Too much work, not enough tarsi: effect of group size on Bombus impatiens queen-less worker reproduction

Bombus impatiens is a highly efficacious wild and commercial pollinator in North America. As pesticides generally remain a necessary resource for agricultural pest management, bumble bees are potentially at risk of exposure while visiting treated crops. Pesticides can induce detrimental sub-lethal changes to bumble bee life span, reproduction, or development, ultimately resulting in reduced pollination. To assess pesticides for such sub-lethal effects, a queen-less micro-colony method has been developed for Bombus terrestris, a species native to Europe. Groups of three workers, when isolated, begin to function as a colony, with one worker assuming the role of queen and laying unfertilized eggs destined to become males. The group then can be provided with treated pollen or nectar and observed for a variety of chronic, sub-lethal changes. We applied this cost-effective method to B. impatiens, but observed a marked reduction in male production compared to B. terrestris. Differences between these species in queen-right reproductive behaviour have been documented, and this may translate into a species difference in queen-less reproduction. Thus, to assess pesticide impacts on brood production, a modified, species-specific micro-colony method may be necessary for B. impatiens. Group size is known to influence reproduction in bees, and a higher number of B. impatiens workers may be required to achieve reproductive capacity comparable to B. terrestris. We investigated the influence of group size on B. impatiens queen-less worker reproduction in the context of developing a standardized method for assessing pesticide sub-lethal impacts on this species.