Temporal relationship between the prey spectrum and population structure of the weevil-hunting wasp Cerceris arenaria (Hymenoptera: Crabronidae)

Specialized predators must face the problem of reductions in resources, and variations in the prey spectrum can be expected to be reflected in predator population traits. Cerceris arenaria L. (Hymenoptera: Crabronidae) is a solitary wasp that hunts weevils (Coleoptera: Curculionidae), with the individual range of prey sizes varying according to the size of the female wasp. A nest aggregation in Castiglione d'Adda, northern Italy was studied in 1997-1999, 2001, 2003, and 2005 in order to investigate variations in the prey spectrum across several years, while a 2nd aggregation in Castell'Arquato was investigated in 2001 to look for possible differences in prey selection between the 2 populations. The captured weevils belonged to 23 species. At both sites, the most often collected genera were Otiorhynchus Germar and Sitona Germar, which include several pests of cultivated plants. The prey spectrum (in terms of taxonomic identity and specimen frequency per species) varied among the years of study in Castiglione d'Adda, with Sitona spp. being more abundant in 1997-1999 and Otiorhynchus spp. in 2001, 2003, and 2005. According to this temporal shift in the prey spectrum, the annual average prey size increased from 1999 to 2005. Annual average prey size in Castiglione d'Adda was always lower than that in Castell'Arquato, where the frequency ratio of Otiorhynchus/Sitona was higher than that in Castiglione d'Adda. Prey size and the frequency ratio of Otiorhynchus/Sitona in Castiglione d'Adda were negatively correlated to the wasp population size, and the average wasp size increased when the frequency of larger prey increased. We concluded that interactions between extrinsic (prey availability) and intrinsic (wasp size distribution) factors may strongly influence fluctuations and persistence of specialized predatory wasp populations.