Something's Bugging Purple Loosestrife
What happens when a beautiful invader meets its natural enemy.
By Luke Skinner
It was early June of 1995 and I was headed to Rice County to check on some beetles released a year earlier to control infestations of purple loosestrife, an exotic plant that is choking Minnesota's wetlands. Nervous anticipation was setting in. We hadn't had much luck in establishing these loosestrife-eating insects at several other test sites. As I approached the spot along the shore of Circle Lake where the insects had been released, I could see that the loosestrife plants looked liked they had been hit with a shotgun blast. The beetles had chewed tiny holes all over the plants. Looking more closely, I saw little brown beetles crawling all over the plants.
Finally a breakthrough! The leaf-eating beetles had survived the winter and were reproducing rapidly, as only insects can. Everywhere I looked, they were feeding, mating, and laying eggs on the loosestrife. I felt an overwhelming sense of satisfaction: We had turned the corner in our efforts to establish this biological control for purple loosestrife.
Purple loosestrife (Lythrum salicaria) a showy perennial plant of Eurasia, is invading wetland and lakeshore habitats across North America. It can form dense stands that crowd out native aquatic plants, from common cattails and sedges to rare species found in calcareous fens. Without native vegetation, the wetlands cannot provide as much food, shelter, or nesting sites for waterfowl, songbirds, muskrats, and other wildlife.
Purple loosestrife was introduced into North America through several different routes. Loosestrife seed was inadvertently transported in the ballast of ships carrying goods and people between the Old World and New World in the 1800s. European immigrants, who valued purple loosestrife as an ornamental plant, also brought and planted it intentionally.
Purple loosestrife became established on the East Coast in the early 1800s and spread westward. Purple loosestrife now occurs in every Canadian province and every state except Florida and Hawaii. It was discovered in Minnesota, in a Ramsey County wetland, in 1924. Since then, it has spread to 68 of Minnesota's 87 counties, infesting nearly 40,000 acres of wetlands, lakeshores, streambanks, and roadsides.
Loosestrife's appeal as a garden perennial undoubtedly hastened its spread in Minnesota. It is hardy, tolerant of a variety of moisture and soil conditions, and, in North America, virtually free of insect pests and disease. In the 1920s several garden clubs in Minnesota planted purple loosestrife to beautify swamps. In many cases, loosestrife simply spread from gardens into nearby wet areas, then advanced to lakes, streams, and wetlands.
Millions of Seeds
Wildlife managers soon learned that loosestrife was entrenched and no friend to wildlife. Attempting to eradicate the exotic pest, they tried cutting, burning, manipulating water levels, and applying herbicide. Such conventional control methods were largely unsuccessful. Only small, isolated stands could be effectively controlled by uprooting plants or treating them with herbicide.
Though conventional treatment methods killed individual purple loosestrife plants, established stands produced huge numbers of seeds that fell into wetland soil. This reserve of seeds, often half a million per square meter, allowed loosestrife to rapidly re-establish. Repeated attempts to control infestations proved costly and ineffective in the long run.
Managers needed a method that was long-term, environmentally friendly, and cost-effective. For a solution, they turned to nature -- in this case, the insects that help keep purple loosestrife in check in its native setting in Eurasia.
In the mid-1980s the U.S. Department of Agriculture hired European researchers to identify insects as potential controls of loosestrife. To be a suitable agent, an insect would need to: (1) be host specific -- to survive and feed exclusively on purple loosestrife and not pose a threat to native American plant species; and (2) cause significant damage -- either kill purple loosestrife, reduce shoot growth, suppress flowering, or reduce seed output.
Of the 120 species of insects associated with purple loosestrife in Europe, researchers identified five as the most promising for biological control. They were a root-boring weevil (Hylobius transversovittatus), two leaf-eating beetles (Galerucella calmariensis and G. pusilla), and two flower-feeding weevils (Nanophyes marmoratus and N. brevis). After eight years of testing revealed these five species eat nothing but loosestrife, the USDA cleared them for release in the United States. There is always an element of risk in introducing a new species to the environment. In this case, the risk appeared low when compared with the negative impacts of purple loosestrife. However, N. brevis was not brought to the United States because it was found to carry a parasite.
