Submitted to: XI Symposium on Biological Control of Weeds
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 29, 2003
Publication Date: June 1, 2004
Citation: Smith, L. 2004. Impact of biological control agents on diffuse knapweed in central montana. In: Cullen, J.M., Briese, D.T., Kriticos, D.J., Lonsdale, W.M., Morin, L. and Scott, J.K. editors. Proceedings of the XI Symposium on Biological Control of Weeds, April 27-May 2, 2003, Canberra, Australia. p. 589-593. Technical Abstract: Diffuse knapweed (Centaurea diffusa) is a major weed in semiarid regions in the northwestern United States. Investigations on biological control began in the 1960s and have resulted in the release of 13 species of insect biological control agents (most of which also attack spotted knapweed, C. maculosa or C. stoebe subsp. micranthos). In Central Montana, three capitula insects (Urophora affinis, U. quadrifasciata and Larinus minutus) and three root-feeding insects (Sphenoptera jugoslavica, Agapeta zoegana and Cyphocleonus achates) have become well established. Populations of diffuse knapweed have rapidly declined at study sites at two locations in the presence of high densities of biological control agents. The root-feeding insects, Cyphocleonus achates, Agapeta zoegana and Sphenoptera jugoslavica infested up to 64%, 57% and 31% of roots, respectively. The capitula insects, Larinus minutus and Urophora species (U. affinis and U. quadrifasciata), infested up to 62% and 37% of capitula, respectively. By the summer of 2000, some study sites had no mature plants that could be sampled.