehiscence at thirty trees. Tree Date of Fruit Dehiscence 1 January 59 1980 1a February 29. 1980 lb January 17. 1980 1f February 19. 1980 1g February 19, 1980 1i March 4, 1980 1j February 19, 1980 1k February 12, 1980 ll February 25. 1980 2 November 24, 1979 2c March 3, 1980 3 January 10, 1980 4 December 28, 1979 5 December 20, 1979 8a February 5. 1980 8b January 59 1980 Be February 15. 1980 8d February 25. 1980 8P February 17. 1980 15 November 30, 1979 15a January 30, 1980 15b January 5. 1980 16 January 4, 1980 17 November 22, 1979 18 January 4, 1980 C3 February 10, 1980 05 February 14. 1980 C7 February 10, 1980 8 January 13. 1980 9 March 8, 1980 Table Tree 15 Table Tree APPENDIX B Size Measurements of Dysdercus bimaculatus B1. Size measurements of female adults. Date 11/30/79 1/ 5/80 1/12/80 1/28/80 2/12/80 2/29/80 12/16/79 12722§g9 1 0 1/30/80 B2. Size Date 11/30/79 1/ 5/80 1/12/80 1/28/80 2/12/80 2/29/80 12/16/79 127227g9 1 0 1/30/80 # of Bugs Meas. # of Bugs Meas. 54 >4 mNWO-Ptml-‘HU‘ \OHVNWCDVUO‘Wh m:::muw:¢¢ X uuwwmuuwwu Hm mxo 0014 N .l:\0\o (S'H 4:00 #0 000K») 77 Pronotum Width: 8090 0.32 0.16 0.25 0.30 0.15 0.06 0.30 0.26 0.23 0.16 measurements of male adults. Pronotum Width: s.e. 0.20 0.15 0.41 0.25 0.10 0.06 0.17 0.10 0.18 0.15 Wing Length: R 11.83 12.05 11.95 11.26 10 .46 10.25 11.98 11.67 11.93 11.36 (D 0000000000 (0 PCDCDHNNO‘WJ‘VF‘O 000V0\\0\}O\-{:‘\0 - Wing Length: E 10.69 9-95 10.18 9.52 9-23 8.48 11.98 10.28 10.29 9-17 (D (D t¢¢&§w&%¢o 0000000000 \OOOQNVmw-P'UI 78 Table BB. Size measurements of female fifth instars. Tree Date # of Bugs Pronotum Width: Measured — x s.e. 2 12/28/79 15 2.55 0.58 1/ 5/80 13 2.53 0.67 1/12/80 19 2.52 0.64 1/21/80 29 2.56 0.44 1/28/80 20 2.47 0.44 2/ 4/80 10 2.40 0.35 8 2/ 4/80 26 2.48 0.41 2/10/80 25 2.31 0.35 2/18/80 19 2.24 0.44 15 12/28/79 9 2.59 0.35 1/ 4/80 12 2.59 0.47 1/11/80 13 2.56 0.28 1/19/80 10 2.43 0.24 1/28/80 18 2.42 0.42 2/ 4/80 25 2.38 0.42 2/12/80 22 2.29 0.30 Table B4. Size measurements of male fifth instars. Tree 2 15 Date 12/28/79 1/ 5/80 1/12/80 1/21/80 1/28/80 2/’4/80 zéfi4égo 2 10 0 2/18/80 12/28/79 17.1738 1/19/80 $723728 2/12/80 # of Bugs Measured 9 22 23 31 25 41 50 5O 50 10 26 17 27 28 29 40 Pronotum Width: X 2.18 2.21 2.18 2.18 2.16 2.09 2.14 2.02 1.94 2.28 2.25 2.16 2.14 2.12 2.11 1-99 s.e. U1 0\ 000000000000 {rmmtmmmpmJ—‘m 'flpompo Eopm mpmv mcflmmpom Hmuuflsvm .HU manna 83 mm.: 00.00 mm.m 66.0 66.0 06.6 50.60 66.6 60.60 60.60 66.0 65.6 00.5 66.00 66.0 00000200 .Ummm Q .Hucwm HNNd‘M Copmm mcmmm mo.% dMPOB 06.6 66.6 66.0 66.0 60.0 mm.o 0809 .vmmm 0000 .00066 00000 vmxomnu 60006360 mo % 60.66 60.50 06.0 66.0 mm.6 mu.m om.0 om.m mm.0 om.H W66.6 0mm.m oo.m mN.H 665.6 666.6 om.o 606.0 mN.0 Bflfiod m 0869 condom 06000 00606 00.60.6 66.60.5 00.60.5 00.60.5 oo.mm.m 06.66.6 06.66.6 66.066 60.606 0N0m605 00060.5 ON0N005 mmuomum 06.6006 00.66.5 06.50.5 0600605 oo.Nmum 0606605 .0000 0006 0869 Hmphaswm H % 06\6 \6 06\0 \6 06\0m\0 06\6 \6 06\06\0 006m 60: 56: :m: m 000 666 mcom mo % 0009 .A.U.Pcoov Ho manme 84 666.0 0 650.5 00066.5 66.5 6 06.6 66.: 6 66.6 60060.5 60.0 6 06.6 56.6 6 66.0 06066.5 6 60.0 6 60.0 60 6 66.6 06006.6 0 06\60\6 600 006 6 66.0 06.66.5 00.0 6 60.6 65.6 006 00 66.0 66.6605 66.0 0 66.0 60 6 66.0 0066.5 0 06006 566 06.0 0 06.0 60 0 06.0 66.506 66.6 0 66.0 00.6 50.0 00.0006 66.5 6 06.6 66.6 50.0 06.66.5 50.6 0 60.0 66.6 00 06.6 00066.5 66.6 6 06.6 60.0 6 00.6 60.6605 66.0 6 66.0 m0 6 60.0 66:06.5 05.6 6 05.6 o o 50.o 66.0605 66.6 6 66.6 60 6 65.0 6606605 06.6 6 06.6 0066 0 0 66.66.5 66.0 0 66.0 0 6 66.0 00066.5 0 06\6 \6 666 66.6 6 56.6 60. 