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Biological Control of Spotted Knapweed (Carson)

Spotted knapweed (Centaurea stoebe var. micranthos) is invasive throughout much of the U.S., and has been recognized as a significant threat to plant and animal communities in Michigan. Efforts to use specialist insects from the plant’s native range as biocontrol agents have been ongoing in the Western U.S. for decades, and several insect species have emerged as the most effective candidates. Three of these species, all weevils, were released at several study sites in Michigan. These weevils are Cyphocleonus achates, which feeds on knapweed roots, and Larinus minutus and Larinus obtusus, which both feed on knapweed seeds and foliage. I am studying the insects’ establishment and impact on knapweed populations at these sites.

This research is being conducted at sites in Jackson, Ionia, Schoolcraft, Crawford, and Roscommon counties. Each release site is paired with a control site where spotted knapweed is present. This will enable us to differentiate changes caused by the biocontrol insects from natural fluctuations in the knapweed population.

Even though spotted knapweed threatens many aspects of our ecosystems, it has been identified as a significant nectar resource for honeybees and many native nectar-feeding insects, including wild bees and the federally endangered Karner blue butterfly. Because of this, we are conducting revegetation experiments at our biocontrol sites to try to establish native flowering plants. This will ensure that there is a continued nectar flow if biological control proves to be successful. In order to test how our plantings might influence the bee fauna at biocontrol sites, I will also be testing the relative attractiveness of spotted knapweed and plants used in our seed mixes to different bee species. This research will be conducted through observations of flower visitors in a common garden experiment.

See our new Spotted Knapweed Fact Sheet and Frequently Asked Questions resources.

Spotted knapweed flower and bee

Seeding plot

Cyphocleonus