Proximity to non-crop vegetation increases estimates of predation frequency but not beetle numbers

Abstract

Removal of non-crop vegetation (e.g., forest and grassland patches) to expand crop fields is detrimental to the natural enemies of crop pests, like the ground beetle family (Coleoptera: Carabidae), that use these areas for foraging and overwintering. An argument for maintaining non-crop vegetation is its potential to support the supply of ecosystem services, like pest control, to the surrounding crop. Pest control services from non-crop areas are usually measured using the activity-density of natural enemy species, a proxy measure for predatory activity. Evidence that predators such as ground beetles are attacking pests in crops has been less frequently studied. We distributed sentinel prey (fake ‘caterpillars’ made to look like potential prey items) along with pitfall traps at 300 stations using a spatially and temporally replicated design across 20 field sites in Alberta, Canada. The potential for pest attacks by carabids was estimated through bite marks on the sentinels. Using generalized additive modelling, we found that non-crop vegetation positively impacts carabid predation, but activity density of carabids does not explain the relationship. Instead, the non-crop vegetation affects predation frequency, likely by promoting predatory behaviours. These findings exemplify the importance of closer indicators of ecosystem services delivery than activity-density and provide evidence of the potential that ground beetles have in supplying pest control ecosystem services within agroecosystems.