Sharing the wealth: pollen partitioning in a Cucurbita crop pollination system with reference to the wild hoary squash bee.

Abstract

Cucurbita pepo crops (pumpkin, squash) are entirely dependent upon insect pollinators for reproduction. In Ontario, Canada, their most important pollinator is the hoary squash bee (Eucera pruinosa), a wild ground-nesting, solitary bee whose only source of pollen in the region is Cucurbita crops. As such, in this context, we have a unique opportunity to study pollen partitioning in a cropping system in which a wild bee is the main pollinator. To evaluate pollen partitioning in the system, we measured pollen production by the crop, the pollen lost as waste due to the activities of bees in staminate flowers, pollen loads collected by female squash bees, and the number of pollen grains in fully provisioned hoary squash bee nest cells, and we compared these to the crop’s pollination requirements as reported in the literature. From the perspective of both plant and bee reproduction, about 13% of the pollen produced by staminate acorn squash flowers was wasted, but it may be harvested by other organisms like ants. After waste is accounted for, about 9% of the pollen left is needed for plant reproduction leaving the remaining 91% available for hoary squash bee reproduction. We also evaluated the mass of pollen a female hoary squash bee could carry in a single foraging trip relative to her own body mass (~4%). The information contained here is useful for understanding the relationship between a crop and an oligolectic wild bee species or to set up controlled, field realistic experiments involving the hoary squash bee.