Spotted Knapweed

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Spotted Knapweed, Centaurea stoebe L. ssp. micranthos (Gugler) Hayek, syn. = C. biebersteinii, old = C. maculosa

Spotted knapweed infested field before and after visible decrease in knapweed density.

Spotted knapweed flowers can range in color from purple to light pink.

Spotted knapweed flower closeup Spotted knapweed is becoming a common weed in Minnesota of roadsides and pastures with dry, sandy soil. Native to Europe and Asia, spotted knapweed was accidentally introduced to North America in the late 1800s. Spotted knapweed can be spread by wind, water, wildlife, vehicles, contaminated hay, farm machinery, gravel distribution, logging equipment, and road construction. Since introduction, this invasive plant has spread to nearly all states in the United States.

Spotted knapweed is a biennial or short lived perennial that grows approximately 2-3 feet tall with gray-green hairy foliage and has pinkish purple flowers. It reproduces quickly by seed and produces a chemical that is toxic to other plants and allows spotted knapweed to displace desirable vegetation. Once established spotted knapweed can become a monoculture and take over large areas. Resulting infestations can reduce forage and wildlife habitat.

Watch a video about biological control of spotted knapweed (MPG: 46.35 MB)

2003 LCMR Spotted Knapweed Biological Control Report (PDF: 521 KB)

Assessing the Impacts of Biological Control on Spotted Knapweed in Minnesota (PPT: 14.34 MB)

Biological control is one method to reduce spotted knapweed infestations. Spotted knapweed proliferated in North America unchecked because the insects and diseases that control the plant in its native range were not in North America. The practice of spotted knapweed biological control reunites specialized insects with their host plant, spotted knapweed. These insects were tested extensively to ensure that they will not harm any plants other than knapweeds. The goal of biological control is not to eradicate the weed, but to reduce the infestation to an acceptable level of control.

Biological control agents

In Minnesota, the predominant biological control agents used include seedhead flies, seedhead weevils, and root boring weevils. These three bioagents work in conjunction to control spotted knapweed. Seedhead flies are no longer actively collected and released in Minnesota because they are commonly recovered at infestations making new releases unnecessary. Seedhead weevils and root boring weevils are collected from established sites and released at new sites.

Seedhead flies, Urophora affinis and U. quadrifasiciata

Seedhead flies reduce spotted knapweed seed production and so decrease knapweed spread and proliferation. Adult seedhead flies emerge in the spring, mate and lay their eggs in the developing spotted knapweed flowers. The eggs hatch in 3-4 days and the larvae tunnel into the base of the seedhead. Larval feeding in the seedhead induces the formation of a gall. Most of the larvae will pupate and emerge the following spring. The result of larval feeding is that less seed is available to produce new knapweed plants.

Seedhead weevils, Larinus minutus and L. obtusus

Seedhead weevils work similarly to seedhead flies in that the eggs are laid on the seedhead and the larvae consume the developing spotted knapweed seed. Adult seedhead weevils overwinter in the plant litter on the ground at the base of spotted knapweed plants. In the late spring and early summer, adults emerge to feed on the foliage, mate and lay their eggs in the knapweed seedhead. The eggs hatch in 3 days and the larvae consume the material in the seedhead for about 4 weeks. Then the adults pupate and emerge to feed on foliage before burrowing in their overwintering sites.

Root boring weevils, Cyphocleonus achates

Root boring weevils are highly effective biological control agents because they weaken or kill existing knapweed plants. From mid summer through early fall, adult females lay eggs on the soil surface at the base of knapweed plants. After hatching, the larvae burrow into the roots where they feed and develop over the winter, spring, and early summer. The developing larvae in the roots use precious plant resources and damage the roots. As a result, the plant is weakened or killed. Adults will emerge from the damaged roots in the mid to late summer to feed on the foliage, mate, and start the cycle again.

Expectations

Many sites in Minnesota are considered controlled by spotted knapweed biological control. All of these sites utilize multiple bioagents in conjunction. Spotted knapweed biological control has proven a long term endeavor – up to a decade for large sites. The MDA is exploring methods to decrease the amount of time to achieve control.

Starting and maintaining a spotted knapweed biological control program

First assess the site. How much spotted knapweed do you have? Is it one large continuous patch or several smaller, isolated patches? If you have only a few plants, you should consider hand pulling (use gloves) or spraying the plants with a herbicide and follow up over the next 3-4 years.
Next, call your County Agricultural Inspector (CAI) or the Minnesota Department of Agriculture (MDA) and explain that you have spotted knapweed on your land and are interested in using biocontrol. The CAIs have a well established network that coordinates with multiple agencies, including the MDA, to ensure that bioagents are distributed where needed. There is no charge or cost to the landowner for the bioagents.
After bioagents are obtained, releasing them is simple. Take a photo of the site so you have an image to compare with several years following the bioagent release. Make a note of the release location or mark the spot with a stake, then open the container of insects and gently pour them onto the spotted knapweed plants. The insects will disperse on their own. Do not worry about touching the insects. They will not harm you.
It is unlikely that you will see immediate changes. It is important to monitor the site over the next few years to determine progress. The second summer after release, go back to the release site and look for bioagents. They can be difficult to find without a sweep net (show photo) so don’t be discouraged if you don’t see any. In following years, you may begin to see a decrease in the amount of spotted knapweed. Again check for bioagents. If at this point you don’t find any bioagents and see no decrease in the amount of spotted knapweed, call your CAI or the MDA for advice. Augmenting the bioagent population may be a good option in this case.
If you see spotted knapweed beginning to decrease, call your CAI or the MDA to report the progress. It is possible that your site has a collectible population of biocontrol agents. Bioagents can be collected and moved to a new location. Don’t worry about depleting the bioagent population. Many will remain after the collection and will continue to eat away at the spotted knapweed.

Minnesota Cooperative Weed Biological Control Cooperators

Biological control programs in Minnesota are cooperative. Multiple agencies, associations, institutions, and private landowners work together to accomplish goals. Resources such as the biological control agents are shared. Cooperators include:

Minnesota Department of Agriculture
Minnesota Department of Natural Resources
Minnesota Association of County Agriculture Inspectors
Minnesota Department of Transportation
University of Minnesota
United States Fish and Wildlife Service
United States Department of Agriculture-Animal and Plant Health Inspection Service

We would like to thank all of the cooperators who have participated with this program in the past and look forward to their involvement in the future.