D3096: Non-aggressive interactions in the squash bee Eucera pruinosa (Apidae: Eucerini) and its brood parasite Triepeolus remigatus (Apidae: Epeolini)

The squash bee Eucera pruinosa (Apidae: Eucerini) is a solitary species whose nests are parasitized by the cuckoo or brood parasitic bee, Triepeolus remigatus (Apidae: Epeolini). Several aspects of the life-history of Triepeolus cuckoo bees are unknown, including modes of parasitism, host identity, and host avoidance strategies. Avoiding detection by the host bee is thought to be one of the main mechanisms used in obligate parasitic lineages to escape aggressive encounters with the host. However, whether host-parasite encounters are aggressive or not is unknown for most bee species and their parasites. Our goal with this study was to assess whether interactions between E. pruinosa and T. remigatus are aggressive, tolerant, or avoidant, and how this might inform the mode of parasitism of T. remigatus. We performed circle-tube assays and took videos using field cameras to determine whether host-host and host-parasite interactions were aggressive in both an experimental setting and around the nesting sites. We found that most interactions were tolerant or avoidant in the circle-tube assays and that E. pruinosa do not attack T. remigatus when flying or standing near the nesting sites. Similarly, E. pruinosa females are not aggressive towards each other in both situations. Moreover, the videos suggest that T. remigatus typically enters the squash bee nest while the host is foraging. The results suggest that either E. pruinosa is a highly tolerant species or T. remigatus might use a strategy to conceal their presence to the host using chemical mimicry or insignificance.