When the bumble bee, Bombus terrestris, was reared under controlled conditions in the laboratory for many generations, queens with a wet weight of 1 g or more were no longer produced after several generations. At present it is unknown what causes the disappearance of these heavier queens. In this paper three possible explanations are given: (1) inbreeding, (2) altered energy allocation strategy such that more but lighter queens are produced, (3) a gradual build-up of a deficiency of scarce nutrients. No proof was found for a negative effect of inbreeding on the weight of queens, and colonies did not produce more queens. This suggests that the disappearance of heavy queens is caused by a deficiency of scarce nutrients.