Field Notes: Mustard Eaters

Lessons learned in the battle with purple loosestrife and leafy spurge-nonnative plants that have invaded and altered Minnesota ecosystems-are now being applied to garlic mustard (Alliaria petiolata), an exotic species that threatens to crowd out native wildflowers in Minnesota woodlands. Although leafy spurge and purple loosestrife are still found in Minnesota, their spread has been substantially reduced by the introduction of leaf- and root-eating beetles.

Biologists from several U.S. states are working with Swiss biologists from CABI Bioscience—a nonprofit international research institution specializing in biological control of invasive species—to find insects that feed on garlic mustard plants and do not harm other vegetation. Such insects could be approved by the U.S. Department of Agriculture for release in the United States by 2006.

"Several species of insects look promising," said Luke Skinner of the DNR invasive species program. "But we need to be as certain as possible that the insects we introduce won't harm native plant species."

The search for natural enemies of garlic mustard began in 1998. Funding has come from natural resources departments in Minnesota, Illinois, Indiana, and Kentucky; the U.S. Forest Service; and the U.S. Department of Defense, which manages large tracts of land in eastern states.

From 69 species of insects found on garlic mustard during an initial field investigation in Europe, scientists selected four that seem to feed exclusively on the plant. Researchers are now feeding plants native to the United States to the four insect species to evaluate whether the insects might harm native vegetation if they were released here.

Garlic mustard was known to exist in Wisconsin and Iowa before the Minnesota Department of Agriculture placed it on its noxious weed list in 1999. The plant has been found on thousands of acres of public and private land in 25 Minnesota counties, most in the southeastern and central parts of the state.

Early settlers probably brought the plant to North America to use as medicine and a flavoring for food. Genetic variation analysis indicates that garlic mustard was introduced several times from Europe.

Floodwaters, wildlife, hikers, and off-highway vehicles can transport the biennial herb's small, lightweight seeds. The seeds germinate in early spring, and the plant produces a rosette of three to four leaves the first year. In May and June of the second year, the plant produces white blossoms and grows up to 4 feet tall. The weed's dense cover overshadows native wildflowers and other herbaceous plants, causing their decline. The disturbance to the native plant communities can, in turn, affect native insects and wildlife.

Traditional management, such as cutting, burning, or the use of herbicides, can control garlic mustard in small, isolated infestations; but it is impractical in larger areas.

"We've accepted that there is no way to completely eradicate some species of noxious weeds," Skinner said. "But we can manage their populations and minimize the damage to native species through long-term control."

Jason Abraham