A STUDY OF SOME FACTORS
AFFECTING THE EFFICIENCY OF
ENCARSIA .FORMOSA GAHAN. AN

APHELINID PARASITE OF THE

GREENHOUSE WHITE FLY.
'I‘RIALEURODES VAPORARJORUM
:wasmfl

Theszs for the Degree of M. S.
MIU‘HGAN STATE COLLEGE

Herbert E. Milliron
1938

 

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A STUDY OF SOME FACTORS AFFECTING THE EFFICIENCY OF
ENCARSIA FORMOSA GAHAN, AN APHELINID PARASITE
OF THE GREENHOUSE WHITE FLY, TRIALEURODES
VAPORARIORIIM (WESTFJ

Thesis

Submitted to the Faculty of the Graduate School
of Michigan State College, in partial
fulfillment of the requirements for

degree of Master of Science

HERBERT E. RILLIRON
1 9 z 8

“HEEHS

‘ACKNOWLEDGMENTS

'The writer expresses his thanks
to Professor Ray Hutson and Associate Pro-
fessor E. I. McDaniel for their suggestions
and guidance during the course of this

work.

115912

CONTENTS

REVIEW OF LITERATURE . . . . . . . .
THE GREENHOUSE WHITE FLY . . . . . .
Synonymy, Origin, Distribution .
Descriptions, Life History . . .
Host Plants, Habits, Parasites .
THE PARASITE . . . . . . . . . . . .
Description, Origin, Distribution
Life History . . . . . . . . . .
Habits, Economic Importance . . .
OBJECT OF THE EXPERIMENTS. . . . . .
EQUIPMENT AND METHODS . . . . . . .
Control Cabinet . . . . . . . . .
Greenhouse . . . . . . . . . . .
Instruments . . . . . . . . . . Q
Plants Used . . . . . . . . . . .
Method of Collecting Data . . . .

METHODS OF CALCULATING PERCENTAGES
OF EFFICIENCY

DATA . . . . . . . . . . . . . . . .
DISCUSSION OF DATA AND OBSERVATIONS
CONCLUSIONS . . . . . . . . . . . .
LITERATURE CITED . . . . . . . . . .

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. 15
. 15
. 16
. 16

. 18
. 21
. 51

REVIEW OF LITERATURE

The life history, habits and host plants of
Trialeurodes vaporariorgg (Westw.) have been given thor-
ough treatment by several authors. Morrill (19), Har-
greaves (l2) and Britten (3), have contributed largely
to the biology of this insect, while Weber (41), has
given the most complete ecological account. Others have
added valuable observations and data on the habits and
control (10, 17, 29, 51, 55, 54, 4o, 45).

The publications of Cockerell (5), and Quain-
tance and Baker (23), concern the classification of white
flies.

No work on the biology of Encargia formosa Gahan,
and nothing regarding the factors which affect the efficien-
cy of the parasite in controlling the greenhouse white fly
have been published in the United States to the knowledge
of the writer.

Speyer (29), of the Experimental and nesearch Sta-
tion, Chestnut-Harts, England, has conducted research on the
insect's biology. He has given a good account of the life
history and habits. Various workers have been chiefly con-
cerned with the observations of plants they believed to be
repellent to the parasite and the extremes in temperature
that it could endure, especially low temperatures (7, 30,
51, 55, 45). A few articles (41, 44), are more specific

in the discussion of temperature as a factor affecting the

-2.
control of the greenhouse white fly by E, formosa.

The literature which treats of environmental
factors that affect insect activities, particularly those
of parasite and host, is extensive and demonstrates the
differential effects of temperature and humidity on the

biotic potential * , the host parasite balanceand the

 

* Sweetman (37, p. 11), patterned his definition of
"biotic potential" after Graham (1955). It is "the in-
herent ability of an organism to reproduce and survive,
within a given time, and under optimum environmental con-
dltions." He further states that its values may be divid-
ed into a "reproductive potential" which takes into account
the number of young, sex ratio, and number of generations;
and into the "survival potential" which is concerned with
the nutrition and protection.

Chapman (Animal Ecology, 1931, p. 194), terms the
opposing value of the above as the "environmental resist-
ance." ‘

 

varying internal changes with reSpect to reproduction.
The conclusions reached are not specifically related to this
problem but are reviewed because they are analageous.

Payne (21), found that under certain conditions of
temperature and moisture, Micngracon hebgtor Say was able
to control its host, Ephestia Runniella Zeller. Weber (41),
experimenting with what was probably Encarsia figrmgga, show-
ed that at low temperatures the rate of reproduction of the
parasite is much lower than that of its host, T. vaporarior-
Em and thus, it follows that the percentage of parasitism was
low and complete control could not be eXpected.

Webster and Phillips (42), working with Toxoptera
graminum Rond. discovered that it was able to oviposit at

-3-
Slightly below 4.400 while its principal parasite,

Aphidius testaceipes Cress., was inactive below 13.50C.

The biotic potentials at that given set of conditions
varied considerably and even the slightest control was

out of the question.

