Selection for non-diapause in the bumblebee Bombus terrestris, with notes on the effect of inbreeding

Bumblebees are generally believed to be annual insects. However, here we will show that under laboratory conditions the bumblebee Bombus terrestris (L.) can produce a second generation without a period of cold storage (diapause) or CO₂ narcosis (a method to break diapause). It is also shown that this so-called non-diapause trait can be selected for. The percentage of non-diapausing queens increased from 8% (minimum) to 97% (maximum) in two generations of selection. However, it was not possible to maintain isofemale non-diapause lines. Colonies of the fourth and fifth generation remained small (expressed in worker number) and produced a small number of queens. Also the percentage of queens that started laying eggs (defined as the percentage non-diapause) decreased in the fourth and fifth generations. To study whether this decline of the non-diapause lines was caused by inbreeding, a control experiment was conducted. In this control experiment queens were mated with their brothers (full-sib mating) for several generations and the number of queens that start egg laying was measured. This revealed that inbreeding can have a negative effect on the egg-laying capacities of queens thus causing the decline of inbred (non-diapause) lines.