Communal Nesting in the Digger Wasp Cerceris australis (Hymenoptera : Sphecidae)

Abstract

Study of 39 nests of Cerceris australis at nine localities in eastern Australia has demonstrated that most nests are occupied by two or three successive generations of wasps and may ultimately contain well over 100 cells. Nests are dug deep in the soil and are provisioned with scarab beetles, which are allowed to accumulate in the burrow before from two to six are placed in a cell. Nests are usually occupied by several females, some of which are provisioners, bringing in beetles day after day and each time leaving the nest usually after only a few seconds, and others are non-provisioners, leaving the nest for a short period once a day and returning without prey. During the day, nest entrances are occupied by females (believed to be usually non-provisioners) stationed facing out; they are effective in deterring the entry of ants and mutillids. The factors that determine what role a female will play remain obscure. Both provisioners and non-provisioners show progressive mandibular wear as well as essentially similar ovarian development; there are no consistent differences in body size between members of the two groups. In any one nest, considerable variation in the appearance of the ovaries is apparent, and oosorption appears to be common. More than one female often appears to be in egg-laying condition, and the fact that cooperating provisioners bring in enough beetles each day to provision several cells suggests that more than one female lays an egg each day. However, the presence of oocytes in various stages of resorption suggests that in some individuals oviposition is suppressed. No correlation was found between extent of oosorption and the provisioner-nonprovisioner dichotomy. The necessity to guard these large, multicellular nests from parasitoids and predators has evidently brought about selection for the development of a caste of guards which, however, continue to play a role in nest construction and presumably in laying eggs on beetles provided by other females.