Attack and aggregation of a major squash pest: parsing the role of plant chemistry and beetle pheromones across spatial scales.
Abstract
Successful management of insect crop pests requires an understanding of the cues and spatial scales at which they function to affect rates of attack of preferred and non-preferred host plants. A long-standing conceptual framework in insect-plant ecology posits that there is hierarchical structure spanning host location, acceptance and attack that could be exploited for integrated pest management. We investigated how plant- and insect-derived chemical cues affect successive decisions of host choice in aggregating insects, and tested predictions in the Cucurbita pepo-Acalymma vittatum system. Acalymma vittatum is an aggregating specialist beetle pest that strongly prefers zucchini (C. p. pepo) to summer squash (C. p. ovifera), two independent domesticates of C. pepo. We hypothesized that subspecies-specific plant traits, especially volatile cues, interact with the male-produced aggregation pheromone to amplify beetle preference for C. p. pepo. Differential beetle attack of C. pepo subspecies in the field is not determined by plant traits that affect host finding or differential aggregation due to pheromones: across two years, beetles had strong density-dependent attraction to both subspecies when male beetles were feeding, and no interactions between plant volatiles and the male-produced pheromone were detected. In the absence of male pheromone emission, beetles were equally unattracted to plants with or without beetle feeding. In contrast, plant traits that mediate insect acceptance appear to underlie differences in preference. At a local scale, beetles did not accept and emigrated from C. p. ovifera compared to C. p. pepo. Distinct volatile emissions were observed between subspecies, but further work is needed to identify if these volatiles promote emigration. Synthesis and applications. By dissecting pest preference during successive host choice decisions, we isolated a trait with implications for pest management. Beetles on cucurbits can be managed by employing cultivars with differential susceptibility (e.g. trap cropping), and the mechanistic knowledge presented here informs best practices and limitations for on-farm applications. More broadly, pest management in diversified cropping systems can be enhanced through understanding how plant preference gradients affect herbivore movement and behaviour, and plant breeders can target traits to reduce herbivory in such systems.