Parasitism by this wasp is quite variable. During a four-year study in Wisconsin, parasitism by C. glomerata averaged only 14.5% with a range of 0-21.2%. However, the wasps parasitized 86% of the caterpillars in one commercial red cabbage field in 1983.
C. glomerata populations often have very little effect on the first generation of imported cabbageworm. Densities of parasites in the spring are low because of mortality during the winter and hyperparasites, so the parasites rarely increase at a sufficient rate to suppress subsequent populations before economically important injury occurs. Parasitized caterpillars continue to feed, consuming approximately one and a half times as much as unparasitized caterpillars before they are killed by the emerging parasite larvae. Cotesia causes significant mortality later as parasitism increases over the subsequent generations, although host defense mechanisms frequently kill the parasite eggs. Rates of parasitism reach high levels only late in the season, after the caterpillars have injured the crop. This parasitism does reduce the overwintering population, however, and therefore the subsequent initial spring population.
There is a related species of parasite, Cotesia rubecula, that is a solitary instead of gregarious parasite. It is a more efficient parasite than C. glomerata and also kills the host sooner, which reduces pest damage.
However, this species is established only in isolated pockets in the Americas, including British Columbia and Ontario, Canada. A strain of the species imported from Yugoslavia has been established in Michigan since releases in 1985. It has displaced C. glomerata there and has spread at least 20 miles from the original site. Even if this species does become established, however, additional releases may be necessary to provide economic control. Currently, neither wasp is commercially available.
- Susan Mahr,University of Wisconsin - Madison
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