Abstract
For over 30 years foreign exploration and trap gardens have been used in Europe to find biological control agents for yellow starthistle (Centaurea solstitialis) and five knapweeds (Centaurea spp.) in North America. Two seedhead flies and two seedhead weevils have been imported from Mediterranean Europe and released against the annual, yellow starthistle. A gall wasp, another seedhead fly, two flowerhead weevils, and two pathogens are being studied as potential biological controls for this weed. Two seedhead flies and a seedhead moth have also been released against diffuse knapweed (Centaurea diffusa) and spotted knapweed (Centaurea maculosa) since the 1970s. However, research emphasis is now on two root boring beetles and three root boring moths that attack these biennial and perennial weeds in Eurasia. Some of these insects also attack squarrose knapweed (Centaurea virgata ssp. squarrosa), a weed that has recently increased in northwestern states. A rust fungus already attacks diffuse knapweed in North America. The most promising biological control for Russian knapweed (Centaurea repens), a perennial that grows from an extensive root system, is a stem and leafgalling nematode that is currently being used in the USSR as a bioherbicide. A gall mite, a gall wasp, and a rust fungus may be imported in the future to use against this weed.
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