Assistant Professor Cornell University Ithaca, New York
Infectious disease is a major threat for social organisms, as increased contact between groupmates increases the risk of disease transmission. However, many social organisms possess “social immunity” traits that allow group members to (1) avoid, control, or eliminate infection, and/or (2) reduce the fitness costs associated with infection. Here, we investigated whether two measures of social life, group size and density, impact disease transmission and fitness in the common Eastern bumble bee (Bombus impatiens) and a trypanosome gut parasite, Crithidia bombi (hereafter Crithidia). Although this is an emerging model system for the ecology and evolution of infectious disease, little prior research addresses how Crithidia spreads within bumble bee colonies. We experimentally manipulated the number (n=15 or n=45) or density (1 bee/in2 or 3 bees/in2) of worker bees in microcolonies, inoculated 1/3 of the workers in each microcolony with Crithidia, and screened each bee for infection over three weeks. Additionally, we measured how Crithidia infection affected worker survival, nest construction, and colony fitness compared to healthy microcolonies. We found that Crithidia infections spread more quickly in colonies with more workers compared to colonies with fewer workers. Similarly, Crithidia infections spread more quickly in colonies with greater worker density but the same total number of workers. Finally, Crithidia infection had significant costs, reducing worker survival, nest construction, and colony fitness in infected compared to control microcolonies. Thus, our results suggest that group size and density impose fitness costs on bumble bee colonies by increasing their susceptibility to disease.
