Technical Abstract: Global pollinator declines have been attributed to habitat destruction, pesticide use and climate change. Managed honey bee, Apis mellifera, colonies are part of these worldwide declines and are exposed to a wide variety of contaminants as they forage in the environment. Factors blamed for their decline range from parasitic mites, poor nutrition, and pesticide exposure in agricultural crops, pathogens or some combination of these factors. Here we exposed honey bee colonies during three brood generations to sub-lethal doses of a widely used pesticide, imidacloprid, and then subsequently challenged newly-emerged bees with the gut parasite, Nosema ceranae. The pesticide dosages used were low but within the range of residues found in nectar and pollen of crops following treatment with imidacloprid. We demonstrated an increase in pathogen growth within individual bees reared in colonies exposed to the pesticide. Interactions between pesticides and pathogens could be a major contributor to increased mortality of honey bee colonies worldwide.