Highly male-biased investment ratios (1:2.86, F:M, biomass wet weight) were found among reproductives reared by 32 Bombus terricola colonies set out in the field and 35 B. melanopygus colonies held in laboratory confinement. The numerical sex ratios were almost identical for the two species (1 queen to 6.1 males). Most of the males were produced early in the season by small weak colonies. It is argued that in a population where some colonies are ‘orphaned’ and rear only males, female-biased sex ratios are expected among the surviving queen-producing colonies.