Wild bee abundance and pollination service in cultivated pumpkins: farm management, nesting behavior and landscape effects.

Recent declines in managed and feral honey bee populations have greatly increased interest in the current and potential role of wild pollinators in agricultural pollination. Pumpkin (Cucurbita pepo L.) has great potential to be served by wild pollinators because of a reliable and widespread group of bee species that are commonly associated with their flowers, including bumble bees (Bombus spp.), and, in the Americas, two genera of specialist ground nesting bees (Peponapis and Xenoglossa). We examined the effects of several key farm management practices and landscape variables on bee abundance in pumpkin on 20 farms in Virginia and Maryland during summer 2006. We evaluated bee abundance with respect to tillage, irrigation practices, soil properties, natural habitat (forest and grassland), flowering crop, and disturbed areas. Additionally, we examined nest site preference (within or outside of crop areas) of Peponapis pruinosa (Say) at one farm and in a large screenhouse. We found P. pruinosa nesting preferentially within crop areas and near the vines and leaves of their host plant. Although these bees typically place some of their brood cells within tillage depth, we did not find a tillage effect on their abundance at flowers. We found a negative effect of soil clay content (R2 = 0.24, P = 0.03) and a positive effect of irrigation (F1, 15 = 12.2; P < 0.001) on P. pruinosa abundance. Using published data on pollinator visitation requirements, we found wild bee densities were sufficient to fully pollinate all pumpkin flowers on 13-17 of the 20 farms studied.