Nest sharing and provisioning activity of females of the digger wasp, Cerceiis rubida (Hymenoptera, Crabronidae)

Using individually marked females and nests, we investigated aspects of the nesting biology of Cerceris rubida, the only European species of its genus suspected to cooperatively share nests. Up to 11 females were marked from a single nest, but only 2-6 shared a single nest at a given moment, thus probably not all the wasps remained in their natal nests after emergence. Only 1-3 of the females sharing a nest collected brood cell provisions at a given moment, sometimes joined by other females that took few additional prey to the nest. The wasp has two generations a year, and the first generation females were larger than those of the second generation. A guard wasp was constantly present at the nest entrance and rejected non-nestmate females and parasites. Females typically made 1-18 provisioning trips in quick succession before stopping their hunting activity. They then remained in the nest for 1-148 min before re-starting provisioning. Provisioning females generally foraged simultaneously. All provisioning females showed well-developed ovaries with small to large oocytes ready for oviposition. Wasp size was not correlated with the number of provisioning flights it performed. Mandible wear and wing wear seemed to be independent of provisioning activity, suggesting that females are involved in digging and provisioning across their life span. We discuss these findings in relation to the known biology of other social Cerceris and other social apoid Hymenoptera.