Body size mediates ground beetle dispersal from non-crop vegetation: Implications for conservation biocontrol

Intensifying agricultural landscapes by removing non-crop vegetation threatens ecosystem services like pest regulation. Non-crop areas may serve as overwintering habitat for natural enemy arthropods that disperse into and predate on insect pests in the adjacent field. However, managing this service requires greater understanding of the mechanisms driving this dispersal and the subsequent control of pests by arthropod predators. A functional trait framework, i.e., studying the traits of predators that influence their foraging behaviour and dispersal, supports generalization across cropping systems where conditions differ (e.g., which arthropod taxa are present). Predator body size, a trait known to influence both dispersal distance and prey consumption, is a plausible mechanism governing the supply and the effective delivery of pest control. We focused on ground beetles (Coleoptera: Carabidae), common insect predators found globally in agroecosystems. We measured 27,815 beetles collected in 20 crop fields from 180 sampling stations to examine how body size changes with distance from non-crop vegetation. We tested the effect of predator body size on foraging behaviour by exposing 77 Pterostichus melanarius ground beetles to different sizes of the model pest Trichoplusia ni (Lepidoptera: Noctuidae). The smallest six deciles of carabid body size increased in size with distance from non-crop vegetation, demonstrating that more smaller beetles are found closer to the field edge and that body size is a predator trait mediating the distance of dispersal. Larger P. melanarius show a trend towards predating larger prey than smaller prey, though we were unable to reject a null hypothesis of no effect (α=0.05; P=0.08). This affirms that body size is a plausible trait governing the effectiveness of pest control, and that size-based foraging behaviour requires in-field investigation. Our findings reinforce calls for more and better-protected non-crop vegetation areas in agroecosystems intended to control a diverse array of insect pests. Placing non-crop vegetation patches closer to crops (e.g., through restoration or reducing field size) is a critical lever for manipulating the body size distribution of predators in the crop, and subsequently may affect the prey that can be controlled without the use of pesticides.