Diploid males in the bumble bee <i>Bombus terrestris</i>

Abstract The frequency of colonies that produce diploid males after brother‐sister (50%) and nephew‐niece (37.5%) matings proves that in B. terrestris the sex is determined by a single multi‐allelic sex locus. The diploid males which develop normally into adults make up 50% of the diploid brood. In the laboratory the growth rate of colonies with diploid males is influenced only slightly. Of 41 colony founding queens caught out of a natural population, all produced a colony without any diploid males. Therefore, the number of sex alleles in this population is estimated to be at least 24. This means that in commercial rearing systems for bumble bees, involving several generations, the occurrence of diploid males can largely be prevented by a good scheme for crossings.