The contrasting response of cavity‐nesting bees, wasps and their natural enemies to biodiversity conservation measures

Abstract

Abstract The implementation of agri‐environmental schemes (AES) is a major tool to sustain biodiversity and ecosystem services in European agricultural landscapes. The benefit of different types and features of novel biodiversity measures needs to be assessed in order to successfully conserve insect communities in agroecosystems. We set up trap nests in 3 × 3 km landscapes in Germany, in which farmers implemented different novel biodiversity measures. We tested the effect of different features of the closest three biodiversity measures (i.e. distance to measures, measures' area, measures' quality) and of the proximity to forest and oilseed rape and of landscape connectivity on the abundance and species richness of cavity‐nesting bees, wasps and their natural enemies. We found total cavity‐nesting insect abundances to increase with measures' quality and to decline with increasing distance to oilseed rape and forest. Parasitism and mortality rates declined with edge density. Cavity‐nesting bees were not affected by biodiversity measures. Total wasp and herbivore‐predating wasp abundances increased in proximity to biodiversity measures. The abundance and species richness of all wasps, natural enemies and spider‐predating wasps declined with increasing distance to forest. Cavity‐nesting wasps benefit from biodiversity measures, while effects on solitary bees might only be detectable after several years after measure establishment. The proximity to forest is a major factor promoting insects as forest edges provide food, nesting sites and shelter. We emphasise the need for long‐term insect monitoring and an increasing focus of future AES on the creation of nesting habitats for effective insect conservation.