‘ The host-parasite balance has likewise been in-
vestigated by several workers, a few of whom are Blunk,
Bremer and Kaufman (2), Barnes (1), Shelford (27), and
Payne (21). The general contention is that the hosts were
able to overcome their parasites at low temperatures as a
result of the conditions retarding the development of the
parasites. The opposite of this has been reported by
Ruzicka (26), in the case of the parasites of Eogthetria
monacha Linn., provided a high humidity accompanies the
low temperature.

In two instances noted here, the experiments of
Ruzicka (26), and Hefley (15), indicate that the host in
each case, was favored by lower humidities than those of
its parasite, but probably many more such cases could be
found.

Physical factors, whether of climate or of a
host, are responsible for stimulating or inhibiting the
desire for oviposition and therefore control to some ex-
tent the amount of parasitism. The summary of a few phy-
sical factors is made by Richardson (24).

Publications dealing with the mathematical treat-

ment and the calculation of percentage of parasitism is

limited. Larrimer and Noble (16), have devised a method

-4-

whereby the relative percentage of parasitism can be cal-
culated for the parasites of the Hessian.F1y, Phytophaga
destructo: (Say), by using a system of means and standard
variation. The formula preposed by Thompson (38), and later
improved by Chapman (4), does not provide a means of determin-
ing the differential action of environmental factors since
the formula alone is intended to calculate the number of gen-
erations that are necessary before a parasite can entirely
overcome its host, and takes into account only the values
eXpressed by biotic potentials (See footnote, p.2).

Methods of measuring temperature and humidity (28,
11, 6), means of maintaining constant humidities in control
cabinets and cages (56), and laboratory equipment (22), were
reviewed, but it seemed necessary to devise special methods

and procedure for the eXpeerents discussed in this thesis.

-5-

THE GREENHOUSE WHITE FLY
Trialegrodes vapgrariorum (Westw.)

m, 22454.4 =and =4==i===D str bution

When Westwood described this insect in 1856,
(Gardener's Chronicle, p. 852), he assigned it to Latri-
elle's genus Aleyrgdgs, which has been altered by some in
spelling to Aleurodes. This genus has been divided into
several genera and at various times the insect has been
referred to the genus Astgrochitgg (23), and to the sub-
genus Trigeurodes (5).

Either Brazil or Mexico is thought to be the
original home (5, 23), bum since this is not a definitely
established fact, it can only be said that the species is
probably indigenous to tropical America.

The greenhouse white fly has a general distribur

tion in both Europe and the Western Hemisphere.

Descr t ons and Li£§_g;stggz

The adult which averages about 1.5 mm. in length,
has four wings and is entirely covered with white waxy
powder (Fig. 2). The mouth is fitted for piercing and suck-
ing. fiales are identical in appearance except that they are
smaller.

The average period which constitutes the adult
stage is 30 to 40 days (10, 19, 41), and during this time

each female lays an average of two eggs per day (41). Vary-

ing conditions will affect egg production which account for

-6-

the recordings of individual females from 28 to 554
eggs (lo, 17, 19).

The eggs are 0.2-.25 mm. long, when first laid,
are yellowish green but after 2-4 days (10, 41), they be-
come black, (Fig. 3 and 4). This stage, depending upon
the environment, requires from 6-13 days (3, 10, 19, 41).

Eggs are attached to the tissue of the under
surfaces of leaves by a short petiole. It is not a common
occurrence to find them on the upper surfaces of leaves or
attached to the stems. When depositing the eggs, the fe—
male uses her beak as a pivot, thus, on smooth-leaved
plants, one often observes perfect circles of eggs (Fig. 4).
The number of eggs composing each circle varies. It ranged
from 21-46 per circle on the fuchsia leaves examined by the
author. Half-circles or quarter—circles are frequently found
on ageratum leaves; while on such pubescent leaves as holly-
hock, Nicotinia spp., tomato and similar plants, the eggs
are scattered, because of the interference of hairs with
the pivoting movement.

Larva; instars (10, l2, 17, 19)

The remainder of the cycle is composed of four in—
stars, the first three of which are known as the larvae; lst,
2nd, and 3rd. The fourth stadium is often called the pupa,
(Fig. 10). A few writers have recognized five separate in-
stars, maintaining that the stadium following the third is
in reality the fourth larval stage. For all practical pur-

poses, we may assume there are four instars between the egg

and imago, designating the fourth as including the immature

-7-
insect from the third molt to the time the adult emerges.
First instar (5—6 days)

The newly hatched larva is oval, flat and trans-
parent light green in color and about 0.29 mm. long. It
possesses functional legs and antennae and during the first
2-3 days moves about in search of a suitable place to settle,
but seldom wanders farther than a half-inch from its egg-shell.
Each lateral margin of the body is provided with 18 spines,
graduated in length; the posterior ones being longest.

Second instar (4-6 days) Sedentary

After the first molt, the insect flattens out on the
leaf; is more transparent than in the first stage; and is dif-
ficult to detect. Legs and antennae are vestigial. The mar-
gins of the body are finely crenulated and the marginal spines
number three on each side. Dorsal bristles, one anterior and
two caudal pairs, which appear at this time, are short. The
insect measures about 0.39 mm. during this instar.