Why did researchers choose so many insects to control loosestrife? To increase the likelihood of success, they chose insects that attack different parts of the plant, thus reducing its vigor and reproductive potential. Root-boring weevils attack the plant's roots, leaf-eating beetles defoliate and curtail flower production, and flower-feeding weevils destroy flowers and reduce seed production. The root-boring weevil and the leaf-eating beetles will probably do the greatest damage and have the greatest effect in controlling loosestrife.
In 1992, as Department of Natural Resources purple loosestrife program coordinator, I worked with researchers from the USDA, the Minnesota Department of Agriculture, and the University of Minnesota to release loosestrife-eating insects near Pig's Eye Lake in St. Paul. It was the first release in Minnesota and one of the first in North America. We had high hopes for using this site as a nursery to raise insects for controlling loosestrife throughout Minnesota. Unfortunately, after one year, prospects did not look good.
In summer of 1993, I stood along a road near Pig's Eye Lake, overlooking a wetland flooded by the rising Mississippi River. As our bugs were being flooded out, researchers clad in waders slogged through waist-deep water, trying to collect and save as many as possible.
Soon after that fateful day, the Minnesota purple loosestrife biological control program bought more leaf-eating beetles from Europe. With this new stock of beetles, resource managers at the DNR, University of Minnesota, and MDA started to raise their own loosestrife-eating insects. In 1995 they produced and released 30,000 leaf-eating beetles into loosestrife infestations in Minnesota. In 1996 they released more than 160,000 beetles, and by 1997 nearly 1 million.
The leaf-eating beetles are the easiest of the loosestrife insects to raise. Potted loosestrife plants placed in kiddie swimming pools provide the host. Sleeves of mosquito netting cover each plant. Ten leaf-eating beetles placed on each plant in the spring will produce up to 2,000 offspring in just six weeks. In 1997 the DNR provided "starter kits" to DNR field managers, county agricultural inspectors, and MDA staff. That year, 27 counties reared and released 600,000 leaf-eating beetles into 100 loosestrife-infested sites statewide.
Beetles at Work
The insects are starting to do their job in Minnesota. At several sites the beetle populations are booming, sometimes reducing loosestrife infestation by 50 percent or more. On Pelican Lake near Orr, for example, leaf-eating beetles released in 1994 all but wiped out a patch of 100 loosestrife plants by 1997. Now the insects have moved nearly a mile down the shore to a much larger infestation. We will be watching to see how big a bite the control agents will be able to take there. Other infestations of insects are stunting loosestrife, reducing seed production, and even killing plants from Mille Lacs to Winona.
The most spectacular control using the leaf-eating beetles so far has occurred in Guelph, Ontario. There researchers released 2,000 beetles in a two-acre loosestrife infestation in 1993. By 1996 the flourishing beetle population had reduced the loosestrife biomass by more than 90 percent.
What are the insects going to eat when all the loosestrife is gone? The answer is nothing. The insects will move off site to look for more loosestrife. If they don't find any, they will die. The insects never eat all the loosestrife; consequently, some insects remain. If the loosestrife population rebounds, the insect population will also multiply, keeping the plant in check.
While the leaf-eaters are doing their job, researchers have yet to determine the effectiveness of root-boring weevils, which are much slower growing than the leaf-eating beetles. The weevil can take up to two years to go from egg to adult. Because raising the root-boring weevils is slow and difficult, they have been released in only about a dozen Minnesota locations. It will take many years to release the root-boring weevil around the state.
The Legislative Commission on Minnesota Resources has paid for much of the research on raising loosestrife-eating insects, including a project under way to grow root-boring weevils in only three to four months. This project is deemed particularly important because root-boring weevils can wreak such havoc on loosestrife, stunting the plants and often killing them.
The flower-feeding weevil has been released on only five sites in Minnesota. Researchers will devote more effort to propagating this insect once they have developed a more efficient method of rearing root-boring weevils.
Biological control of purple loosestrife will take time. In most cases, insects will need five to seven years to significantly reduce loosestrife in small wetlands. Eventually, biological controls such as these insects could permanently suppress the profusion of loosestrife that now infests Minnesota wetlands. Bernd Blossey, a German researcher who oversees national loosestrife biological control at Cornell University, has predicted up to 90 percent reduction of purple loosestrife abundance across most of North America. If this prediction is only half correct, most researchers would consider the control program a success.
Maybe 10 years from now I'll look at the Mississippi backwaters near Pig's Eye Lake and be happy to see a wetland with only a little loosestrife and full of native plants.
Luke Skinner is the purple loosestrife program coordinator for the DNR.