Bumble Bees and Parasitoid Flies: Manipulation of Host Behavior and the Importance of Host Body Size, Blandy Experimental Farm, Boyce, VA, 2012

Many parasites and parasitoids are able to manipulate the behavior of their hosts for their own benefit. Parasites are also known to select their hosts based on certain physical characteristics, such as body size, that optimize their own fitness. Conopid fly larvae (Conopidae: Diptera) are endoparasitoids of bumblebees in Europe and North America. Previous research on the interaction between conopid flies and a single European bumblebee host species, Bombus terrestris, demonstrated that bees infected with a conopid larva would bury themselves in soil just prior to death, and that this behavior may confer fitness benefits on the conopid developing within the bee. Additionally, it has been shown in several studies out of Europe and Canada that conopids are found more frequently in bees that are larger-bodied, a host characteristic that correlates positively with at least two measures of fitness: conopid pupal weight and adult fly emergence. This study, which took place at Blandy Experimental Farm in the northern Shenandoah Valley of Virginia, found that three locally common bumblebee species (B. bimaulatus, B. griseocollis, and B. impatiens) exhibit a digging behavior in response to parasitism by conopid larvae. Unexpectedly, one species, B. griseocollis, experienced a significantly lower rate of detected parasitism than the other two species. I hypothesize that this may be due to reduced survivorship of larva within this host species. In accordance with previous research, this study also found that the likelihood of being parasitized increased with host body size, and that larger conopid pupae developed in larger-bodied bees. The findings of this research are novel: this study documents a parasite-induced modification of bumble bee behavior previously not observed in North American bumblebees and identifies one host species that may possess an ability to resist developing parasites. Further, this study sheds additional light on conopid-bumblebee host selection dynamics.