Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Photo of spotted knapweed © Wade Oehmichen

Spotted knapweed produces thistle-like flower heads late June through August.
© Wade Oehmichen

June 2011

Weevil warriors

Spotted knapweed control coming on strong.

Wade T. Oehmichen

Wisconsin is full of wonderful plants. This story isn't about one of them.

Spotted knapweed (Centaurea stoebe L. ssp. micranthos Gugler Hayek) is a ruthless native plant murderer in the form of a two- to three-foot tall perennial with pinkish purple flowers. It reproduces quickly by seed, produces a chemical that is toxic to other plants and, in Wisconsin, tends to be found in the well drained soils that comprise about 65 percent of the state. Once established, spotted knapweed can take over large areas and reduce forage and wildlife habitat.

Spotted knapweed control coming on strong. Spotted knapweed originated overseas in Asia Minor and was introduced to North America in 1883 as a seed contaminant from Turkmenistan. It then slowly spread into Wisconsin sometime before 1915. For the next 76 years, knapweed enjoyed the easy life: lots of sunshine, plenty of open ground and a life free from predators.

A single spotted knapweed plant can produce 400 to 25,000 seeds that remain viable for eight or more years. Dispersal assistance could come in the form of a tiny ant carrying it away or a giant mower tossing the seed from its deck.

No matter how the seeds are moved, the plant's ability to establish in even the most pristine area without a prior disturbance is something to be feared. Spotted knapweed produces allelopathic chemicals (chemicals that inhibit growth of other plant species). Researchers have isolated the chemical, catechin, which is produced from knapweed roots and can be as effective as the herbicide 2, 4-D on some plants.

The spotted knapweed biological control program is one of the oldest in North America having started in the 1960s. In classical biological control, a natural plant pest is brought from a harmful plant's home range and released into the plant's introduced range. Extensive biological control candidate testing ensures that it is safe to release.

In 1991 the U.S. Department of Agriculture and the Wisconsin Department of Natural Resources released two biological control species, seedhead flies (Urophora affinis and U. quadrifasciata), in the state. While in their larval stage (called galls), the flies thrive in developing knapweed seedheads (florets). The galls drain nutrients from other parts of the plant, which results in less knapweed growth and fewer seeds. During their adult stage the flies disperse, mate and lay eggs into new seedheads.

Since 1991, five other spotted knapweed biological control agents have been used by the Wisconsin Department of Natural Resources.

During their adult stage, seedhead weevils (Larinus minutus and L. obtusus), defoliate the knapweed plant and lay eggs in the seedhead. The weevil larvae then feed on the knapweed's pappus hairs, seeds and receptacle. For the next four weeks the larvae construct pupal chambers inside the seedhead. The weevils reduce plant growth and destroy seeds in the seedhead. Collecting the weevils with sweep nets isn't difficult, but sorting them from all other invertebrates (specifically ants) is challenging. In 2010 the Department of Natural Resources tested a new design for sorting the weevils – a drastic improvement from old methods.

Root-mining larvae, Agapeta zoegana and Cyphocleonus achates, also are effective biological controls that bore into the root of the knapweed plant. As adults, Agapeta zoegana live up to 13 days, but mating takes place in the first 24 hours. The eggs are laid on knapweed plant leaves and once they hatch, the larvae immediately mine into the outer root. The larvae feed on the root overwinter and emerge mid-summer the following year. They devastate the knapweed plant, but they have been difficult to raise and distribute as adults. The Department of Natural Resources is developing methods to harvest this agent in the larval stage. Successful collections in 2009 and 2010 raise hopes for the 2011 collection season.

The Cyphocleonus achates beetle is a brown gray mottled color and about ½ inch in size. It also targets spotted knapweed. The adults live eight to 15 weeks and during warm sunny days climb to the top of the knapweed plants to search for a mate. After mating, the female lays her eggs at the base of the root crown. When the eggs hatch, the larvae bore into the root core. The larvae feed on the root over winter and emerge mid-summer. Their effects on the knapweed plant are similar to that of the Agapeta zoegana, but collecting adult beetles is still difficult in the field.

In 2009 and 2010, the Department of Natural Resources built eight Cyphocleonus achates propagation corral pens and plans to bring that total to 25 pens by 2012. The concept is to enclose an area with aluminum flashing, so the beetles can't escape.

Biological control is an effective, long-term answer to suppress spotted knapweed in Wisconsin. But the process is slow and spotted knapweed is a formidable enemy. This shouldn't discourage you but help you put into perspective the severity of this plant's dominance and the difficulty in controlling it. To get involved in stopping spotted knapweed, join your local weed district, adopt a highway or local conservation group and help map infested areas. For more information email Wade Oehmichen.

Wade T. Oehmichen is a DNR wildlife biologist located in Horicon.