Trapped honey bees reduce floral visitation on milkweed flowers

Trapped arthropods have been shown to benefit plants in several ways, but few studies have examined the potential costs of arthropod entanglement. Milkweeds (Asclepias spp.) have an unusual pollination system that requires relatively large packets of pollen (pollinia) to become attached to the appendages of insect pollinators and be pulled through a narrow opening in the flower. Honey bees (Apis mellifera) commonly become trapped and die with their legs still attached to milkweed flowers. In this study, we conducted a field experiment to examine how dead trapped honey bees affect floral visitation. We expected that the presence of a dead trapped bee would reduce floral visitation via two non-mutually exclusive pathways: (1) a direct deterrent effect on floral visitors and (2) an indirect deterrent effect mediated by an increased abundance of scavenging predators such as ants. The presence of a dead bee reduced floral visitation by 37% compared with controls, and this effect was stronger and more robust for honey bee visitors than non-Apis visitors. While ant densities were 51% higher on floral umbels with a dead bee and ants reduced floral visitation by 30%, our path analysis indicated that the direct deterrent pathway explained 91% of the total effect, consistent with an aversion to dead conspecifics among honey bees. Our results suggest that the lethal entanglement of honey bees is likely to incur an ecological cost for milkweed flowers, although the deterrence of honey bees could also shift the pollinator communities on milkweeds with unexpected consequences.