Wild Bees Benefit from Low Urbanization Levels and Suffer from Pesticides in a Tropical Megacity

Abstract

How urbanization affects crop pollination has scarcely been studied, especially in the tropics. Here, we focus on the diversity of wild bees and their pollination services to 30 small-scale fields of Lablab purpureus, a globally wide-spread grain legume, in the Indian megacity Bangalore. Farms were selected along a gradient of intermediate to low urbanization, measured as percentage of impervious surface (grey area) at the landscape scale, ranging from 0% and 30%. We found that the abundance of lablab-visiting wild bees increased with increasing grey proportion in the fields’ surroundings and that, in particular, ground-nesting, large-bodied and Xylocopa bees benefited. The higher availability of forage and nesting resources in low-density urban areas appeared to enhance bee populations. When pollinating insects were experimentally excluded, lablab plants produced 36% less and 31% lighter fruits. Yet, we did not detect any changes of pollination outcomes along the urbanization gradient. Finally, we found that the local bee richness was negatively affected by the number of on-field pesticide applications, resulting in 35% fewer species after 3 applications round. In summary, we conclude that low density urbanization can be beneficial for wild bees in lablab farms, but intensive pesticide use counteracts this positive effect. Large and ground-nesting farmland bees benefited most from urbanization, but more studies on different crops in tropical cities are urgently needed.

Keywords: bee, pollination, pollinators, pesticides, urbanization, ecosystem service