Identity, distribution, and seasonal phenology of parasitoids of the swede midge, Contarinia nasturtii (Kieffer) (Diptera: Cecidomyiidae) in Europe

The swede midge, Contarinia nasturtii (Kieffer) (Diptera: Cecidomyiidae) is an invasive gall midge of Eurasian origin that has recently become a pest of crucifer (Brassica oleracea L.) crops and canola (Brassica napus L.) in North America. In order to identify possible candidates for the classical biological control of this pest, we conducted an extensive survey of Europe to determine what species of parasitoids attack the swede midge. In addition, weekly monitoring of an oilseed rape plot in north western Switzerland allowed the observation of the seasonal phenological relationships between the swede midge and its parasitoids. Synopeas myles (Walker) (Hymenoptera: Platygastridae) and Macroglenes chalybeus (Haliday) (Hymenoptera: Pteromalidae) were found to be the two primary parasitoid species present throughout the surveyed range and, in Switzerland, attacking every generation of the swede midge. In the survey and the monitoring of the oilseed rape plot, total percent parasitism of samples ranged from 0% to 41%, but was typically quite low (<15%). Both S. myles and M. chalybeus have been reported to attack several other species of gall midges in Europe, casting doubt on their host specificity. However, before classical biological control of the swede midge in North America using its parasitoids from Europe can be ruled out, more research is needed to measure their importance as a mortality factor for natural swede midge populations and to properly assess their host specificity.

Highlights► We conducted an extensive survey of Europe to determine what parasitoids attack the swede midge. ► We also monitored an oilseed rape plot in Switzerland to observe host/parasitoid phenological relationships. ► Two species, Synopeas myles and Macroglenes chalybeus were found to be associated with the swede midge. ► Neither parasitoid consistently attacked the swede midge at high rates and neither appears to be host-specific.