Weevils released in connection with spotted knapweed workshop

Judith Basin County Extension Agent

A root weevil called Cyphocleonus achates was dispersed in the Geyser area last week to help control knapweed.

Photo by Katie Hatlelid


A seed head weevil was released several years ago. This weevil makes its home in the seed head of a mature knapweed plant and exits in the summer through a hole in the top of the head. 


Thirty people attended two workshops held Wednesday, Aug. 24, to learn more about biological control of a common weed pest here in Central Montana. Spotted knapweed was the focus of the meeting, along with information about houndstongue, Canada thistle, and cheatgrass biocontrol agents. 

Guest speaker Melissa Maggio-Kassner, the Montana BioControl Coordinator, discussed some of the common misconceptions and myths surrounding biocontrol. After the classroom presentations in Raynesford and Geyser, we headed to the field and made two releases. The Cyphocleonus achates, a root weevil, was the insect dispersed. (Pictured in the cup.) 

Some in the area may remember a previous release of a seed head weevil of the Larinus spp. years ago. In searching the knapweed stands, the Larinus larvae were discovered in many plants. The Larinus makes its home in the seed head of the mature knapweed plant, and then exits in the summer of the following year through a hole in the seed head. 

The releases this week will be the last of the season in this area. If any other landowners are interested in releasing some root weevils next year, please contact the Judith Basin County Extension Office for more information.  




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