Native parasitoids associated with the biological control agents of Centaurea stoebe in Montana, USA

Abstract

Under classical biological control, agents are released against target non-native organisms based on the assumption that invasiveness occurs due to a lack of adapted natural enemies. Biological control agents themselves, however, can become prey to native predators, parasites and parasitoids in their introduced environment with the potential to inhibit their effectiveness. The goal of this study was to identify parasitoids that attack the biological control agents of Centaurea stoebe L. in western Montana, United States. Roots and seedheads of C. stoebe were collected from 45 sites over a two-year period, and monitored for insect emergence. Of the thirteen biocontrol agents released against C. stoebe in Montana, ten were reared from the plant material collected. Nine species of parasitoid emerged, four of which were previously unknown associations with these biocontrol agents: Bracon sp., Pronotalia carlinarum Szelényi & Erdös, Eupelmus vesicularis Retzius, Scambus brevicornis Gravenhorst, Pteromalus cardui Erdös, Pteromalus elevatus Walker, two unknown species of Pteromalus, and one unknown species of Mesopolobus. Host associations were determined for three of the parasitoid species, and others were inferred based on previous studies. Parasitism rates of Urophora affinis by P. carlinarum were variable by location and time of sample collection, but were surprisingly high (reaching 100% in some cases) considering this is the first record of this host–parasitoid relationship and previous studies of U. affinis in this region found low levels of parasitism by other species. The long-term vulnerability of biocontrol agents to parasitism and predation by native organisms is a concern for the practice of classical biological control, especially for agents that have been established for several decades, and thus merits further research attention.