Female–female attraction influences nest establishment in the digger wasp Stizus continuus (Hymenoptera: Crabronidae)

Although digger wasps are good models for the study of nest site selection, the behaviour of females in relation to time of nest establishment has rarely been considered despite this being the key to discrimination between substrate availability-based and a female attraction-based selection. We carried out a study on nest establishment by the digger wasp Stizus continuus (Klug), giving to all nests dug by the wasps both spatial (nearest neighbour distance to nests, proximity of vegetation, soil hardness) and temporal (day of excavation) attributes. Stizus continuus females dug more nests under bush edges, where the soil was much softer. There was no tendency for females to maximize distance between nests at the moment of establishment. On the contrary, as shown by different nearest neighbour statistics, females preferentially tend to dig close to conspecific nests (if one considers both the whole study area and two subareas free of vegetation and at different times of the nesting season). Moreover, those nests that were active (still provisioned) attracted more females than the inactive nests at the moment of nest establishment. We conclude that female S. continuus are attracted by conspecifics at the moment of nest establishment and that clumped patterns of nests are a result of such social attraction rather than a result of limited available substrate; moreover, such attraction could be better detected when comparative analyses are employed and best at a multiple-scale view.