Third instar (4—6 days) Sedentary

This stage closely resembles the second in all re-
Spects except size (averages about 0.52 mm.).

Fourth instar (12-16 days, including the pupa) Sedentary

When newly emerged from its third molt, the larva
is whitish green and closely set to the leaf surface. As it
matures, it separates itself farther and farther from the
leaf by submarginal, perpendicular wax rods, which are so
close together they form a complete wall or palisade of

striated secretion. It is always distinguished from other

stages by the possession of seven pairs of long dorsal

-8-
spines or rods. The margins are evenly fringed with
shorter spines. When mature, it measures about 0.75 mm.
It is from the pupa which forms inside, that
the adult emerges through a T-shaped opening. This last
instar is undoubtedly the most important as far as E.
formosa is concerned, since the parasite almost entirely
limits its attack to this stage in the white fly cycle.
The life history from the time the egg is laid

to emergence of the adult requires about five weeks.
Host Plants, habits and Parasites

The greenhouse white fly adults prefer the more
tender leaves and invariably are found by hundreds on the
undersides of leaves at the top of the plant where most of
the eggs are deposited at any one time in the growth of the
plant. Lower leaves, especially of a plant which makes rapid
growth, seldom show many eggs or first and second instar lar-
vae. Adults have been noted congregating to a considerable
extent on the under surfaces of pale or chlorotic leaves, or
on normal leaves under intense light. This suggests a photo-
tropic response even during the period of oviposition.

Britten (3), has listed 58 plants upon which he has
observed young stages, and many more could be added. In these
experiments the host preference was in descending order; tomato
Nicotiana spp., heliotrope, fuchsia, ageratum and lantana.
Conditions of temperature, light and humidity appear to alter
this order, however.

Weber (41), reports the greatest activity of the

-9-

greenhouse white fly is at temperatures between 25-30 C.,
and that the optimum for development of all stages is 30 C.

Natural parasitic enemies of the white fly are
several species of fungi (25, 40, 15, 29), and several Chal—
cids belonging to the genus Encarsia: g. formosa Gahan,
(a, 29, 7, 51, 41, 54, 52, 55, 45, 59, 44, 45, 20), p.
versicoloz Gir., (18), fl. pergandiella How., (10), E.
partenopea Masi., (29, 14), E. luteola How., (35) and
Encarsia Sp. (41).

-10-

THE PARASITE

Encarsia formosa Gahan

Description, Origin and Distribution

Encarsia foerSa was first described by Gahan
in 1924 (9), (family Aphelinidae of some authorities, or
EulOphidae, subfamily Aphelininae of others).

The female (29), is about 0.6 mm. long and 0.5 mm.
broad (Figs. 5 and 6). Most of the head is brown, the thorax
black and the abdomen pale yellow; the legs and antennae are
light brownish yellow and the wings opalescent, fringed with
relatively long hairs.

Males are usually slightly larger and may always
be distinguished by the dark brown or black abdomen. Since
the Species reproduces thelyotokously, males rarely occur,
generally making their appearance after a prolonged period
of low temperature.

The bionomics of the insect suggests a tropical
origin. It is thought that India is the location from which
the parasite spread (29, 41); however, this has not been
definitely established and is likely only if a recent host
association was initiated.

Within recent years, the parasite has been report—
ed as occurring in England (7, 29, 31), Australia (39), New

Zealand (20), Germany (41), Canada (correspondence with Dr.

A. B. Baird of the Dominion Parasite Laboratory, Belleville,
Ontario) and the United States (9). Its distribution is

-11-

is largely restricted to the greenhouse in the Temperate

Zone.

Life Historg- (29)

ESE. Observations thus far indicate that only a single egg

is deposited in each white fly pupa at a point anterior and
Just to the side of the operculum. mach oviposition requires
from two to four minutes. Females, which live for about 28
days, have been observed ovipositing over a period of nine
days and in this time deposited 50 or more eggs each. Ton-
noir (39), reports that females of this parasite have deposit-
ed an average of 100 eggs per individual during the oviposi-

tion period.
EGG

The egg itself is large, measuring 0.08 mm. long
and 0.03 mm. in diameter. It is devoid of the "neck" so often

noted as a part of the eggs of chalcidoid parasites.

LAEEA

 

The incubation period is approximately four days.
Upon hatching, the young larva is dilated at the anterior end,
but soon becomes elongated, and finally semi-circular. It
molts three times as is shown by the number of cast-off lar-
val skins. The total length of time necessary for these

stages is unknown, but is probably 10—14 days.

-12-
PUPA

After attaining maturity, the larva trans-
forms into a pupa which usually faces the anterior end
of the white fly pupa. Pupation requires about 10 days,
after which the adult emerges from the pupa and escapes
from the white fly scale by cutting a circular opening
in the top.

The life cycle from egg to adult is about 26—30
days. Tonnoir (39), has observed it to be as short as 20
days, intimating that it varies widely depending upon con-

ditions.of temperature.

