Food availability is a major component of habitat quality. For bumblebee field colonies, it is unknown to what extent reproductive success is limited by food availability relative to other factors such as parasites. To assess the importance of food availability, we carried out a field experiment in the Quebec City area, Canada, in 1999 and 2000, using 45 colonies of Bombus impatiens and B. ternarius. Colonies whose nectar and pollen supplies were increased regularly throughout the season reached larger sizes (in number of workers) and had a higher reproductive success than controls, by 51% and 86% respectively. In particular, food supplementation increased the number of males produced and the probability of producing gynes (young queens). The sex ratio was highly skewed in favour of males overall, and the relative proportion of gynes increased with food supplementation in B. ternarius, but not in B. impatiens. These results suggest that colonies ensure reproduction by producing some males and, given the opportunity (sufficient food availability), will produce gynes. Possible reasons for the increased success of food supplemented colonies are explored. However, despite some clear advantages of having larger food supplies such as the build-up of larger worker populations, food supplementation did not appear to help colonies defend themselves against macroparasites because experimental and control colonies experienced similar levels of parasitism by Psithyrus, Fannia canicularis, Brachicoma devia and Vitula edmandsae.