Degree

Doctor of Philosophy (PhD)

Department

Entomology

Document Type

Dissertation

Abstract

Soybean looper, Chrysodeixis includens (Walker), has established itself as the primary lepidopteran defoliator in Midsouth soybean agroecosystems. Pesticide-induced outbreak events associated with this pest have been observed by soybean producers. The insecticides believed to prompt these incidents are routinely used for the control of other co-occuring pests, such as stink bugs. Outbreak conditions present soybean producers with conflicting management decisions for pests. Laboratory, greenhouse, and field experiments were conducted to provide a better understanding of insecticide usage impact on populations of C. includens and the arthropod community. To discern the potential mechanisms behind C. includens outbreak, a baseline for the composition of the endemic pest and natural enemies was first established. A moderate positive correlation between C. includens and foliar-residing predators was found, revealing an association between these two groups. Parasitoids of C. includens were also identified across the Midsouth to gauge their composition in soybean systems. The diversity of species parasitizing C. includens was similar to prior assessments 30 years ago, with four primary species still presently collected. These findings show the stability of the parasitoid community and their association with C. includens. After determining the importance of natural enemies, field trials evaluating the impact of insecticides were conducted to validate the hypothesis that a release from predation was causing C. includens outbreak. These trials confirm the capacity of pyrethroids, organophosphates, neonicotinoids to induce population increases in C. includens. Increased densities of C. includens also coincided with reductions in predator populations, specifically predaceous hemipterans. These findings support a release from predation hypothesis, but presents an alternative mechanism regarding neonicotinoid applications. Neonicotinoids can disrupt plant defense against herbivores, which may lead to increased susceptibility and conditions that may benefit C. includens development and fecundity. Laboratory and greenhouse trials verifying the effect of neonicotinoids indicated a negative impact on C. includens development and behavior, likely a result of toxicity. Field applications of neonicotinoids demonstrated a potential positive impact of imidacloprid on C. includens survival and fecundity, but the effect was minimal. Outbreaks of C. includens that resulted from neonicotinoid application are due to a loss of natural enemies.

Date

4-5-2023

Committee Chair

Davis, Jeffrey A.

DOI

10.31390/gradschool_dissertations.6081

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