A North American invasive seed pest, Megastigmus spermotrophus (Wachtl) (Hymenoptera: Torymidae): Its populations and parasitoids in a European introduction zone

Abstract

Megastigmus spermotrophus (Hymenoptera: Torymidae) is a seed pest of Douglas-fir ( Pseudotsuga menziesii) in its natural range of North America. Along with seeds, this invading species was accidentally introduced to Europe where it became the major pest in Douglas-fir seed orchards and stands. The high rates of seed parasitism observed in Europe were first ascribed to a reduced control by natural enemies which apparently did not followed M. spermotrophus from their native American range. However, our survey showed that the parasitoid censuses carried out so far were incomplete. Several other chalcid parasitoid species were present in the stands surveyed in Belgium. Four parasitoid species emerged from Douglas-fir seeds infested by the Douglas-fir seed chalcid. Two pteromalids of North American origin, Mesopolobus spermotrophus and Mesopolobus americus, were dominant whereas a few specimens of an eupelmid, Brasema urozonus, and of another pteromalid , Anogmus hohenheimensis, were also observed. Our results strongly suggest that the previous evaluations of parasitoids populations were biased by an inadequate timing in the censuses.