Paralobesia viteana (the grape berry moth) and Venturia inaequalis (causal agent of apple scab) are two key pests of perennial fruit crops in Michigan and the Northeastern US. Both of these pests provide examples of the feedback potential of management decisions: they both overwinter within the cropping system and have multiple reproductive generations per growing season. This allows their populations to carry over from season to season, exacerbating any failures in management from previous years. Both pests overwinter on the ground in leaf litter. During that time, these organisms are affected by physical changes of the ground habitat and interactions with other ground-dwelling organisms. Manipulation of that habitat to alter the physical properties and community dynamics to decrease overwintering survivorship of these pests may provide growers with additional management tactics. Indeed, there is historical precedent for such tactics originating in the early 1900s. Publications from that time mention grape growers in Northeastern, PA throwing furrows over the leaf litter beneath vine canopies in late fall or early spring and observing reduced grape berry moth emergence. During this same time period, it was discovered that spraying a urea solution onto fallen leaves in apple orchards decreased the spore density released by V. inaequalis in the following season. Unfortunately, our understanding of the mechanisms underlying these techniques are incomplete and their adoption by growers remains low. Furthermore, our general understanding of the activities of floor dwelling organisms during the overwintering period is also limited. Increasing knowledge both of how these specific tactics affect their associated target pests, and of how overwintering populations are structured are essential steps in the development and improvement of winter management tactics. With regard to P. viteana and grape vineyards, I explored the effects of physical damage and burial resulting from a rotary cultivator used at the end of the season. Survivorship of pupae recovered from the vineyard immediately after tillage and held until emergence was not significantly different from those recovered from an untilled control area, indicating little effect of mechanical damage on this pest. However, a single pass of the tillage implement buried three quarters of pupae under at least 1 cm of soil. A laboratory experiment to recreate these conditions resulted in significant increase in mortality when pupae were buried in more than 1 cm of sand. I conclude that interference with adult emergence of diapausing pupae via burial is the primary mechanism by which tillage controls grape berry moth.With regards to apple V. inaequalis and apple orchards, I observed the response of overwintering, ground-dwelling arthropods to 1) the application of urea to fallen leaves; and 2) organic versus conventional management strategies. In addition, I screened the gut contents of collected arthropods for the presence of V. inaequalis to identify potential natural enemy taxa. My primary finding was that orchards host a diverse, winter active arthropod community. Management strategy did not affect family richness or intra-community complexity (alpha diversity). There was also considerable overlap in the dominant families detected under both management strategies. However, the relative abundances of those families did respond to management strategy. These changes were associated with differences in the secondary and tertiary decomposer sub guilds. There was also evidence that organic management supported a greater arthropod population. Urea application caused an up-regulation of tertiary decomposers and a down-regulation of primary decomposers during the first month following application. I also found evidence that the absolute decomposer populations were greater in the urea treatments. I propose that urea application caused a trophic cascade in which increased microbial growth leads to a recruitment of fungal feeding arthropods into leaf litter from surrounding areas of the orchard.
Read
