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Biology

The biology Lasioglossum baleicum is discussed by Cronin and Hirata (2003). This species can be either solitary or eusocial. The authors studied the life cycle of this bee at two sites in Hokkaido, Japan: one warmer site in Nishioka (near Sapporo) and one colder site near Kawakita. L. baleicum nests consiste of a tunnel into the soil which opens onto a cavity containing a cluster of earthen cells. Each offspring is reared in individual cells in the earthen cluster, which is supported by earthen pillars inside the cavity. Nests were typically 7-12 cm deep.

The Nishioka population was weakly eusocial: spring foundresses produced a non-reproductive worker brood, and these workers then provisioned a second reproductive brood. Nests were initiated in April or May, after snow melted. Nests were initiated by a single female, except one which contained two females. The males and females from this second brood then mated, and the females hibernated overwinter to initiate nests the following spring, while the males died. However, this division was not absolute: ~15% of the first brood was male (only females are workers) and 28% of workers were mated with some ovarian development, suggesting that the worker brood is not completely non-reproductive after all. Additionally, there were some solitary colonies in the Nishioka population. The average number of workers during second brood provisioning was 1.7, although one nest contained 6. Two (of 50) nests contained two queens, (both with a single worker) and 4 nests contained no queens but two workers, suggesting that workers could not always develop into replacement queens. Further support for this comes from the fact that 57% of orphaned nests contained brood, whereas 84% of queen-containing nests contained brood. Workers averaged 5% smaller than their queens, and larger-bodied queens had larger ovaries.

In contrast, the Kawakita population was solitary, and produced only one brood of offspring per year. Nests were initiated in late May or early June after snow melt. Nests were initiated by a single female, although one colony contained two females. The offspring of the overwintered females emerged as adults in July and August. The males and females of this offspring generation mated, with the females then overwintering to initiate nests the following spring, and the males dying before winter. However, some colonies at Kawakita appeared to produce a partial second brood: some nests collected in August included newly provisioned cells, and some included offspring females still present at the nest. In a warmer environment, these could have functioned as workers and reared the second generation of offspring, but in the Kawakita population, it would have been impossible for these brood to develop before the onset of cold weather.

Thus, social organization is polymorphic both between and within populations.

Hirata and Higashi (2008) further investigated the differences between the two sites. They showed that the two populations were not genetically diverged or heterospecific, thus the social polymorphism was environmentally determined. However, they also demonstrated that even at the Nishioka (warmer) site, temperature variation affected social organization: nests in sunny sites tended to be eusocial, where as those in shady sites, with slower brood development, did not have functional workers.

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Creative Commons Attribution 3.0 (CC BY 3.0)

© Smith, Adam

Source: Halictidae

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