Skip to main content
ARS Home » Pacific West Area » Albany, California » Western Regional Research Center » Invasive Species and Pollinator Health » Research » Publications at this Location » Publication #197193


item Smith, Lincoln

Submitted to: Western Society of Weed Science Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 12/1/2005
Publication Date: 3/14/2006
Citation: Smith, L. 2006. Biological control of knapweeds and yellow starthistle. Western Society of Weed Science Meeting Proceedings, March 14-16, 2006, Sparks, NV. pp.

Interpretive Summary: Yellow starthistle, spotted knapweed, diffuse knapweed, and squarrose knapweed are important alien weeds that have invaded more than 30 million acres in the western U.S. They are primarily rangeland weeds, and have displaced desirable vegetation, negatively impacting livestock grazing, biodiversity, soil erosion and outdoors recreation. These weeds are closely related but differ slightly in their biology and ecology. Because these weeds are alien and cover large regions, they have been the target of classical biological control programs. Most of these programs are now mature, and scientists are beginning to see substantial control of many of these weeds in many regions. A symposium is being held at the Western Society of Weed Science annual meeting to review the status of these biological control projects in the western U.S. and Canada. Successful biological control will provide self-perpetuating long-term management of these weeds, reduce the need to apply pesticides, and increase the productivity and utility of millions of acres.

Technical Abstract: The plant genus Centaurea (family Asteraceae) includes many species that are important invasive alien weeds in the western U.S. These include spotted, diffuse, squarrose and meadow knapweeds and yellow starthistle. Russian knapweed (Acroptilon repens) is closely related and was once included in the genus. Only two species of Centaurea are considered to be native to N. America (C. rothrockii and C. americana); however, these are distantly related to the invasive species and have been placed in a separate genus, Plectocephalus. Bachelor's button (C. cyanus) will likely also be placed in a separate genus, Cyanus. The absence of other closely related native or agronomic plants in N. America makes knapweeds and yellow starthistle suitable targets for classical biological control. Despite their taxonomic similarity, the weedy species represent annual, biennial and perennial forms that are adapted to a range of biomes, including Mediterranean, steppe and coniferous temperate forest. It has been over 30 years since the first biological control agents were introduced for some of these weeds. Now, a total of 20 insects and one pathogen have been introduced. The symposium, "Status of Biological Control of Knapweeds and Yellow Starthistle" reviews the status of various projects in 5 states and Canada, noting successes, failures and situations needing further research.