HABITS AND ECONOMIC IMPORTANCE

 

Speyer (29), states that the adult parasites are
able to cover considerable distances in quest of their host.
Weber (41), who worked with what was probably E, formosa, con-
cluded that even "crowded parasitized larvae” on a leaf did
not induce wandering. Observations during these experiments,
especially at low temperatures, confirm the belief of Weber.

The parasites are more active at higher temperatures (20-3000.)

and under these conditions many can be seen deserting the low-

er leaves for those higher up on the plants, but they seldom
fly great distances. They are not easily disturbed and often
remain quiescent, even upon close examination.

.Several plants are naturally repellent to the adult
parasites but they are discussed in another part of this

thesis.

-;3-

E. formosa seems to be the most valuable of
all Species of Encarsia for biological control of the
greenhouse white fly and has been reported with at least
some degree of success, in a number of countries where
it has been introduced and colonized (29, 7, 31, 43, 34,
55, 59, 44, 45, 20).

-14-
OBJECT OF THE EXPERIMENTS

The experiments attempt to determine; some
of the factors which increase or decrease the total para-
sitism of the greenhouse white fly, I. vaporariorum (West),
'by E, formosa; the optimum conditions for parasitism; and
the economic importance of E. fgrmosa in an average green-

house environment.

EQUIPMENT AND METHODS

Contro; Cabinet

A cabinet 4' x 54' x 2' was converted into a con-
trol cabinet (Fig. 1). Since it was necessary to use plants,
light was essential, and accordingly, two 500—watt lights
were adjusted outside the cabinet, about eight inches above
the glass tap. Later it became apparent that regulation of
the cabinet temperature required the installation of two
water trays (Fig. l), placed over the top Just under the
lights. These trays were equipped with glass bottoms
and were kept filled with running water by means of an
inlet hose connected to a water supply and an outlet hose
leading into the sink.

The inside conditions of temperature and humidity
were satisfactorily controlled by passing water through a
copper line running across the ceiling of the cabinet. Vari-
ous conditions were obtained by the relative amount, or speed

of flow, of water that passed through the line. Humidity was

maintained by the moisture supplied the potted plants. Thermo-

-15-
static control apparatus was installed which maintained
a temperature that did not fluctuate more than five degrees.
Electric coils at one end of the cabinet supplied heat.

The air currents set up by a fan disturbed the
flights of both host and parasite adults from plant to
plant.

The cabinet functioned satisfactorily between

18°C. and 26°C.

greenhouse

 

The Entomology Greenhouse was used to conduct
several of the experiments. Temperature was controlled
there by thermostatic apparatus.‘ The problem of maintain—

! ing desired humidities was the most difficult factor in at-
tempting to determine the most ideal conditions for para-
sitism. The only means of obtaining various degrees of at—
mospheric moisture were the use of 10-15 shallow pans fill-
ed with water and placed on the steam pipes under benches,
and by wetting plants and the steam pipes. Transpiration
of the many kinds of plants that occupied the house assured
a rather constant though lower humidity when either of the
foregoing practices were performed less frequently.

Care was taken never to sprinkle the tops of plants

used in these experiments.

Instruments

Temperatures were recorded continuously by thermo-

graphs and checked at intervals with accurate thermometers.

-16-

A hygrograph which was periodically checked with a sling

psychrometer, was used to measure the relative humidity.

Plants 232g
Tomato plants were selected at the beginning of
these experiments as the most ideal plants because they were
rather easily cultivated and they appeared to be the favor-
ite greenhouse host plant of the white fly. Data were also
collected from the following kinds of plants, either in the
control cabinet or the greenhouse or both; ageratum, fuchsia,
heliotrope, lantana, hollyhock, Nicotiana tabacum and fl. lu-

tinosa.

Method 2; Egllecting Data

Parasitized white fly fourth stadium individuals
may be detected as a usual thing 10-12 days after parasitism,
since they become black (Figs. 8 and 9). Figure 7 shows the
difference in appearance between an unparasitized pupa (white),
and one that has been parasitized (black), after an exposure
of 10-15 days. Due to the succession of white fly generations
that commonly appear on a single leaf, data was never collect—
ed from.a new plant until a period had elapsed equal to that
required by the parasite to transform from egg to adult, and
only after any initial parasitism had taken place under a

normal parasite population. * In most cases, data was taken

 

* (3-4 parasites per leaflet or leaf, appearing on the
majority of leaflets or leaves, were the general criteri—
on followed in regards to a normal population of the para-

site).

 

-17-

only after exposures of from 50—60 days during which time
an endeavor was made to maintain a large parasite popula—
tion.

Leaflets \tomato) or leaves (other plants) were
taken in a randomized manner. No attempt was made to choose
those leaves showing the greatest number of blackened pupae
or those with the least number.

