Effects of annual cropland and season on pollen diversity and pesticide exposure in honey bee colonies

Honey bees face multiple stressors, with cropland expansion, pesticide use, and lack of floral resources being major contributors. However, few studies have jointly investigated altered floral resources and pesticide contaminations in landscapes with increasing dominance of annual crops. Therefore, we studied pollen diversity and current-use pesticide (CUP) residues in pollen samples from 36 honey bee colonies in Lower Franconia, Southern Germany. The nine study sites covered a gradient of annual cropland ranging from 43 % to 97 % within a two-kilometre radius. Pollen was sampled from July to mid-August 2021. Using DNA metabarcoding, a total of 140 different pollen species were identified in 540 pollen samples. Contrary to expectations, pollen richness and Shannon diversity did not decrease with increasing annual cropland area. Instead, richness and diversity showed high temporal turnover and site-specific variation, likely influenced by crop diversity, agro-environmental schemes, and the heterogeneity of surrounding habitats. We conclude that site-specific factors drive the richness and diversity of pollen foraged by honey bees. Among the 16 CUPs detected, fungicides were the most prominent, with notably high concentrations in pollen (3300.5 µg/kg fludioxonil and 971.6 µg/kg azoxystrobin). The probability of detecting fungicides was positively associated with the relative abundance of sunflower pollen, although the detected CUPs were not authorized during flowering. Overall, we observed low pesticide contaminations (39 % of samples), which could be attributed to reduced pesticide applications to the end of the cropping season. Our data indicates that brood in late summer, when long-living winter bees are developing, is less exposed to pesticide residues in pollen compared to earlier phases in the cropping season.