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The knapweeds comprise a diverse complex of species that predominantly infest rangelands in the western United States and Canada. This manual considers the biological control of six species of knapweeds: 1) spotted knapweed, 2) diffuse knapweed, 3) squarrose knapweed, 4) meadow knapweed, 5) black knapweed, and 6) brown knapweed. Spotted knapweed, Centaurea stoebe, is perhaps the most widespread species, followed in abundance by diffuse knapweed, C. diffusa. A third species, squarrose knapweed, C. virgata var. squarrosa, has a more limited distribution in the West. Three less widespread species, meadow (C. pratensis), brown (C. jacea) and black knapweed (C. nigra), are host to some of the same biocontrol agents and thus are included. Another well known species is Russian knapweed, Acroptilon repens (formerly known as C. repens). While a serious rangeland weed itself, Russian knapweed is not considered in this manual because it is sufficiently different from the other knapweeds to be considered separately and it has a unique complex of biocontrol agents.