Successful biological control of diffuse knapweed, Centaurea diffusa, in British Columbia, Canada

The biological control program for diffuse knapweed, Centaurea diffusa Lamarck, a Eurasian plant that has invaded large areas of grasslands in western North America, has gone on for over 35 years. This program involved the release of 12 biological control agents of which four are numerous and widely distributed; two species of Tephritid flies, Urophora affinis and Urophora quadrifasciata, the root boring beetle, Sphenoptera jugoslavica, and the most recently established weevil Larinus minutus. Field observations show that diffuse knapweed densities declined at sites in British Columbia Canada where the weevil L. minutus became established. Decline in diffuse knapweed density did not occur where densities of L. minutus were low. Field cage experiments showed that feeding by L. minutus damaged rosette leaves and bolting stems, and reduced seed production, seedling density and the density of rosette and flowering diffuse knapweed plants.