Don’t poop where you eat: Hygienic nest architecture and behavior in bumble bees

Infectious disease is among the major contributors to pollinator declines. Consequently, understanding how disease spreads through pollinator communities is a major goal for pollinator conservation. Yet while bumble bees, among of the most abundant native pollinator species, are eusocial, the social processes that shape disease transmission within bumble bee colonies have received little attention. Infectious disease is widely considered a cost of social life, but social species can also cooperate to defend against disease through “social immunity” behaviors. Many social insect species, including bumble bees, defecate outside the primary nest area, and this cooperative behavior is widely hypothesized to serve a social immunity function. However, this hypothesis has rarely been empirically tested. Here, we use manipulative laboratory experiments to investigate whether the common eastern bumble bee (Bombus impatiens) uses hygienic defecation to defend against the common gut pathogen, Crithidia bombi. We address the following questions: (1) Do bumble bees perform hygienic defecation behavior? (2) Is this behavior altered by infection with C. bombi? (3) Does this behavior slow the rate of C. bombi transmission? (4) Does this behavior improve colony performance? We found that bumble bees do defecate in an additional “outhouse” chamber more than the primary nest cavity, and that this behavior was robust against C. bombi infection. This behavior also significantly slowed C. bombi transmission at low initial infection rates and was correlated with greater survival and reproduction. Overall, our results suggest that bumble bee defecation behavior functions as a behavioral defense against C. bombi.