All leaves so selected were placed under a binocur
lar microsc0pe and the following stages counted and recorded
separately: "Parasitized, fourth stadium, "Unparasitized,
fourth stadium", First to third stadia," and "Eggs". in
nearly all cases a column was made also of the number of
adults which had emerged as indicated by the larval skins
with resemblances of a T—shaped opening. This last figure
was also included in the column "Unparasitized, fourth sta—
dium", and was tabulated separately only to indicate the
thoroughness of the work of the parasite at the time counts
were made. Such recording would have been of little impor—
tance, which will be pointed out later, had all larvae been

of the same age.

-18-
METHODS OF CALCULATING PERCENTAGES 0F EFFICIENCY

After experimenting for sometime with the
various methods that might be used to arrive at a repre—
sentative figure for the percent efficiency ofthis para-
site, it was concluded that no one method is absolutely
accurate, due primarily to the several peculiarities al—
ready mentioned in connection with both host and parasite.

If one were to count every stage of the host on
a plant exposed for the time required for parasite estab—
lishment, he would discover that the average percentage
figure obtained was anywhere from one-half to two—thirds
lower than the figures tabulated under "Data". This is
illustrated by the complete counts from Experiment A (see
also Table I for data given below under "lower leaflets,

fourth stadium only"), made in analyzing the problem.

Entire Plant, All fitages

All 8tages Parasitized Percent
Lot 1 2065 - 265 15.0
Lot 2 2101 558 27.0
Lot 5 2588 565 24.0
Lot 4 1866 581 17.0
Lot 5 970 510 52.0
Lot 6 654 159 24.0

-19-

Lower Leaflets, Fourth Stadium Only

Fourth Stadium Parasitized Percent
Lot 1 546 261 48.0
Lot 2 712 554 79.0
Lot 5 669 565 84.0
Lot 4 586 579 65.0
Lot 5 505 509 61.0
Lot 6 251 156 54.0

The difference in the percentage figures is
due to the much greater value of combined counts of the
egg, first, second and third instars and adults; all of
which are stages said to be ignored by the parasites.

Because the results obtained by considering all the stages
were not believed to be representative of the work of the
parasite, the method was not used.

The second method considered was that of regard-
ing only the empty larval skins from which white flies emerg-
ed, in relation to the number of parasitized individuals.

One obvious drawback appeared to be that of often confusing
the empty larval skin from which a white fly emerged and that
from which a parasite emerged, since they frequently resemble
each other. Another detriment was the unusually long time re-
quired for maturation of all the stages on a leaf, because of
the overlapping generations. During the course of these ex-

periments few leaves were found without some of the immature

larval stages. flince it was advisable that many counts be made,

this method alone was unsuitable, yet under conditions where

-20-
’time permits its application, with moderate white fly
infestations, it appears to be an efficient way of determin-

ing the value of the parasite.

The method found most applicable, especially since
a number of different kinds of plants were used, was that of
considering only the fourth stadium. All percentages given
in the following tables were based on the number parasitized
of the total fourth stadium found on the number of leaves in-
dicated on a certain plant. It was assumed at the time counts
were made that any individuals in the fourth stadium, not dis-
colored, were free from parasitization.

Additional information, i.e. "Emerged" (adults), is
often given for the purpose of indicating the proportion that
the parasites have failed to parasitize. Counts were made of
all stages in the case of the earliest experiment, "Experiment
A", and they are included for reference in "Discussion of Data
and Observations".

As was intimated previously, the method adopted is
not without defects; any parasitism occurring approximately 10
days or less prior to counting would in many cases fail to show,
and there is the possibility, when making hundreds of counts,
that the early fourth stadium and the late third may be confused
and designated in the wrong columns. In order to partially over-
come the first difficulty, the exposure period was made rather
long (as stated elsewhere), and series of counts at intervals
were made when possible, and then totaled.

Despite the complication encountered in the collection
of data, it is felt that reasonably accurate conclusions may be
drawn from the following tables, because the procedure which was

f‘nnnd mngt Rlfi tnh19 WHS used 001151813311th throughout the work--

-21-

DATA

Environmental factors which affect the para—

site's efficiency* are interpreted in terms of the per-

 

* The parasite's efficiency as used in this thesis
is the effect of the parasite on the host, and it
is usually thought of as the degree control accom—

plished.

—_.._

centage of parasitism incurred on its host. In reference
tn: most tables, therefore, the number parasitized in rela-
tzion to the number unparasitized, is of prime importance.
Crther items such as number of "First to third instars" and
'Ffiggs" were omitted in all tables except Table I because
iihey apply directly to the white fly reproduction and not
tho the calculation of the efficiency of the parasite by
‘tlle adOpted method.

The data forming the basis for these interpre-

tEitions are tabulated and graphed on the following pages.

TABLE I.
Experimgnp A, Fog; Tomato Blants

Average Greenhouse Conditions

A

 

 

 

 

 

 

 

 

No. F°urth Stadium 1-3 Eggs SgggftizL
leaf- Parasit- Unpara- Stadia ed '
lets ized sitized (Approx.)

