 Four exotic thistle species are currently listed as prohibited noxious weeds in Minnesota. Musk (Carduus nutans), plumeless (Carduus acanthoides), Canada (Cirsium arvense), and bull (Cirsium vulgare) thistles are all native to European and Asian landscapes.
Since their introductions into Minnesota, they have established throughout much of the state and have become serious pests in pastures, rangelands, wildlife areas, and along road right-of-ways. Minnesota Department of Agriculture (MDA) field surveys conducted over the past three years have shown that Canada thistle is the most prevalent species because of its wide distribution and high densities across the state. Musk thistle is primarily a problem in the southern region of the state and plumeless thistle affects much of the western half of Minnesota. Both of these species can outcompete valuable forage species in pastures, natural areas, and rangelands, making it hard for livestock and wildlife to obtain high quality forage and habitat resources. Bull thistle has been found sporadically throughout Minnesota and large infestations have been observed along roadsides and within intensively grazed pastures.
Current thistle management centers on chemical and mechanical controls. Many landowners have found both methods to be labor intensive and expensive. Additionally, the need for repeat herbicide applications and regular mowing deters some landowners from developing sound thistle management strategies. Biological control of exotic thistles has also been attempted in Minnesota. In the late 1970s, three biological control agents approved for thistle control in the U.S. were introduced to sites in several southeastern and southwestern Minnesota counties. A seedhead weevil, Rhinocyllus conicus, a rosette weevil, Trichosirocalus horridus, and a defoliating beetle, Cassida rubiginosa, were released with the intentions of controlling musk, plumeless, and Canada thistles. However, few follow-up studies regarding these initial releases were ever performed in Minnesota, and the effects that these natural enemies have on exotic and native thistle populations are not clear.
 In 1999, the MDA began revisiting sites in southeastern and southwestern Minnesota where thistle biological control agents were known to have been released. Our goal was to learn more about the establishment and distribution of these agents. We collected large numbers of the seedhead weevil, R. conicus, and the defoliating beetle, C. rubiginosa, from a majority of the sites. Since then, we have also discovered populations of both agents in the west-central region of Minnesota and the Minneapolis/St. Paul metro area.
Interviews with several southern Minnesota landowners who were involved with releases of the seedhead weevil in the late 1970s and early 1980s have provided us with some interesting insights. Several landowners have attributed the “disappearance” of musk thistle on and along their properties to seedhead weevil releases. Although we have no definitive proof of this, observations by MDA research scientists have shown that both the seedhead weevil and defoliating beetle can attack musk thistle plants in high numbers. Dense stands of musk thistles occurring at seven MDA biological control study sites in southeastern Minnesota diminished significantly over the past three years where large populations of seedhead weevils and defoliating beetles were attacking the plants. Additionally, reports from several southwestern counties have suggested dramatic decreases in musk thistle plants attributed to the success of the seedhead weevil alone.
During a MDA field survey conducted statewide in July 2001, samples of plumeless thistle collected in the southwest and west-central counties were found to have been attacked by the seedhead weevil. To date, samples collected of Canada and bull thistle seedheads in 1999, 2000, and 2001 have not yielded a single seedhead weevil larva. It appears that musk thistle is its preferred host in Minnesota and that it will also attack plumeless thistle plants when available. The effects of this agent on plumeless thistle in Minnesota are yet to be determined. MDA surveys have also shown that the defoliating beetle will attack all four noxious thistle species in Minnesota, but it has been observed more frequently attacking musk thistle plants and less frequently attacking bull thistle. There have been no recoveries of the rosette weevil in the state. However, due to the lack of post release monitoring following the initial releases in the late 1970s, little is known still about this agent and whether it has ever established in the state or not.
Due to recent concerns about non-target effects of the seedhead weevil on native thistle species that have been documented in research performed in other states, the MDA began conducting several preliminary seedhead analyses of four native thistle species including tall (Cirsium altissimum), field (Cirsium discolor), yellow spine (Cirsium ochrocentrum), and Flodman (Cirsium flodmanii) that were found growing within or immediately adjacent to stands of musk thistle harboring large populations of seedhead weevils and variable populations of defoliating beetles. Approximately 500 seedheads were collected from these native species over the past two years (5 seedheads collected per sampled plant) and not a single seedhead weevil has been observed in the samples. However, the MDA is not discrediting other research that has shown non-target attacks nor are we suggesting that non-host attack is not possible. At all the sites where our samples were collected there were still plenty of musk thistle plants to support the seedhead weevil population. Therefore, it may be a matter of preference for musk thistle that has kept the seedhead weevil from attacking these native thistle species.
Larinus planus, another thistle biological control agent that has never been officially documented as being released in Minnesota, was recently discovered in Canada thistle seedheads in Ramsey County. This species is another seedhead weevil and has been released in bordering states as a biological control agent of Canada thistle. Similarly to R. conicus, recent studies have shown that this agents host specificity is also low and that it will attack native thistle species. The distribution and impacts of this weevil in Minnesota are largely unknown at this time; however, we plan to survey for L. planus in upcoming statewide surveys for exotic and native thistle species.
Exotic thistle species are a serious concern for a majority of weed managers in Minnesota. The need for more cost effective thistle management in high density areas, the lack of highly-specific herbicides, and the positive results from purple loosestrife and leafy spurge biological control programs has driven many Minnesota landowners to support the notion of utilizing available thistle natural enemies. However, the MDA has decided to conduct statewide surveys over the next five years to determine the distribution and densities of both exotic and native thistle populations throughout the state. This survey will also evaluate target and non-target effects associated with the two seedhead weevils, rosette weevil, and defoliating beetle, and identify other organisms that are associated with exotic and native thistle plants.
MDA Contact
Noxious & Invasive Weed Unit
Anthony Cortilet
Anthony.Cortilet@state.mn.us
651-201-6538
Monika Chandler
Monika.Chandler@state.mn.us
651-201-6537
Plant Protection Division
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