Host choice and fitness correlates for conopid flies parasitising bumblebees

Two parasitoid flies, Physocephala rufipes and Sicus ferrugineus (Diptera, Conopidae), and their hosts, Bombus spp., coexist at various locations in northwestern Switzerland. A detailed field study showed that both conopid species use the host B. pascuorum to a similar degree, while the host B. terr-luc (a pooled category of B. terrestris and B. lucorum) is more frequently parasitised than expected by S. ferrugineus. The host B. lapidarius in turn is exclusively used by P. rufipes. Furthermore, hosts of B. terr-luc and B. pascuorum parasitised by S. ferrugineus were larger than hosts parasitised by P. rufipes, or than those not parasitised. The findings suggest that S. ferrugineus selects larger hosts and may displace P. rufipes. Pupal weight, a predictor of adult body size and parasitoid fecundity, is positively correlated with host size and larger pupae are more Likely to emerge, while host species had no effect on the probability of emergence in either conopid species. Host species affected pupal weight in S. ferrugineus, but not in P. rufipes, although P. rufipes grew larger in hosts of a given size. Daughters emerged from larger pupae than males, but this did not correlate with larger host sizes. These observations add to the scarce knowledge of dipteran parasitoids.