Agricultural expansion and intensification have been associated with the disappearance of natural and semi-natural areas in croplands. However, small patches of non-crop vegetation still persist in field margins. Preserving them, rather than converting them to crop, can support beneficial insects, contributing to crop pest and weed suppression across nearby fields. Associations between the structural characteristics of field margins (e.g., vegetation height or percent cover) and these natural enemies are poorly known. Further, if beneficial insects are dispersing from field margins into crops, a functional trait perspective (e.g., body size, a trait related to dispersal ability) may explain their distributions within fields. Here, we focus on ground beetles (Coleoptera: Carabidae), a family frequently studied for their biocontrol potential, sampled in or near eight crop fields in Alberta, Canada. We predicted that increased vegetation height and ground cover in the field margin would support a wider range of carabid body sizes with higher activity-density, and that larger carabids would disperse further. We measured the elytra length of 7084 carabids and sampled vegetation height (N = 648) and cover (N = 216) in the field margins. A generalized additive model revealed decreased activity-density of carabids with greater percent cover and taller vegetation. A quantile generalized additive model showed that the smallest carabids were more strongly associated with changes in vegetation structure and distance from the field margin. The predicted range in body size was greater near field margins with greater percent cover and taller vegetation. These findings highlight how field margin management could affect carabid communities. • Beetles and field margin vegetation sampled in eight Canadian Prairie crop fields. • Quantile regression used to connect body size diversity and field conditions. • Taller denser field margins increase carabid body size diversity. • Distance from field margins alters body size diversity, not activity-density. • Conserving complex margin vegetation sustains diverse natural enemy populations.
Neumann et al. (Fri,) studied this question.