National Wildlife Refuge System
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leafy spurge beatle
A black leafy spurge beetle serves as a natural biological control for the invasive non-native plant leafy spurge.
Credit: USDA

Priming Beetles for Battle

While the main growing season is now winding down across most of the country, a less visible but vital growing season is revving up in northwestern Minnesota. At Tamarac National Wildlife Refuge, a new crop of natural enemies of pest plants is incubating for next season. Seedhead moths and knapweed root weevils, two host-specific insects that overwinter in the refuge's grass litter, help control invasive plants throughout the region. Two particularly nasty targets are spotted knapweed (Centaurea maculosa) and leafy spurge (Euphorbia esula), non-natives that have become widespread across Minnesota, Wisconsin and several Western states.

For the past several years, the refuge has collaborated with federal and state agriculture experts to oversee production, collection and distribution of the plant hosts. On the refuge itself, upland birds that depend on native plants for nesting habitat have been among species to benefit as root-boring weevils have broken knapweed’s chokehold by feeding on its roots and seed heads.

"We’ve seen very significant reductions in spotted knapweed over the past 15 years — in the vicinity of 90 percent in some fields where these beetles have been released," says Tamarac Refuge biologist Lowell Deede. Native plants have returned in those areas.

Spotted knapweed is a biennial or short-lived perennial native to Eurasia that reproduces from seed; the three-foot-tall plant with grey-green hairy foliage and pinking purple flowers infests some 7 million acres in the United States, especially in Montana, Idaho and Washington. By chemically inhibiting the growth of native vegetation, spotted knapweed can reduce forage and wildlife habitat.

Leafy spurge is an aggressive perennial herb from Europe that has invaded and ruined thousands of acres of rangelands in the western United States. The stems and leaves of the flowering plant contain a substance that is toxic to most grazing mammals. Like spotted knapweed, leafy spurge also chemically inhibits the growth of competing plants.

While using insects for weed control can take years longer than chemical pesticides, natural control may be cheaper in the long run and pose fewer health risks, say experts. The aim is to establish and sustain populations of natural enemies that will contain the plant pests indefinitely. Other natural control experiments that have shown some limited success have included grazing by goats.

For more information: 218-847-2641 or

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