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Document Type

Peer-Review Article

Abstract

Centaurea stoebe L. ssp. micranthos (Gugler) Hayek (spotted knapweed) is an invasive plant that has been the target of classical biological control in North America for more than four decades. Work in the western U.S. and Canada has shown the seedhead-feeding weevils Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae) and the root-boring weevil Cyphocleonus achates (Fahraeus) (Coleoptera: Curculionidae) to be the most effective C. stoebe control agents. These three weevils have recently been introduced into the eastern U.S., including sites in Michigan in 2007 and 2009. In 2010, we made additional releases at six sites in Michigan, monitoring them for three years 2011-13. Here we report on the establishment, impact, and cur- rent range of L. minutus, L. obtusus, and C. achates in Michigan. We also report on the initial results of native plant overseeding treatments that were applied to biological control release sites with the aim of supplementing the nectar source C. stoebe provides. We found that L. minutus has established at all of its Michigan release sites and is widespread in the southwestern part of the state, while L. obtusus has established at the single site where it was released in 2007 and is spreading to adjoining counties. We also found C. achates to be present at four sites and established at one additional site in Michigan, but in all cases abundances are low and dispersal has been minimal (< 10 m). In the three years following the 2010 releases, we found no measurable impacts of these biological control agents on C. stoebe growth, demographics, or plant community metrics. We also found little evidence of native flowering plant establishment at seeded sites. These baseline data will be useful in monitoring the spread and potential impacts of biological control agents on C. stoebe in Michigan.

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