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Social Behaviour of Lasioglossum apristumAlthough previously reported as solitary (Sakagami and Munakata, 1966; Yamada and Sakagami, 1990), Lasioglossum (Evylaeus) apristum has since been identified as socially polymorphic, where a solitary life cycle exists in northern Japan (Sapporo), and a social in south-eastern Japan (Mt. Daisen) (Miyanaga et al., 1999).
Miyanaga et al. (1999) inferred the social life cycle of L. apristum on Mt. Daisen from specimens captured over four years as follows: foundresses emerge in mid-May, and workers from mid-July to mid-August with reproductives from early September to the first frosts of October. Overall, queen worker dimorphism was found to be 7.7%, indicative of primitive sociality (Packer and Knerer, 1985), however, overall low incidence of both mated workers (6.5%) and workers with developed ovaries (8.7%) are considered more advanced social traits (Packer and Knerer, 1985).
The solitary life cycle was similarly inferred by Miyanaga et al. (1999), from data specimens collected in 1969 from Sapporo as follows: foundresses emerge from mid-May and provision their nests from June to August with reproductives appearing in August and September. The lack of uninseminated females with underdeveloped ovaries and the relative homogeneity of size distribution was taken as evidence for the absence of a social worker phase.
In addition to the above studies, Miyanaga et al. (1999) collected pre-hibernation gynes from Mt. Daisen and released them into a cage in a greenhouse for detailed observation. Of 300 bees released, 16 survived to dig nests, only 7 produced reproductive offspring and from four of these the following information on caste structure were gathered by excavating the nests. Nests contained between 2 and ten workers, and of the three in which the queen was found, caste dimorphism was 4.1-9.0%. Detailed observations of two nest revealed workers undertaking between 2 and ten pollen, and 1-2 nectar, foraging trips per day.
A small number of the uninseminated workers were identified as likely layers of haploid eggs. The excavated nests were classified as type IVb or Va (Sakagami and Michener, 1962), and therefore cells are arranged in clusters rather than individually dug along the length of the main vertical burrow. Of the surviving captive-reared bees, the worker brood contained on average 6.9 offspring and was strongly female-biased, but the sex ratio of the second brood was not investigated, both suggestive of more advanced social level (Packer and Knerer, 1985). However, it is worth noting that these represent a very small sample size in an unnatural environment, and therefore may not reflect the wild condition.
A common reason given for the appearance of solitary populations is contraction in the length of the bee-active season. Thus, solitary populations tend to be found in northern and high altitude areas where spring starts later and summer finishes earlier than where social populations are found (e.g. (Eickwort et al., 1996); Sakagami and Munakata, 1972). Interestingly however, Miyanaga et al. (1999) found that the cumulative temperatures in both Sapporo and Mt. Daisen are very similar, indicating the bee-active seasons are of similar length. It would be useful to know whether L. apristum is socially plastic, as demonstrated for Halictus rubicundus in the UK (Field et al., 2010), or whether these are fixed differences as suggested for L. albipes (Plateaux-Quénu et al., 2000) and H. rubicundus in North America (Soucy and Danforth, 2002).