Bumble bee predators reduce pollinator density and plant fitness

Research in pollination biology has focused on the interactions between animals and the flowers they visit for food reward. However, other selective agents, including predators, seed feeders, and herbivores, may affect pollination systems. Because flowers are predictable food sources for a variety of species, flowers are also reliable sites at which predators can locate flower-visiting animals. Prominent among pollinators' predators are beewolves (Philanthus spp.), common sphecid wasps (Sphecidae) that prey almost exclusively on bees. My fieldwork over three years indicates first, that an area of similar to 50 km(2) surrounding a single bumble bee wolf (Philanthus bicinctus) aggregation had a low bumble bee (Bombus spp.) density caused by intense predation by the wasps, and second, that fruit-set of the bumble bee-pollinated western monkshood (Aconitum. columbianum) was significantly lower at locations and times of bumble bee wolf activity than at control locations and times with no such predatory activity. These results indicate that predation can sometimes alter plant-pollinator interactions.