reduction in knapweed due to insect introduction

Knapweed Root Weevilknapweed root weevil, half inch long, light grey with dark grey spots

Cyphocleonus achates

We are OPEN for the 2021 season.

This insect can safely be considered "The King" of spotted knapweed biocontrol. A large, conspicuous insect, it lays its eggs on the top of the knapweed's root crown. After the eggs hatch, the larvae burrow into the root. This larval feeding destroys knapweed's vascular root tissue and prevents the plant from transporting water and nutrients. Roots become swollen and deformed as a result of this larval feeding. Death of the plant can occur within two years. Several larvae can fat white root weevil larvae in rootinfect the same root. Recent research by Montana State University has documented up to a 99% reduction in knapweed density as a result of the knapweed root weevil introduction, (see paper in supporting research link).



$160 per release of 100 insects. 3 Cyphocleonus achates larva with dime for size comparisonAvailable late July through mid-September.


As the old saying goes, “Montana weather is predictably unpredictable”. Since the insect’s development is dependent on the weather, that means that many years the insects are out early and unavailable early too. We always encourage folks to order their insects as early as possible in the year so you don’t miss your chance at the bugs.


Knapweed root weevil larvae in root with dime for size comparison. Note the frass in the root cavity indicating digested root tissue.



Knapweed Flower Weevilsflower weevil, quarter inch long, black with greenish dusting

Larinus minutus / obtusus

These two species of insects are nearly indistinguishable. Minutus prefers diffuse knapweed while obtusus prefers spotted knapweed. However both species of weevils feed readily on both species of knapweed. Larval feeding in the seedhead destroys the knapweed seed before it matures. One larva can consume 100% of the seed in a single seedhead. When combined with the widespread knapweed seedhead flies, the seedhead weevil can reduce knapweed seed production by 95%. In addition to reducing seed production, defoliation by Larinus minutus on diffuse knapweed causes a significant decline in that weed.

hole in seedhead


$90 per release of 100 insects. Available June through late July.











Emergence hole created by knapweed flower weevil in a seedhead . Presence of this damage indicates destruction of 100% of the seed that was contained in this head.



All the knapweed insects we handle are approved for release in the United States by the US Department of Agriculture. They are also host specific, meaning they only feed on spotted and diffuse knapweed, nothing else. There is no danger of them feeding on native plants or crops. Once established on a site, the insects reproduce rapidly and spread to other weed infested areas (A Cyphocleonus achates female will lay aprox. 100 eggs). One release of insects can grow to over a million in just a few years. They are adapted to cold climates and do not die off in the winter. There is no need to purchase additional insects in following years. Because the insects's only source of food is knapweed, as the density of the weed decreases the insect's population also decreases, a classic predator-prey relationship. Eventually the weed and the biocontrol come into equilibrium with each other at a low population in the environment. Since the insects spread to many acres and achieve permanent control of knapweed, the cost of control is very low, possibly less than a dollar per acre for a large area. This makes biocontrol a very attractive option compared to the high and recurring cost of chemical control. All of these factors make biocontrol "The Smart Choice" for control of spotted and diffuse knapweed.

OPEN for the 2021 season. As we are a family owned business, social distancing is not a problem for us. We work primarily outside and usually speak with customers over the phone. Masks are worn when mailing packages.

Click here to see what the experts have to say in research articles and government publications.

Click here for practical information on the use of biocontrol insects.

Contact us at (406)251-4261 or (406) 552-5024.

Back to top