In the Upper Peninsula of Michigan, spotted knapweed (Centaurea maculosa Lam.) invades disturbed sites in urban areas and along roadsides, as well as natural ecosystems that are subject to disturbance such as dunes along the shores of Lake Superior. Spotted knapweed is an exotic weed that has invaded many millions of acres in the Western and Central United States. Management of this species has included attempts at biological control using seed head flies in the genus Urophora (Diptera: Tephritidae). The larvae of these flies feed in the capitula (seed heads) and reduce seed production.
The seed head fly Urophora quadrifasciata (Meig.) has not been released in the Upper Peninsula of Michigan, but was observed infesting capitula in Houghton County, Michigan, as well as in many other counties on Michiganís Upper Peninsula. To determine the relationship between knapweed patch size and percent of capitula infested, knapweed plants were collected during August through November 2002 from 14 patches within Houghton County, Michigan. Twenty two percent of capitula dissected from these patches contained U. quadrifasciata larvae. An inverse relationship between patch size and percentage of capitula with U. quadrifasciata larvae was found.
During dissections and rearing, a parasitic wasp was observed utilizing these same capitula as the U. quadrifasciata. Adult parasitic wasps emerged from 20 percent of the capitula placed into rearing. This wasp appears to be a parasite of the introduced seed head fly, and may have direct impacts on the success of U. quadrifasciata as a biological control agent.
Species 1: Diptera Tephritidae Urophora quadrifasciata
Species 2: Hymenoptera
Keywords: invasive species
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