0 06.0 66060.0 60.00 6 60.6 0.6 6 66.0 06.0606 00.6 0 00.6 60 0 0 6066.6 60.6 6 60.6 60 6 66.6 60.606 56.5 6 60.6 65.6 6 50.6 66.56.6 6 00000000 00066 0000 0006 0060000 0000 .0000 .0006 m0oom .0006 .00266 60006300 £0060m Psom % m0om 0 .6026: Mo 0 66009 660o9 9o % 660o9 0509 Houhwsdm 0069 mo % 0009 5.0.0.0800 .60 660.69 6 0000 60.0 66.6 06.6 66.6 DECO .000m 0 .H0cmm NN NM m MM“ 0 N 0000m 0000m 60.0 06.0 0000 .000& Mo % HMPOB 60.6 66.6 0000 .00000 00000 \o:ro UNH 0000000 00 0 6 66.6 65.0 60.6 mN.H 60.0 60.0 wo.H 00.0 N:.o 00.0 60.0 66.6 mo.H 00.0 66.0 66.6 66.0 0000 00000500 :0000m 00000 00.0000 60.6m.m 06.06.5 60.06.6 66.60.6 00.00.6 66.06.6 00.06.6 60.60.6 06.60.6 60.06.5 06.06.5 00.6665 00.60.5 06.66.5 06.00.5 00.60.5 .0000 0006 0000 0 6000050m 06\60\6 0000 .A.0.Pfloov Ho manna 50 00cm .HO % @699 LIST OF REFERENCES LIST OF REFERENCES Blackith, R.E., R.G. Davies and E.A. Moy. 1963. A biometric analysis of development in Dysdercus fasciatus Sign. Growth 27:317-334. Bongers, J. and w. Eggerman. 1971. Der Einfluss des Subsozialverhaltens der specialisierten Samensau- ger Oncopeltus fasciatus Dall. und Dysdercus fasciatus Sign. auf ihre Ernahrung. Oecol. 6:293—302. Charnov, E.L. 1976. optimal foraging, the marginal value theorem. Theoro P013111. BiOlo 93129-1360 Derr. J.A. 1977. Population movements of D sdercus bimaculatus (Pyrrhocoridae; Heteroptera} in relation to moisture stress and fruiting cycles of its differ- ent host plants. Ph.D. thesis. Washington Univ., St. Louis. Derr, J.A., B. Alden and H. Dingle. MS. Insect life histories in relation to migration, body size, and host plant array: a com arative study of Dysdercus. J. of Anim. Ecol.(subm. . Dingle, H. 1966. The effect of population density on mortality and sex ratio in the milkweed bug, Oncopeltus, and the cotton stainer, Dysdercus THeteroptera). Amer. Nat. 100:465-470. Dingle, H., N.R. Blakely and E.R. Miller. 1980. Variation in body size and flight performance in milkweed bugs (Oncopeltus). Evol.(in press). Fox, L.R. 1975. Cannibalism in natural populations. Ann. Rev. Ecol. Syst. 6:87-106. Frankie, G.W., H.G. Baker and P.A. Opler. 1974. Comparative phenological studies of trees in tropical wet and dry forests in the lowlands of Costa Rica. J. of Ecol. 623881-919 I 86 87 LIST OF REFERENCES (cont'd.) Fuseini, B.A. and R. Kumar. 1975. Biology and immature stages of cotton stainers (Pyrrhocoridae: Heteroptera) found in Ghana. Biol. J. Linn. Soc. 7:83-111. Glanz, W. MS. Food and habitat use by two Sciurus species in central Panama. Holdridge, L.R., W.C. Grenke, W.H. Hatheway, T. Leang and J.A. Tosi, Jr. 1971. Forest Environments in Tropical Life Zones: a Pilot Study. Pergamon Press. Oxford. Janzen, D.H. 1972. Escape in space by Sterculia apetala seeds from the bug Dysdercus fasciatus in a Costa Rican deciduous forest. Ecol. 53:350-361. Lewis, A.R. 1980. Patch use b gray squirrels and optimal foraging. Ecol.