Although females of Ceratina flavipes are believed to be inseminated in spring on Honshu Island, Japan, 100% of the females were inseminated before hibernation on Ishikari Coast, northernmost Japan. Because most, if not all, of the males also overwintered with the females, this prehibernal insemination may be a local event. In the hibernal season, females were more frequently alone in nests than males, whereas the sex ratio in their main habitat was almost 1:1, suggesting that prehibernal dispersal is more frequent in females, but that the dispersal distance is shorter than previously reported.

Colony structure and eusociality level of the sweat bee Lasioglossum (Evylaeus) duplex were studied in 2001 in Sapporo and Assabu, Hokkaido, northern Japan. Sakagami and his colleagues had also studied this species in Sapporo in 1957-1968. Brood size, sex ratio and queen-worker size dimorphism were geographically and temporally variable, indicating spatio-temporal variation at the eusociality level. Inseminated workers constituted only 7.9% of the populations in 1957-1968 Sapporo but about 60% in 2001 in Sapporo and Assabu. A few of the inseminated workers were believed to leave natal nests for independent colony founding. Thus, partial bivoltinism is likely in this sweat bee species. The presence of workers with developed ovaries and/or corpora lutea suggests the occurrence of worker oviposition.

Polymorphic microsatellite markers can provide essential information on genetic analyses of sociality and mating behaviour of insects. We developed eight microsatellite loci in the small carpenter bee, Ceratina flavipes, using a magnetic bead hybridization enrichment protocol. These loci showed two to eight alleles with expected heterozygosity of 0.21–0.87, and also seemed useful to such congeneric species as Ceratina okinawana distributed in southern Japan. By using these loci, the sociality and mating systems in Ceratina species, that are still controversial, are possible to be analysed.

At 17 localities in Japan, 71-81% of overwintering adult females of the solitary bee Ceratina flavipes mated before hibernation. The occurrence of males with inseminated females in their natal nests suggested the possible occurrence of inbreeding. Therefore, we examined the seasonal trends of inbreeding coefficients, calculated by comparing the band patterns of microsatellite DNA between a female and her mating partners (indicated by sperm in her spermatheca). The mean coefficient of inbreeding was 0.80 on 25 August and 0.60 on 1 November versus 0.31 on 5 June and 0.38 on 28 June. These results demonstrate a tendency towards inbreeding before winter and towards outbreeding during dispersal after hibernation.