Illinois Natural History Survey - University of Illinois

Good Beetle, Bad Plant

Purple loosestrife (Lythrum salicaria) is a nonwoody perennial plant that came to North America from north-central Europe in the early 1800s, most likely in soil used for ballast of ships. Because it arrived without its natural mortality factors (herbivorous insects, plant diseases), loosestrife populations grew unchecked and it became an invasive weed that today occurs in wet areas throughout North America.

Several traits make the plant a formidable invader: hardiness, tolerance of a wide range of conditions, ability to exploit disturbed habitats, large size (over 8 ft. tall) and virtual freedom from insect pests and disease. Also, plants can produce over 2 million seeds the size of ground pepper, which can be transported by wind, water, or animals.

Vast stands of loosestrife defy most control efforts, whether hand pulling, herbicides, burning, or flooding. That's why biological control has been welcomed--it's the only alternative in this case.

Five beetle species were considered most promising and host specific, and have been introduced into North America for biological control. Two species, Galerucella calmariensis and G. pusilla, have been released throughout the U.S. and Canada, and are now showing signs of success.

The Illinois loosestrife story illustrates how a biological control program can evolve to large-scale participation and success. In Illinois, the loosestrife biological control program began in 1994. Several county and state agencies purchased 7,000 adult Galerucella adults, which were released into field cages at seven sites in five counties in northern Illinois. INHS joined the group in 1995, and our part of the partnership has been to rear and distribute the biological control agents, and provide expertise about biological control. At INHS, large-scale production and distribution of Galerucella beetles has been ongoing since 1995. Each year, hundreds of purple loosestrife roots are dug and returned to the greenhouse, roots are potted, and plants are caged in tomato cages and covered with mesh bags. About 20-30 adult Galerucella are added and left to grow for a month until new adults emerge. Beetles are collected daily and shipped (or delivered) to collaborators throughout Illinois.

Since 1995, over 1 million Galerucella adults have been released at over 80 sites in Illinois. Collaborators also are rearing their own beetles on site, educating the public about loosestrife and other exotic plants--and releasing another 200,000-300,000 beetles.

Two Illinois sites are showing dramatically different impacts. At Hosah Prairie, adjacent to Illinois Beach State Park, a few hundred Galerucella calmariensis were released into field cages in 1994. Beginning in 1996, we sampled plants annually, classifying plants as either 1) damaged, and the plant failed to flower; 2) partially damaged, in which reduced flowering occurred; and 3) undamaged, in which full flowering occurred. In 1996, one-third of the 300 plants fell into each category, a sign of initial impact, as no damage occurred before the 1994 releases. By 1998, 42% were completely damaged, 47% partially damaged, and 11% had no damage. Beetle feeding has reduced loosestrife to a "background" plant, a splash of purple.

In Savanna, along the Mississippi River, came a different tale. In 1994, 1,000 Galerucella adults were released into field cages at three sites. In 1997, plants were heavily damaged at all three 


Fredy Cardona of INHS measures purple loose-strife plants.

release sites, even 150 meters from a release point. Each site looked like a bull's-eye --a circle of brown, dead plants, 15-30 meters in diameter, surrounded by green, unflowering plants, which were surrounded by purple flowering plants. Beetles covered many plants, stripping every last vestige of green. And in the midst of the feeding frenzy, stood untouched, or just nibbled, plants of other species, showing us the beetles remained host specific. In 1998, the insects and their damage have moved well beyond the release points. Even more exciting, native plants are making a comeback.

It is too early to tell whether loosestrife is still "on the loose" or "on the run." Although we have made releases at 80 Illinois sites, many more need to be targeted. Many private sites where wetland mitigation projects are ongoing will be new targets. Initial impact seen at the two sites is not yet occurring elsewhere in the state. There will never be zero loosestrife because biological control will not eradicate exotic species. But, if Hosah Prairie is indicative, loosestrife may become a background splash of purple in the wetlands. Biological control, even if it puts loosestrife "on the run," is only a part of the solution. Other exotic species are waiting in the wings, ready to fill the void left by fewer loosestrife plants.

For more information, see the Survey's Loosestrife Web Site:

Robert N. Wiedenmann, Center for Economic Entomology; David J. Voegtlin, Center for Biodiversity

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