Weevils as Biological Control for Knapweed

Weed Weevil Sign
Cartons of weevils




Many years ago, we released weevils as a biological control for knapweed. My dad took a workshop in northern Idaho with Paul Brusven and as part of the workshop received two cartons of weevils. Per instructions, in a grand ceremony, we released the weevils at two different pasture sites where there was dense knapweed. I even made wood-burned signs to designate the weevil-release sites. One site was closest to the driveway and another site was several hundred yards away on an isolated, raised and dense knapweed patch. We released the weevil years ago, probably around 2003 or 2004. Honestly, we did not notice much knapweed control at the first site; however, at the second, denser site, we might swear we still notice a decrease in knapweed. What we should have done is to keep up the weevil release every year! What we also should have done is to keep better notes and records (always an important goal)! There is a knapweed blunthead weevil (Larinus obtusus), and we are fairly sure that we released the blunthead weevil. Nevertheless, there is also the knapweed seedhead weevil (Larinus minutus). Apparently, both kinds of weevils are winter, cold hardy and thrive in dry, hot summers.

Released weevil