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Russian knapweed (Rhaponticum repens) is a nonnative weed in the western United States. It was introduced in the late 1800’s and is now invading and degrading cropland, rangeland, riparian areas, and roadsides. This deep-rooted perennial is persistent and difficult and expensive to control by conventional means, is toxic to horses and outcompetes native vegetation by producing chemicals that inhibit plant growth. Russian knapweed is listed by the State of Colorado as a noxious weed, to be suppressed, contained, or locally extirpated.

The Colorado Department of Agriculture (CDA) offers biocontrol agents to help suppress weeds and insect pests. When two gall-forming agents were developed and tested by CABI, and became available for use against Russian knapweed, the CDA was prepared to use them against the more than 50,000 hectares of the weed that currently infests Colorado.