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The rapid naturalization of Bombus terrestris across the Nemuro Peninsula has led to a decline in two closely related native Japanese species, namely Bombus hypocrita sapporensis and Bombus cryptarum florilegus, both belonging to the common subgenus Bombus. Although it is widely believed that cross-mating of native and non-native species is influenced by the common male sex pheromone in this region, no study has been conducted to substantiate this claim. Thus, we investigated the cross-activities of male sex pheromones between native and non-native bumblebees, as well as the frequencies of cross-mating, using chemical and DNA assays. Our gas chromatography–electroantennographic detector analyses and behavioral tests revealed the presence of sex pheromonal cross-activities between B. terrestris and the two Japanese bumblebees species. Furthermore, DNA analyses revealed the occurrence of cross-mating between native and non-native species in the Nemuro Peninsula. Overall, these results indicate the immediate need for conservation measures to safeguard Japanese bumblebee populations in the Nemuro Peninsula. [ABSTRACT FROM AUTHOR]

A commercialized pollinator introduced from Europe, Bombus terrestris, has colonized Japan. We investigated nest density and genetic structure in two sites based on worker genotypes at 12 microsatellite loci. We confirmed that five workers were triploids using multilocus genotypes and flow cytometry, indicating that queens mated with diploid males and produced triploid workers. The inbreeding coefficient of diploid workers representing individual colonies was significantly positive (FIS= 0.048) in a site where triploids were found. Genetic diversity in the sites was as high as that in native regions in Europe, and genetic differentiation between the sites was low (FST= 0.006). The maximum distance between sampling locations of full-sib worker pairs indicated that the radius of a foraging range was at least 782 m. The estimates of nest density were 31 and 89?km-2in the two sites, suggesting that the nest density in a colonized region can be higher than that in the native regions. [ABSTRACT FROM AUTHOR]