4 261 285 32 21 48.0

4 554 158 5 none 79.0

4 563 106 4 8 84.0

4 379 ' 207 5 s 65.0

4 309 196 8 11 61.0

4 156 115 none 6 54.0
24 2202 1067 . 52 54 67.0

 

 

 

 

 

 

 

Period of Initial EXposure - about 50 days

Period between Series of Counts - 5-10 days

Conditions: Temperature — 18-2600.
Relative Humidity - 40-90%

-23....

Parasm3 6" n s!»
only 80mm"
bablc av rage Curvl
/ 1w

 

 

GRAPH I.

-24-

TABLE II

Experiment 2, Effect 2; Humidity_(0ab;net)

 

 

 

 

 

Fourth Stadium Tem-

No. Para- Humid- pera—
Plant lea- Fara- Unparasitized sit- ity t e
ves sit- ’Unemerg- E— ized ( C)

ized ed merg—
ed

Tomato 3 249 255 11 50.0 50-40% 25
a, glut. 1 181 258 52 41.0 50-40% 25
Heliotrope- 1 1680 506 104 77.0 50-40% 25
Tomato 5 184 75 4 72.0 60-70% 26
Lantana 1 54 46 2 54.0 60-70% 26
Ageratum 2 161 98 58 62.0 50-70% 26
Tomato 8 245 281 68 47.0 80-90% 21
Lantana 5 72 280 65 20.0 80-90% 21
Ageratum 1 48 70 6 41.0 80-90% 21

 

 

 

 

 

 

 

 

Exposure 50-60 days

Additional factors operating —

Cumulative effect of:

(l

2%

Character of different plants
Temperature
Constant light

 

-25-

TABLE III.

 

Experiment B-l. Constant Temperature. (Cabinet)

 

Fourth Stadium

 

 

 

 

 

 

 

 

 

 

 

% Tem-

.Plant No. Para- Unparasitized Para- pera-
Leaves sit- Une- E- sit- ture

ized merged merged ized (00)
Tomato 7 112 246 1 51.0 18
Fuchsia 2 25 27 19 46.0 18
Ageratum 5 97 41 51 70.0 25
Fuchsia l 66 47 51 58.0 25
(Tomato 5 184 75 4 72.0 26

Exposure - 50 days

Conditions - 60-70% r1umidity
Additional factors operating:
(1) Types of plants
(2) Constant light

 

TABLE IV.

-26..

Experiment Q_Mean Temperatures ip_Greenhouse

 

 

 

 

 

 

 

 

 

 

 

 

 

Fourth Stadium
% Tem-
Plant No. Para- Unparasitized Para-- pera-
Leaves sit— Une- E- sit— ture
ized merged merged ized (00)
Tomato 5 78 72 41 52.0 18
Tomato 4 456 458 50 49.0 18
Tomato 5 521 244 56 68.0 18.5
Tomato 15 744 645 78 54.0 20
Fuchsia 2 82 22 6 79 22
Tomato 15 1025 181 56 85.0 24
Ageratum 5 589 114 5 77.0 24 .
fl. lut. l 114 78 11 59.0 24
Tomato 8 812 170 12 85.0. 26
Exposure - 50—60 days

Conditions - Relative humidity 60-70%

 

Parastt'ucd
8

Percent

 

I00

80

70

S

N
0

Parasitism 5‘: [jam-nose at various \empcratum

-27-

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--- m3.» a...“ "‘ m

 

 

£8

 

-28-

TABLE v.

Experiment C-l
Different Plants Subjected pp Fluctuating
Temperatures and Humidity (Greenhouse)

 

 

 

 

 

 

Fourth Stadium
%. Tem-
Plant No. Para- Unparasitized Para- pera-
Leaves sit- Une- E- sit- Fluc-
ized merged merged ized tua—
tions
(0)
Tomato 5 685 107 56 86.0 18.5-25.5
Tomato 4 158 50 9 86.0 15-27
.Fuchsia 5 247 24 11 91.0 15-27

 

 

 

 

 

 

 

 

Exposure - 50-60 days
Relative Humidity - 40-70%
Observations: At temperatures of 15°C. and
below, the adult parasites were sluggish.
Adult parasites were observed
in the act of oviposition on individuals in
the second and third instars (see "Discussion

of Data and 0bservations".)

-29-

TABLE VI.

Experiment D

Effect pf Type pf Plant

(Approximately the same conditions of temperature
, and humidity — optimum 24—2600. and 40—70% rela-
tive humidity. Highest percentage recorded in

most cases.)

 

Plant

Tomato

Fuchsia
Hollyhock
1 (very pubes.)
2 (med. pubes.)
fl, glutiposa
Ageratum
fl. pabacum
Bean,
Heliotrope

Lantana *

 
 

 

 

   

Fourth Stadium I ”h"
Tem-
No. Para- Unpara- Para- pera-
Leaves sit- ' sit- sit- ture
ized ized (CC)
15 1025 181 85.0 24
5 547 24 91.0 24 (mean)
1 1218 1980 58.0 24 (mean)
1 1291 1457 47.0 24 (mean)
1 114 78 59.0 24
5 589 114 77.0 24
1 75 117 58.0 25 (mean)
1 150 185 45.0 25 (mean)
5 482 256 67.0 25
1 54 46 54.0 26

 

 

 

 

 

 

 

* Plant from Cabinet, others from Greenhouse.

