Flexible social organization and high incidence of drifting in the sweat bee, Halictus scabiosae

Abstract

The very diverse social systems of sweat bees make them interesting models to study social evolution. Here we focus on the dispersal behaviour and social organization of Halictus scabiosae, a common yet poorly known species of Europe. By combining field observations and genetic data, we show that females have multiple reproductive strategies, which generates a large diversity in the social structure of nests. A detailed microsatellite analysis of 60 nests revealed that 55% of the nests contained the offspring of a single female, whereas the rest had more complex social structures, with three clear cases of multiple females reproducing in the same nest and frequent occurrence of unrelated individuals. Drifting among nests was surprisingly common, as 16% of the 122 nests in the overall sample and 44% of the nests with complex social structure contained females that had genotypes consistent with being full-sisters of females sampled in other nests of the population. Drifters originated from nests with an above-average productivity and were unrelated to their nestmates, suggesting that drifting might be a strategy to avoid competition among related females. The sex-specific comparison of genetic differentiation indicated that dispersal was male-biased, which would reinforce local resource competition among females. The pattern of genetic differentiation among populations was consistent with a dynamic process of patch colonization and extinction, as expected from the unstable, anthropogenic habitat of this species. Overall, our data show that H. scabiosae varies greatly in dispersal behaviour and social organization. The surprisingly high frequency of drifters echoes recent findings in wasps and bees, calling for further investigation of the adaptive basis of drifting in the social insects.

A halictid bee with sympatric solitary and eusocial nests offers evidence for Hamilton's rule

“…Direct reproduction by unrelated intruders has been reported in several bee species 19,33,34 . In our data, the intruders take their own share of direct reproduction; however, the members of the host nests obtained similar level fitness comparing with the eusocial nests without intruders (Fig.…”

“…Thus, worker behaviour in L. baleicum would be beneficial to the queens and workers. The F0 females of halictid bees maintain their nests without helpers during the first reproductive period 12,13,16,19 . It has been suggested that the lack of lifetime monogamy may prevent the halictid bees from lifetime eusociality 26 .…”

“…For example, L. baleicum populations inhabiting cold areas rear only a single brood 14,17 , suggesting that early diapause is an optional strategy for F1 females. In addition, adult females occasionally intrude into an unrelated nest 14,15,19 . In a previous study, bees nesting in a shady area did not produce a second brood 14 , which indicated that a F1 female can choose early diapause or intruding.…”

Solitary bees reduce investment in communication compared with their social relatives

“…Recent behavioral studies may shed some light on this observation. Although this species is eusocial, individuals also show a large degree of drifting among nests, and females will often leave their natal nests to reproduce in nests of unrelated individuals (56,57). Many halictids mark nests with chemical cues, and these odors are used by guards to distinguish nestmates and nonnestmates (58,59).…”

Impact of helpers on colony productivity in a primitively eusocial bee

“…In this species, foundresses rear a first brood consisting principally of relatively small females that help their mother to rear a second brood of males and gynes (Brand and Chapuisat 2012). The first-brood females have the possibility to mate and reproduce, either in their natal nests or in other nests (Ulrich et al 2009). Hence, it is interesting to examine whether helpers really increase colony survival and productivity, and by how much.…”