Impacts of Climate Change and Temporal Variation on Multitrophic Interactions in a Brassica oleracea Agroecosystem
Open Access DepositedCaterpillars are some of the most voracious pests of crop plants worldwide, causing hundreds of billions of dollars in crop losses annually. Caterpillar pest populations are strongly regulated by parasitoid wasps that develop in or on their caterpillar hosts, eventually killing them. Effective pest management requires a detailed understanding of the environmental drivers of multitrophic interactions in agroecosystems and how these interactions are likely to respond to climate change. At an organic farm, I quantitatively sampled the caterpillar community feeding on 11 cultivars of Brassica oleracea in both the early and late season for three consecutive years, rearing over 1400 caterpillars and their various parasitoids and hyperparasitoids. These field data were used to construct the first set of temporally explicit trophic interaction networks for these globally important crops (Chapter 1). Ecological interaction networks can provide novel insights into community structure and dynamics and inform pest management strategies used by growers; however, very few trophic interaction networks currently exist, particularly for agroecosystems. My data showed that the scale of temporal aggregation of network data influences network structure and dynamics, with network parameters becoming increasingly skewed the more the data is aggregated over time. Seasonal and interannual variation in both the frequency and identity of interactants (interaction turnover) alter network structure. As ectotherms, insects are particularly vulnerable to climate change; however, host plants, caterpillars, and parasitoids are differentially affected by temperature, which could change the outcome of their interactions and alter the effectiveness of biocontrol. In chapters 2 and 3, I investigated how climate change affects the relationship between cruciferous crops, the caterpillar pest Plutella xylostella, and one of its key parasitoids, Diadegma insulare. I first examined the effects of increased temperature on the bitrophic interaction between P. xylostella and D. insulare using growth chambers and found that increased temperatures resulted in decreased parasitism rates and increased parasitoid mortality (Chapter 2). I then carried out greenhouse mesocosm studies that more closely replicate field conditions to examine the effects of increased temperature and cultivar type on the interaction between P. xylostella and D. insulare (Chapter 3). The results revealed that higher temperatures had sublethal effects on both the caterpillar host and parasitoid, and these impacts were similar across cultivars. Contrary to the results of chapter 2, mortality of the parasitoid was not strongly affected by temperature, suggesting that in field conditions parasitoids may be able to mitigate the negative effects of temperature by controlling host behavior. Overall, this work provides novel ecological insights that can inform the future of biological control in cruciferous crops by anticipating how increased temperature will influence species interactions in the system and by using interaction networks to understand community dynamics.
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Ryan_Spahn_Dissertation_Approved_for_Formatting_-_Ryan_Spahn.pdf | 2023-04-27 | Open Access |
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