Lady beetle chemical cues shape parasitoid foraging decisions and aphid parasitism

Abstract

Chemical cues play a central role in shaping insect behavior, particularly in the context of predation risk and resource use. In parasitoid systems, detecting predator-associated cues may be critical, as developing offspring are vulnerable to intraguild predation within shared hosts. Although predator odor cues are known to influence herbivore behavior, their effects on parasitoid foraging and parasitism are poorly understood. Our study aimed to determine how predator-derived odor cues from the lady beetle Harmonia axyridis (Pallas, 1773) affect the behavior and parasitism success of the aphid parasitoid Aphidius colemani (Viereck, 1912) in a collard (Brassica oleracea L.)-green peach aphid (Myzus persicae Sulzer, 1776) system. To address this aim, a series of laboratory and greenhouse experiments were conducted across increasing levels of ecological complexity. We isolated chemical cues from direct predation effects by restraining lady beetles inside metal mesh tea infusers, allowing us to evaluate the non-consumptive effects of H. axyridis odor cues on both parasitoids and aphids. Parasitoid host- searching behavior was evaluated using olfactometer and whole-plant choice assays, and aphid parasitism was assessed in both simple and complex arena experiments. These approaches allowed us to examine both short-term behavioral responses and longer-term effects on parasitism under realistic conditions. Parasitoids initially avoided plants containing predator- associated odor cues, indicating sensitivity to predation risk during host searching. However, this avoidance behavior diminished over time, and parasitoids did not maintain a consistent preference between plants containing caged predators and plants without predator cues. Despite these behavioral changes, overall parasitism was not reduced in the presence of predator cues. Instead, parasitoids redistributed their foraging activity, resulting in lower parasitism on plants with lady beetle odor cues but higher parasitism on nearby cue-free plants. These results demonstrate that predator-derived chemical cues influence parasitoid behavior without disrupting parasitism at the population level. Rather than reducing parasitoid effectiveness, predator cues alter the spatial distribution of foraging and parasitism. Our study highlights how chemical ecology and predation risk can influence spatial patterns of pest suppression, suggesting that predator-associated cues may be compatible with parasitoid-based biological control strategies.