Tillage regime shapes ground beetle distribution and their potential for weed control under drought conditions

Tillage affects soil invertebrates such as ground beetles (Coleoptera: Carabidae), with potential repercussions on the pest and weed control services they provide. Knowledge about the effects of tillage on weed control through seed predation is particularly important in the context of climate change-induced drought, which may exacerbate weed problems. In this study, we investigated the effects of tillage (conservation vs. conventional) on the abundance, diversity and in-field distribution of ground beetles in 9 pairs of annual crop fields in Northern Italy. We also tested the effect of tillage, artificially induced drought (-50 % precipitation) and arthropod seed predator exclusion on weed biomass and diversity within the same fields. We expected conservation tillage to increase ground beetle abundance and richness, and seed predator presence to buffer the projected increased weed incidence in conservation tillage and drought conditions. In conservation tillage fields, ground beetle activity density was overall higher, and decreased from field margin to field center, while in conventional tillage it was lower and had an opposite spatial trend. This pattern, reflected also in the in-field distribution of the dominant species Pterostichus melas, is likely caused by differential distribution of habitat and food resources in the two management systems. Conservation tillage also positively affected species richness, further underlying its importance in ground beetle conservation. Unexpectedly, seed predator exclusion reduced weed biomass in conservation tillage. This may be linked to higher seed density, a situation in which the removal of competing seeds can trigger compensatory mechanisms in the remaining seed bank. As drought also increased weed incidence, strategies such as thick mulching, sowing density manipulation, crop rotations and cover cropping might become increasingly necessary in order to limit weeds to a level at which seed predation can efficiently contribute to weed control in climate change scenarios.