(in pressy. Lewontin, R.G. 1965. Selection for colonizing ability. In: The Genetics of Colonizing Species. pp. 77-94. H.G. Baker and G.L. Stebbins(eds.). Academic Press, New York. MacArthur, R. and E. Pianka. 1966. On optimal use of a patchy environment. Amer. Nat. 100:603-609. Melber, A. 1979a. Influence of abiotic factors and physiological conditions on the formation of aggre- gations in cotton-bugs (Dysdercus spp., Heteroptera). Ent. exp. appl. 25:196-202. Melber, A. 1979b. Influence of population density and rearing in roups on the bionomics of Dysdercus gardinalis Heteroptera, Pyrrhocoridae). Z. Angew. Entomol. 88:144-158. Pearson, E.O. 1958. The Insect Pests of Cotton in Tropical Africa. Commonwealth Inst. of Ent., London. Rockstein, M., ed. 1964. The Physiology of Insecta, 640 pp. Academic Press, New York. Saxena, K.N. 1963. Mode of ingestion in a heteropterous insect Dysdercus koeni ii (F.) (Pyrrhocoridae). J. of Insect Physiol. 9: 7—71. Schoener. T.W. 1974. Resource partitioning in ecological communities. Science 185:27-39. 88 LIST OF REFERENCES (cont'd.) Smith, 0.0. 1970. The coevolution of pine squirrels (Tamiasciurus) and conifers. Ecol. Monogr. 40:349-371. Smith, J.M.N. and H.P.A. Sweatman. 1974. Food searching behavior of titmice in patchy environments. Ecology 55:1216-1232. Smythe, N. 1970. Relationships between fruiting seasons and seed dispersal methods in a neotropical forest. Amer. Natur. 104:25—35. Stearns, 8.0. 1976. Life history tactics: a review of the ideas. Quart. Rev. Biol. 51:3-47. Van Doesburg, P.H. 1968. A revision of the New World species of Dysdercus guerin Meneville (Heteroptera: Pyrrhocoridae). moologische Verh. 97:1-213. Werner, E.E. and D.J. Hall. 1977. Competition and habitat shift in two sunfishes (Centrarchidae). Ecology 58 3 869-876 0 Werner, E.E., G.G. Mittelbach and D.J. Hall. MS. The role of foraging profitability and experience in habitat by the bluegill sunfish. Ecology(subm.). Wiens, J.A. 1976. Population responses to patchy enVironmentSo Arm- Rev. E0010 sySto 7381-1200 Withycombe, C.L. 1924. Factors influencing the control of cotton stainers (Dysdercus spp.). Bull. ent. Res. 15:171-190. Youdeowei, A. 1966. Laboratory studies on the aggregation of feeding Dysdercus intermedius Distant (Heteroptera: Pyrrhocoridae). Pro. Roy. Ent. Soc. Lond. 41:45-50. Youdeowei, A. 1967. The reactions of Dysdercus intermedius (Heteroptera, Pyrrhocoridae) to moisture, with special reference to aggregation. Ent. exp. appl. 10:192-210. Youdeowei, A. 1967a. Observations on some effects of pop- ulation density on Dysdercus intermedius Distant (Hetergptera: Pyrrhocoridae). Bull. Ent. Soc. Am. 1318-2 0 Youdoewei, A. 1968. The behavior of a cotton stainer Dysdercus intermedius (Heteroptera, Pyrrhocoridae) in a temperature gradient and the effect of tempera- ture on aggregation. Ent. exp. appl. 11:68-80.