Survival and productivity benefits to social nesting in the sweat bee Megalopta genalis (Hymenoptera: Halictidae)

Abstract

Facultatively solitary and eusocial species allow for direct tests of the benefits of group living. We used the facultatively social sweat bee Megalopta genalis to test several benefits of group living. We surveyed natural nests modified for observation in the field weekly for 5 weeks in 2003. First, we demonstrate that social and solitary nesting are alternative behaviors, rather than different points on one developmental trajectory. Next, we show that solitary nests suffered significantly higher rates of nest failure than did social nests. Nest failure apparently resulted from solitary foundress mortality and subsequent brood orphanage. Social nests had significantly higher productivity, measured as new brood cells provisioned during the study, than did solitary nests. After accounting for nest failures, per capita productivity did not change with group size. Our results support key predictions of Assured Fitness Return models, suggesting such indirect fitness benefits favor eusocial nesting in M. genalis. We compared field collections of natural nests to our observation nest data to show that without accounting for nest failures, M. genalis appear to suffer a per capita productivity decrease with increasing group size. Calculating per capita productivity from collected nests without accounting for the differential probabilities of survival across group sizes leads to an overestimate of solitary nest productivity.

Socially induced brain development in a facultatively eusocial sweat beeMegalopta genalis(Halictidae)

“…These recordings allowed us to determine which bees were queens and workers (Smith et al 2008(Smith et al , 2009). Solitary and social nesting are distinct behavioural strategies, rather than different points on the same developmental trajectory (Smith et al 2007(Smith et al , 2009). To distinguish solitary reproductives from social queens waiting for offspring to emerge, we monitored single-bee nests for at least five weeks (the time needed for offspring to complete development) after they were modified.…”

“…Social queens are older than their workers because the latter are the foundresses' daughters. Social queens and solitary reproductives come from the same cohort of bees and establish nests at the same times of year (Wcislo et al 2004;Smith et al 2007), so there is no reason to expect any systematic bias in age of one class or the other. Each class consists of adult females in post-emergent nests; among post-emergent nests, older nests are not more likely to be social than are younger nests (Smith et al 2007; K. M. Kapheim, A. R. Smith & W. T. Wcislow 2008, unpublished data).…”

“…Comparative interpretations are speculative because MB development has not been studied in other solitary bees. To avoid these problems and directly test the effect of sociality, we used a facultatively social sweat bee, M. genalis (Halictidae), which can nest either socially or solitarily (Wcislo et al 2004;Smith et al 2007Smith et al , 2009, to directly compare the brains of social and solitary individuals within the same species.…”

Social competition but not subfertility leads to a division of labour in the facultatively social sweat bee Megalopta genalis (Hymenoptera: Halictidae)

“…Approximately one-half of M. genalis nests are social, and these typically contain a queen, primary forager, and sometimes young female(s) possibly waiting to disperse (Smith et al 2003(Smith et al , 2007(Smith et al , 2008Wcislo et al 2004;Wcislo & Gonzalez 2006). Even in twobee groups, division of labour is strong (Wcislo & Gonzalez 2006;Smith et al 2008), and foragers can gain indirect fitness through helping (Smith et al 2003(Smith et al , 2007.…”

“…Such taxa are critical for studying the origins and evolution of reproductive division of labour because they permit direct examination of the factors that promote a switch from solitary to social life (Wcislo 1997(Wcislo , 2000Field et al 2000;Smith et al 2003Smith et al , 2007Smith et al , 2008Schwarz et al 2007). The facultatively social bee Megalopta genalis (Hymenoptera: Halictidae) shows strong reproductive division of labour associated with body size: foragers tend to be smaller than queens, dispersers and solitary reproductives (Smith et al 2008).…”