1st Place: Taking root: Plant-herbivore coexistence history alters belowground plant defense

Plant defense against herbivory is typically characterized as resistance or tolerance. Resistance deters herbivore feeding, often through plant production of toxic chemicals, whereas tolerance promotes plant regrowth following herbivory. A history of coexistence with an herbivore is predicted to optimize plant defense over evolutionary time. When agricultural crops are introduced to novel geographic regions, coexistence histories are often disrupted as plants encounter unfamiliar herbivores, but our understanding of how coexistence history influences crop plant defense, particularly against belowground root herbivores, remains limited. Therefore, we aimed to fill this knowledge gap by investigating plant defense against belowground cucumber beetle (Acalymma vittatum) herbivory in plant species with and without a coexistence history: zucchini squash (Cucurbita pepo) and watermelon (Citrullus lanatus), respectively. We predicted belowground herbivory would activate tolerance strategies for zucchini but resistance for watermelon, as tolerance can be a more optimal strategy to deal with specialist herbivores. To test our hypothesis, we conducted separate resistance and tolerance bioassays for both zucchini and watermelon. For resistance assays, herbivores fed for 24 hours and we collected root volatiles. For tolerance assays, herbivores fed for 7 days and we harvested plant tissue. Overall, we found herbivory increased watermelon root volatile emissions but did not change zucchini volatiles. Herbivory also reduced aboveground biomass in zucchini but not watermelon. Taken together, our results support our hypothesis and indicate zucchini relies on resources from aboveground tissues to tolerate root herbivores, whereas watermelon resists belowground herbivores likely through induced chemical cues.