Session 9 Post-release Evaluation and Management 475 Determining the Efficacy of Larinus minutus (Coleoptera: Curculionidae) in Spotted Knapweed Biological Control: The Silver Bullet? C. R. Minteer, T. J. Kring, Y. J. Shen3 and R. N. Wiedenmann4 University of Arkansas Department of Entomology, Fayetteville, AR 72764 USA [email protected] [email protected] [email protected] [email protected] Abstract Spotted knapweed, Centaurea stoebe ssp. micranthos (Gugler) Hayek, is an exotic, invasive weed that has caused significant damage in the northwestern United States and can reduce forage production by more than 88%. In spite of a successful biological control program for this in the northwest, the weed is expanding rapidly throughout the southeastern United States, where no comprehensive control program exists. One of the insects thought largely responsible for the weed’s decline in the northwest is Larinus minutus Gyll. Adult L. minutus were collected from areas around Colorado Springs, Colorado, from 2007 through 2011. Adult weevils were returned to Arkansas and released at 39 sites at an average of 700 weevils per release. Studies to determine the effect of L. minutus on knapweed seed and plant densities were conducted in 2010 and 2011. The only differences in knapweed population variables at release and non-release sites were seed density and plant height. At weevil release sites, the number of seeds produced was significantly lower and the plants were significantly shorter. Other impacts on stands of spotted knapweed are likely to become evident after populations of L. minutus have had more time to increase. XIII International Symposium on Biological Control of Weeds - 2011.