PARENT SESSION
Oral Session 117: Invasive Species IX: Knapweed.
Presiding: J Jacobs
Thursday, August 5, 1:30 PM to 5:00 PM, Meeting Room C 120.

A silver bullet in the biological control of diffuse knapweed.

Myers, Judith^*,1, ¹ Dept. Zoology and Faculty of Agricultural Sciences, Vancouver, B.C., Canada

ABSTRACT- The landscape of western North America has been substantially changed by introduced plants such as diffuse knapweed, Centaurea diffusa. In an attempt to reduce the dominance of knapweed in many rangelands, since 1972, twelve exotic species of insects have been introduced as potential biological control agents. Although some of these insect species rapidly became abundant and widespread, knapweed continued to dominate invaded sites. The initial agents primarily reduced seed production of knapweed. However, models predicted that biological control would only be successful if an agent killed knapweed plants rather than just reducing seed production. This prediction has been supported by the most recently introduced control agent, a weevil, Larinus minutus. This species kills knapweed plants by feeding as adults on the epidermis of stems and branches. A dramatic decline of diffuse knapweed has occurred in southern British Columbia since Larinus has become abundant and widespread. Recent years of drought may have accentuated the success of this biological control agent. Long-term data on knapweed will be presented that document the history of this biological control program. This example demonstrates the need for greater parsimony in the selection of biological control agents to reduce the number of exotic species introduced to gain successful control.

Key words: diffuse knapweed, invasive plants, biological control, Larinus minutus