FREQUENCY AND ECOLOGICAL CORRELATES OF PARASITISM BY CONOPID FLIES (CONOPIDAE, DIPTERA) IN POPULATIONS OF BUMBLEBEES

We present field data on the ecology of a host-parasite system, consisting of several species of parasitoid flies (Conopidae, Diptera) and their bumblebee hosts (Bombini, Apoidea, Hymenoptera). Host animals were systematically sampled from different study sites throughout a season and checked for successful infestation in the form of puparia of these endoparasitic flies. Such dissection of the bees revealed that infestation occurs primarily during the summer months (June to September), with an observed maximum frequency of parasitization of 46.7% in workers in one of our study sites. On average, 13.2% of all workers (range 0-46.7%) and 7.1% of all males (range 0-28.6%) contained the puparium of a conopid. Two conopid genera Sicus (64% of cases) and Physocephala (36%) accounted for the infestation, with the latter being more abundant later in the year. A multivariate analysis identified host species, sex (male or worker), and study area as the most important factors accounting for the observed variance in the probability of being parasitized during the summer months. On average, males were less affected than workers. The marked seasonal appearance of conopids seems to account for differences among species, in particular for low levels of infestation among species completing their life cycles early (e.g. B. pratorum) and among the early flying, hibernated quens of Bombus and Psithyrus species. The results are discussd with respect to the impact of conopids on host ecology and evolution. Additional observations on the occurrence of further endoparasites (Sphaerularia bombi and Syntretus sp.) are reported. © 1990 Masson.