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The knapweed             root weevil, Cyphocleonus achates, feeds on knapweed roots.

The knapweed root weevil, Cyphocleonus achates, feeds on knapweed roots.

 

The lesser knapweed flower weevil, Larinus minutus, feeds on knapweed seed heads.

The lesser knapweed flower weevil, Larinus minutus, feeds on knapweed seed heads.

The two insects that have been released to attack spotted knapweed are the root-feeding weevil Cyphlocleonus achates and the seedhead-feeding weevil Larinus minutus. According to Michigan State University entomologist Doug Landis, both have proven successful in achieving suppression of spotted knapweed in British Columbia, (Canada), Montana, Colorado, and, more recently, Minnesota and ­Wisconsin. These insects have been extensively tested for safety and have been approved for release in Michigan by theUSDA Animal and Plant Health Inspection Service.

 Cyphocleonus achates is a large (3/4-inch), gray and black mottled weevil that attacks the roots of spotted knapweed. Collected in Austria and Romania, the first United States release of the weevil was made in Montana in 1988. Dispersal is slow as adults
don’t fly.

Larinus minutus is a small (3/16-inch) weevil that attacks flower heads of diffuse and spotted knapweed. Collected in Greece and Romania, the weevil was introduced into the United States in Montana, Washington, and Wyoming in 1991. Weevils overwinter as adults in soil and become active in early June. Eggs are deposited into freshly opened flower heads during late July through early September. Larvae hatch in about three days and immediately feed downwards into the flower head, where they eat seeds and pappus hairs. Population increase on spotted knapweed has been slow.