Tillage compromises weed seed predator activity across developmental stages

Abstract

Granivorous ground beetles (Coleoptera: Carabidae) are ubiquitous throughout temperate agricultural systems, and reduce weed seedbanks. However, trade-offs may exist between tillage frequency and ecosystem services of invertebrate seed predators, especially those in the larval stages, which have relatively poor resistance to disturbance. While much research has focused on adult activity patterns and the conservation biocontrol services they provide, almost nothing is known about carabid biology and habitat requirements during larval stages, despite the fact that adult recruitment is determined by factors that promote larval survival. We present data on larval and adult Harpalus pennsylvanicus Dej., a common weed seed predator across North America, from two experiments examining its activity density across tillage and cover-cropping treatments in organic tomato systems. Larvae emerged 4–6weeks after the adult activity peak, and larval activity density was up to 10 times higher in no-till crop environments than in cultivated areas. After a long disturbance interval, seasonal cultivation had no effect on foraging activity of adults, but reduced larval activity density in both experiments. Additionally, larvae positively correlated with living weed biomass in no-till treatments, suggesting the importance of plant-based resources in oviposition site choice. Compared with adults, larvae are relatively immobile and vulnerable to disturbance; thus, weed management strategies that rely on frequent cultivation may undermine the ecosystem services provided by granivorous insects.