As parasites grow: Relationships between Conopid parasite size and Bombus worker behavior

Background/Question/Methods

Parasitoid conopid flies are a common antagonist of bumble bees through much of their geographic range and can infect up to 70% of foragers. While conopids typically kill their host in 10-12 days, they can alter host behavior prior to death, thereby contributing sublethal effects as well as lethal effects to colony dynamics. In Europe, it has been shown that conopid infections lead to an increased likelihood of bees spending the night away from the colony, apparently as a way to combat the infection, and a reduction in pollen foraging as conopids grow larger. These effects have been little examined in North America despite a diverse fauna of both bumble bees and conopids. We compared the frequency and size of conopid infections, while controlling for wing wear (a proxy for age) between bumble bees found torpid on plants at nightfall with those found actively foraging during the day. We also compared the frequency of pollen collection between bees spending the night on plants vs. foraging during the day, and pollen collection between parasitized and unparasitized bees. We predicted that growing conopids would increase the likelihood of bees staying out of their nest and reduce the likelihood of pollen foraging.

Results/Conclusions

We examined parasitism status and wing wear of 225 bumble bees (172 foraging on flowers, 53 clinging to plants). Foraging bees and torpid bees showed similar infection levels (foragers --51.1%, torpid bees --52.8%), but among parasitized bees, staying out behavior increased with increasing size of parasitoid larvae, even after controlling for bee age (wing wear). Pollen was seldom found on bees spending the night outdoors, indicating that staying out behavior was not generally a result of interrupted foraging. Among parasitized bees caught on flowers during the day, there was a negative relationship between pollen collecting behavior and the size of the conopid larva.  Thus, similar to results from Europe, we find evidence that conopids both reduce pollen foraging and cause workers to stay away from the colony. Because workers staying away from the colony cannot contribute either to colony defense or thermoregulation, both reduction in pollen foraging and reduced contributions to overall colony function may constitute important sublethal effects of conopid parasitism.