The presence of both triploid males and females in Bombus terrestris was detected by distinct chromosome observation. These cytological features are novel among the higher Hymenopteran insects. We thus strictly applied the complementary sex determination (CSD) model previously proposed for Hymenopteran insects. Three out of 60 sibling queens that were mated with diploid males produced both triploid males and females, and founded colonies. The male to female ratio of the bees which emerged from the fertilized eggs of the queens was approximately 1 to 1. Thus we reconfirm that the sex in B. terrestris is determined by a single multi-allelic locus. The body size of the triploid males was smaller than that of the diploid and haploid males. We found hatched eggs laid by one triploid female (worker). One of these developed into a 3rd instar larva, however most of the triploid individuals were sterile.

The European bumblebee B. terrestris was recently introduced in Japan for agricultural purposes and has now become naturalized. The naturalization of this exotic species may have great detrimental effects on closely related native Japanese bumblebees. The Japanese bumblebee Bombus florilegus is a rare and locally distributed species found in the Nemuro Peninsula of Hokkaido, Japan. In order to assess its population genetics, we estimated the genetic structure of B. floriA C H T U N G T R E N N U N G legus in 16 breeding colonies (queen, workers, and males) and 20 foraging queens by analyzing microsatellite DNA markers. Of the 36 queens analyzed by genotyping and dissection, 32 had been inseminated by a male. The remaining 4 had not been inseminated at all. Of the 4 nonmated queens, one was triploid. Diploid males were found in 4 breeding colonies. Based on the microsatellite data, it appears that B. florilegus has low reproductive success. Since matched mating and nonmating within local populations are high, the extinction risk is correspondingly higher. Our results suggest that conservation of the Japanese B. florilegus is required in order to protect it from both habitat destruction and the naturalization of alien species.