Observations - Epidermal hairs of E, glutinopa secreted

a viscous, sticky substance that appeared
to be detrimental to the activity of the
parasite.

Larvae of the white fly often excreted
globules of "honey dew" on hollyhock, and

especially on heliotrope (see Fig. 11 and

-50-

TABLE VII.

Effggp pf Intense Light

(500 watt bulb about 12—18" from plants)

 

 

 

 

 

 

 

 

 

 

 

Fourth Stadium

73 Tem—

Plant No. Para- Unpara- Para-‘ pera-
Leaves sit- sit- sit- ture

ized ized ized (0C)
Tomato * 8 169 541 55.0 25
Tomato 4 65 41 61.0 24
fl. glutinosa 1 754 1785 29.0 22

(See also Fig. 12)
Fuchsia 4 572 185 67. 25
Conditions: 40-70% relative humidity

* Plant from cabinet, others from Green-

house.

 

-31-

DISCUSSION OF DATA AND OBSERVATIONS
‘ The data presented in Table I (and plotted
in Graph 1) were taken from lower leaf samples.

It is evident by studying the table that few
eggs and immature stages existed on these leaves at any
one time the counts were made. Likewise a gradual deser-
tion by both host and parasite of lower leaves is indicat-
ed. There is an increase in the percentage of parasitism
over a period of 10-15 days and then a gradual decline.

This tendency for a build—up followed by a decrease was

evident in most experiments conducted. It was, however,
more marked in this eXperiment; probably due to the fact
that the counts were made beginning in early summer when
both the populations of parasite and hosts were observed
to be less abundant. Otherwise, under normal conditions
the high percentage should be more extended.

The activity of the parasite (figured by the para—
sitism) and white fly (figured on the basis of numbers pre-
sent) is plotted in Graph 1. The "probable curve" is indic-
ative of the presumed average degree of parasitism over the
given time and bears no other significance.

According to Richardson (24), the humidity may or
may not have an effect on the oviposition by parasites. Weber
(41), has found the optimum relative humidity for the white
fly to be 80%. The figures in Table II, show that the para-
site is little affected by extremes of humidity between
50-90%, if one takes into consideration the compensating dif—

ferences in the temperatures and the types of plants sampled.

-32-

Owing to the difficulty in controlling humidity, wide
ranges had to be considered when tabulating data rather
than means or constants. The most consistent high per—
centages occurred within the range of 50-70% except that
5 on heliotrope, which was unusually high.

Tables III, IV and V, are concerned with the
results of temperature experiments. The percentage fig-
ures obtained from the experiments conducted in the con-
trol cabinet were always lower than those in the green-
house. This was probably due to one or more of three
factors; that of intense light (Table VII), periodical
operation of a fan for air circulation and the lower para-
site p0pulation. The outstanding effect of lower tempera-
tures expressed in Tables III and IV is the low percentage
of parasitism, which seems to indicate a decreased stimulus
for oviposition at those temperatures. At 18qC., 51-52% of
all fourth stadium individuals were parasitized on tomato,
while at 24°C., there was 85 per cent.

At temperatures above this, the parasitism seemed
to decline. Fuchsia showed a greater percentage of parasit—
ism than tomato at all given degrees of temperature.

Table VI indicates the parasitism under rather wide-
ly fluctuating temperatures, and at the same time demonstrates
the effect of plant types. The 85 per cent parasitism on
tomato and 91 per cent on fuchsia were the highest percentages
obtained in any of the experiments.

Graph 2 shows parasitism at various temperatures bas—
ed upon the percentages of parasitized fourth stadium on tomato,

from Table IV. Similar curves could be plotted for other plants

used in the experiments. The "probable curve" in this graph is

-55-

the presumed parasitism, and is similar to that in
Graph 1, in derivation.

p. formosa was sluggish at temperatures below
15°C. and it is not, so far as Table V is concerned, ap-
parent that any fluctuations below l80C. aided its effic—
iency, except to lengthen the period of oviposition. Vari-
ous publications (29, 55, 45, 45) have suggested the ineffi-
ciency of the parasite at lower temperatures and another (44),
recommended an average greenhouse condition of not lower than
70°F., otherwise it would be useless to introduce it. Wilson
(45) pointed out that successful control by the parasite de—
pended upon temperature between 60-75QF.

