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Weevils to the Rescue!

Updated: Apr 29, 2022

On August 5, 2021, root weevils were released by Friends of the Blufflands on the Mathy property in the blufflands near La Crosse. This root weevil, Cyphocleonus achates, was purchased from a business in Bozeman, Montana and released to help control a noxious weed called spotted knapweed, Centaurea biebersteinii.

Spotted knapweed was first recorded in North America in 1893 and in Minnesota in 1918. It probably hitchhiked here in a batch of alfalfa seed brought from Europe. Lacking its natural predators, like many other imported nonnative plants, the knapweed quickly spread crowding out many native species along the way. The plant produces an attractive flower that provides nectar for many butterflies and bees, but otherwise provides little habitat value compared to the native plants it displaces. Its spread is facilitated by its hardiness in dry habitats with its long tap root, prolific seed production, and by exuding a toxic or allelopathic chemical into the soil that inhibits the growth of many native plants. Here is a photo of knapweed:

Control of spotted knapweed infestations is difficult but has been greatly enhanced in recent years by biologic agents, including weevils. The evaluation of insect predators prior to release for biologic control of a nonnative plant is now very comprehensive to avoid unwanted damage to other native species such as occurred with the release of tachinid flies for control of the gypsy moth that instead favored and attacked giant silk moths. This evaluation can take up to 10 years. There are two types of weevils that have been identified as predators of spotted knapweed- the root weevil and the seedhead or flower weevil, Larinus minutus. The experience with the knapweed weevils has been very positive since the first release over 30 years ago with good control of knapweed without any significant collateral damage.

Initially the plan was to release both a root weevil and a flower weevil as recommended by the Wisconsin DNR. But, close inspection of the planned release site on Mathy revealed that the flower weevils were already present in significant numbers. This was great news and meant that the root weevil was all that needed to be purchased helping decrease the cost. Both the root and flower weevils are needed for effective control. Here's a photo of the flower weevil found on Mathy:

The root weevil was first released in British Columbia in 1987 and in Montana in 1988. It is hard to see as an adult as it is very well camouflaged. Adults are weak fliers or flightless. They lay their eggs on the root crown of the knapweed and the larvae burrow into the root causing damage to its structure. The flower weevil, first released in 1991 in Colorado, lays its eggs on the flowers and the larvae feed on the seeds and florets. In contrast to the root weevil, it is a strong flier and can spread on its own into new territory as occurred on Mathy. Acting as a team, these weevils, as well as other insects that use knapweed as a food source, will not completely get rid of the knapweed, but will hopefully keep it at a low enough level that it will not adversely affect the rest of the ecosystem. This will take several years and generally does not become noticeable until three years after release. When the population reaches a certain point, it is possible to collect them from the seeded site with a sweep net and move them to other sites as needed. Friends of the Blufflands will continue to monitor this site and move the insects from the site to new ones if possible.

So, as you hike the trails of Mathy and other bluff properties, hopefully you will see less and less spotted knapweed and more of the native plants that are more valuable for the species that depend upon them.

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