Plant Mediated Effects on Tritrophic Interactions in the Solanaceae-Hornworm System
Top-down pressure from parasites is thought to be a key driver in herbivore diet breath, but studies investigating the evolution of food plant shifts as a defense against natural enemies in the environment are still lacking in the literature. I examined how plants alter insect-enemy interactions for a specialist herbivore utilizing solanaceous food plants, Manduca sexta (the tobacco hornworm) and the parasitoid wasp, Cotesia congregata, as a model. In this study, I documented parasite infections in a field population of M. sexta, and then investigated from an eco-immunological perspective how plant toxins influence susceptibility to parasites in order to explain food plant choice. My research demonstrates that M. sexta exhibits a negative preference-performance relationship with plants in the Solanaceae. This is likely to gain protection from parasitoid attack via direct and indirect effects from plants on herbivore physiology. I show that herbivores are unpalatable and toxic to natural enemies when they consume more noxious host plants, but also provide a subsequent explanation for the adaptive value and maintenance of this interaction; specific plant secondary metabolites alter herbivore immune activity, where for M. sexta nicotine demonstrates immunotherapeutic properties by enhancing this insect’s phenoloxidase activity. I also examined phenotypic plasticity in caterpillar immune responses to nonlethal cues from natural enemies. Upon studying non-consumptive effects of natural enemies on M. sexta in the presence of C. congregata and the spined soldier bug, Podisus maculiventris, my work suggests that M. sexta generally accelerates their development in the presence of natural enemies at the cost of some immune defenses, implying a resource allocation tradeoff to physiological development and immunity. Placed within a community level context, M. sexta can mitigate the consumptive and nonconsumptive effects parasitoids have on this herbivore’s physiology by utilizing a food resource in parasite burdened habitats that increases direct resistance to parasites and also improves immune activity, even at the cost of development. Following this, I investigated the consequences of crop domestication on plant-insect-parasitoid interaction via changes in plant traits for direct defense against herbivores and altered plant volatile signaling of natural enemies. I demonstrated that domesticated chili peppers showed no loss of plant direct defenses to M. sexta compared to wild peppers and that crop peppers had increased attraction and efficiency of parasitoids. This highlighted the context-dependent nature domestication has on trophic interactions and emphasizes the need for dedicated investigation in each unique crop system.
Creighton, Purdue University.
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