Other workers (7, 51, 45), have listed eucalyptus,
Nicotiana tabacum, fl. virginica,;regal, scented and zonale
pelargonium, abutilon, pelargonium, abutilon, bouvardia and
datura as repellent to the parasite. Table VI shows the para—
sitism on various plants under what is believed to be Optimum
conditions. It should be noted that the white fly upon the
two species of tobacco showed considerable parasitism. Under
the same conditions of temperature and humidity the percent-
age of parasitism appears to be correlated with such factors
as; density and character of leaf pubescence and the amount
and type of plant secretions. Fuchsia (smooth) displayed the
highest percentage of parasitism; while tomato (medium pubes-
cent), and ageratum (medium pubescent) displayed the next high—
est percentages. It is also true that the white fly larvae
excreted less "honey dew" on those plants, or the excretion
was eliminated in a normal way as described by Hargreaves (11);
thus, very little accumulation in the form of droplets was evi-

dent. On such plants as heliotrope (Fig. 11), and lantana,

-54-

drOplets of "honey dew" were large and disturbed the
activity of the parasite and frequently a dozen or more
adult parasites could be seen embedded in this excretion.
The epidermal hairs of E, glutinosa excreted a viscous,
sticky substance, (Fig. 12), that greatly impaired the
efficiency of E, formosa and was reSponsible for the de-
struction of great numbers of the adults. Nevertheless,
the apparent attraction of this species of Nicotiana was
so great that the parasite was always found in large num-
bers on the leaves.

Results presented in Table VII are indicative
that constant, intense light reduces the amount of para-
sitism.l This substantiates similar observations in other
experiments, but lacking additional data, it is merely sug-
gested that light is one of the factors which affects the
efficiency of the parasite. The cumulative effect of other
factors operating was most difficult to adjust in working
with the light factor. The representative leaf of E. gig;
tinosa showing 29.0% parasitism is pictured in Fig. 12.

Undoubtedly the early fourth stadium of the white
fly is the most frequently attacked stage. During the course
of the experimental work, adult parasites were observed attack-
ing younger stages, particularly the third stadium. And on
March 11, a single female was seen attacking five different
second stadium larvae within 50 minutes, averaging 2-4 minutes
for each act of oviposition. German and Jewett (10), have ob—
served females of a related Species, E. pergandiella oviposit-

ing in "pupae and larvae".

-35-
CONCLUSIONS

E. formosa has never controlled its host
during these experiments to a greater extent than 91.0
per cent on fuchsia and 86.0 per cent on tomato.

According to the data presented here, the
greatest control (percentage of parasitism) of the white

fly on the majority of plants was reached between 24-2690

(under nearly constant temperatures) and between 50-70 per
cent relative humidity. Widely fluctuating temperatures
gave somewhat higher percentages, Table V. The parasite

accomplished very little control at temperatures of 18°C

or below, for at such temperatures it was sluggish.

Weber (41), has expressed the Opinion that the
optimum temperature for both the host and parasite is 50°C.
The data and observations here recorded show that, if the
percentage of parasitism is any indication of the optimum

conditions of the parasite, it is lower than 50°C.

Even in an environment that seems most ideal for
the parasite, and despite the fact that each generation is
composed of 100 per cent females in most cases (thelyotoky),
the biotic potential of the parasite is not equal to that
of its host, eSpecially during heavy infestations. Where a
variety of plants are grown together, therefore, it seems
unlikely that complete control can be rendered by the para-
site alone, under normal greenhouse conditions.

Of the physical environmental factors affecting

the efficiency of the parasite, temperature seems to be the

-56-

most important. Other factors which appear to play
an important part are humidity and light.

The effect of such physical factors as char-
acter of the host plant is eXpressed by the degree of
pubescence and the amount of excretion of both plant and
white fly larvae (Table VI, and Figs. 11 and 12).

The parasite does not appear to restrict its
oviposition entirely to the fourth stadium; it has been

observed attacking the second and third as well.

 

 

 

 

 

 

 

 

Figure l.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

’///-J

 

 

 

.0
COntTOl CQanCI

1' Wu!“ inkt O - Watt: Outfit

L' Lights W- Water Trags

_ H— ——

Figure 2.
White Fly Adultsi_
Approximately 8x ‘
(After Britten,
Conn. Agr. jiiXP- ;, .
Sta., Bul. 140)u,du'

    

l
,Lw'.
l- ..

 

 

 

 

Figure 5. White Fly
Adults and Egg Circles
on Fuchsia Leaf.

 

Figure 4.
Circle of White
Fly Eggs on Fuchsia
Leaf. (Much Enlarged

  

 

 

Figure 5.

Parasite Adults (Approx. 50x)

 

Figure 6.

Parasite Adult (Approx. 40x)

 

Figure 7.

Unparasitized and
Parasitized White
Fly Pupae (Greatly
Enlarged)

   

 

Figure 8.

Parasitized White Fly Pupae
(Greatly Enlarged)

 

Figure 9.

Tomato Leaf, Showing Para-
sitized and Unparasitized
White Fly Pu ae (About

Normal Size

 

 

Figure 10.

Unparasitized White Fly Pupae
(Enlarged)

 

 

 

Figure 11.

HeliotrOpe, Showing Pubescence of the
Leaf and the White Fly Larval Excre-
tions.

 

 

 

Figure 12.

Tobacco Leaf (N, glutinosa) Showing Relative
Amount of Parasitized and Unparasitized Pupae.
Note the Secretions of the Epidermal Hairs.

(l)

(2)

(3)

(4)

(5)

(6)

-37-

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‘3‘.

 

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