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Knapweed

The Montana State University Extension Office is once again coordinating an order for knapweed biological controls. 

Biological controls are one tool in the weed control toolbox that is especially useful for long-term control of a weed infestation. Biological weed control uses living organisms to control a weed infestation and attempts to recreate a balance of plant species with predators. They are best used in large infestations of a single weed species and in areas that may be hard to get to with an herbicide application.

Biological controls, or the deliberate use of natural enemies, have been used against spotted knapweed in Montana since the mid-1970s. They have been tested to prove they won’t attack non-target plants, thus they feed only on the weed being targeted. They can generally suppress a host population and are most effective on dense weed infestations over large areas.

The most effective agent for spotted knapweed is a root feeding weevil, Cyphocleonus achates. This weevil overwinters as a larva in the root of the knapweed. Larvae pupate within the roots and feed on the roots of the knapweed, which causes considerable damage to the knapweed and a premature death. 

The knapweed root weevil is available for $125/carton, with 105 insects/carton. The order deadline is July 25.

The knapweed seedhead weevil feeds on the seedhead of the knapweed flower, reducing its ability to spread seed. They are available with 105 insects/carton at $50/carton, and the order deadline is July 15.

The root feeding weevil and seedhead weevil together make the most effective combination, as both the knapweed roots and knapweed seedhead are being attacked.

Order forms are available on our website at msuextension.org/glacier, and you can call our office with questions. Orders will be delivered in late July/early August.

Editor’s note: Kari Lewis is MSU Extension Office agent for Glacier County.

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