The influence of soil temperature on the nesting cycle of the halictid bee Lasioglossum malachurum

The physiology and behavior of ectothermic organisms is strongly influenced by temperature. For ground nesting species like the primitively eusocial halictid bee, Lasioglossum malachurum, soil temperature might influence the life cycle as well as the complexity of the social group since the number of broods that can be fitted into the flight season might increase with increasing temperature. Our study population of L. malachurum at Wuerzburg exhibits a remarkable variability with respect to the number of broods and the pattern of sexual production. Broods are separated by activity pauses during which the larvae develop. In this study we investigate the influence of soil temperature on the pattern of nesting activity (duration of broods and pauses) and on the number of broods in L. malachurum. We observed a total of 1138 nests in 13 aggregations near Wuerzburg. As expected, soil temperature shortened the duration of the pauses, resulting in an overall shortening of the nesting cycle. This is most probably due to a physiological effect of soil temperature on the development of the larvae. With regard to the nesting strategies, we hypothesized that a shortening of the nesting cycle within the limited flight season should enhance the success of a strategy with more worker broods. In fact, patches with higher soil temperature showed more broods. However, this effect was rather weak, suggesting that other factors might have a stronger impact on the variability in nesting strategy within our study population of L. malachurum.