Bitrophic Ecological Interactions in a Brassica System Provide Insights into Evolutionary Adaptations in Oviposition Decisions
Open AccessCaterpillar pests pose a threat to global food security, costing producers billions of USD in lost yield and pest management costs annually. Understanding the factors affecting caterpillar pest population sizes, which include adult oviposition decisions and host-parasitoid interactions, can help mitigate their drastic effects of crop damage. With climate change and increasing temperatures threatening to change the agricultural field, the oviposition response of one of the most abundant pests, Plutella xylostella, was measured by counting the eggs oviposited on ambient-grown and temperature-treated lacinato kale (Brassica oleracea var. “Palmifolia”: Brassicaceae) in choice tests (Chapter 1). Additionally, aspects of plant chemistry were measured to investigate the mechanisms behind P. xylostella oviposition decisions. My results showed that the female adult showed no oviposition preference between temperature treatments. Kale plants showed no difference across temperature treatments in percent carbon nor in water content but a difference was found in percent nitrogen. Our measure of nitrogen included glucosinolates (defense compounds) and proteins. However, a previous offspring performance study using our same protocol suggests that the adult female is not able to detect nutritional quality differences in the kale and the higher levels of nitrogen found in the temperature-treated kale is allocated toward glucosinolates and not protein. Parasitoids are important biocontrol agents of insect pest populations and understanding their evolutionary ecology is essential for pest management. During the summer of 2021, 360 caterpillars of the cabbage white butterfly (Pieris rapae) were collected biweekly at different life stages from a local organic farm (Potomac Vegetable Farms in Vienna, VA, USA) and brought back to rear in the laboratory until butterfly emergence or parasitoid adult emergence. Rather than killing their hosts upon emergence, local populations of P. rapae keep their host alive after emergence, presumably to help protect parasitoid immature stages. This ‘bodyguarding behavior’ - a form of host manipulation - is poorly understood and likely entails fitness tradeoffs for the parasitoids exhibiting it. I explored the relationship between clutch size (the number of parasitoid eggs laid in a host) and bodyguard lifespan for the parasitoid, Cotesia glomerata, and found that increasing clutch size decreased bodyguard lifespan and reduced the proportion of immature parasitoids that successfully developed into adults (Chapter 2). These findings suggest that populations of C. glomerata that suffer high post-emergence mortality should have reduced clutch sizes to increase the effectiveness of their bodyguards.
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