Individual prey specialization in wasps: predator size is a weak predictor of taxonomic niche width and niche overlap

Intuitively, larger predators in a population may be favoured in subduing, handling and carrying to the nest a wider range of prey compared to smaller ones. In absence of any individual chemical or behavioural bias in prey choice, and independently from size-biased preference, it may be predicted: (1) that smaller wasps would have both narrower niches and lower niche overlap than larger wasps, (2) a positive correlation between individual niche width and niche overlap, and (3) a positive correlation between predator size and prey size. We tested these predictions using available data in literature on individual prey spectrum for 10 populations/generations of apoid wasps (Hymenoptera: Crabronidae). The analysis showed that individual specialization is widespread. Smaller individuals were more specialized than larger ones, as we hypothesized, in only one population, while the opposite was found in two cases; on the other side, niche width correlated (positively) with wasp size only in one population. In five out of the 10 examined cases predator size correlated with prey size. Overall, the predictions were all true for just one population. In conclusion, different factors probably influence often more than predator size the dynamics of individual prey specialization in wasps. Circumstantial evidences suggest that individual-biased exploitation of different hunting areas, some behavioural traits of prey, and different ratios between prey and wasp size may partially account for the observed patterns.