Carol Mortensen, Leech Lake Botanist/Invasive Species Project Coordinator, 218-335-7428

A simple definition of biological control is “the use of one living organism to control another living thing.” Biological control, or “bio-control” for short, is frequently an important component of many land management agencies’ invasive species management plans. Leech Lake Division of Resource Management (DRM) has been using bio-control to manage invasive plants such as purple loosestrife and leafy spurge since 1996. In fact, the DRM received funding through the Bureau of Indian Affairs Noxious Weed Program to build a greenhouse that is used as a regional facility to rear a beetle that feeds on purple loosestrife.

About now you may be thinking, “So what? What do I care about purple loosestrife?” Well, purple loosestrife is just one of about 100 non-native plants that are now found on the Leech Lake Reservation, but it is one of the worst. While it is lovely, and not a problem in its native range of Europe, northern Africa, and Asia, none of its natural enemies are in North America. When it was introduced here, probably early in the nineteenth century in soil used as ship ballast, it found nothing to stop its spread. Spread it did, from New England all across the Upper Midwest, infesting wetlands and shoreland, crowding out native plants and degrading wildlife habitat. On the Leech Lake Reservation it has a foothold on Leech Lake, Cass Lake, Big Lake, Bowstring Lake, and many other smaller lakes, as well as in ditches and wetlands. Unchecked, purple loosestrife can spread and wipe out native plants, reduce open water in wetlands, and possibly reduce wild rice stands.

Releasing trapped beetles into a loosestrife infested area.

Now, back to bio-control. For years, herbicides were the weapon of choice in the battle to control purple loosestrife, but loosestrife was winning. So scientists had an idea. They looked overseas where loosestrife was a natural part of the landscape, and discovered a number of insects fed on the plants and kept them from getting out of control. They figured perhaps some of these insects would work in North America as well. So they started testing them to see, for example, if they used other plants besides purple loosestrife for food or at any stage in their development from egg to adult. This testing went on for years, because the last thing anyone wanted was to bring an insect into the U.S. to control purple loosestrife and discover that— oops!— it also had a taste for soybeans or tomatoes.

Eventually, several species of insects were found that damaged loosestrife but didn’t present a problem to other plant species. One of these, Galerucella calmariensis, the black-lined loosestrife beetle, is the one we raise at the DRM. This beetle is about a quarter-inch long, brown, and has darker lines along the outside edges of its body. It doesn’t bite humans. All it wants to do is eat purple loosestrife, mate, and lay eggs. The adult beetles feed on the leaves of loosestrife plants. They lay clusters of tiny whitish eggs on the stems and leaves, which hatch in about two weeks into tiny yellow caterpillars (larvae) that begin to feed on the leaves and flower buds. The beetles reproduce at a rate of about 100 to one, so when the eggs hatch, the larvae can do a lot of damage to the loosestrife plants. By the time they reach full size (about 5/16 inch) they can reduce the plants to shriveled brown skeletons. The plants can’t make and store as much food in their roots as usual, and over the course of a few years, many die. Larval feeding on flower buds reduces the number of seeds produced, so fewer new plants sprout.

Biological control is not the answer to every weed problem. Only a few species of insects have “made the cut” and been approved by the U.S. Department of Agriculture-Animal and Plant Health Inspection Service for use. No bio-control agents have been found for most invasive plants. However for some, such as purple loosestrife, bio-control works very well and is a state-of-the-art component in an integrated weed management plan, which combines biological control, chemical control, and manual techniques. The insects we use have been rigorously tested, are widely-used across North America, and are no more likely to change their eating habits than are milkweed-eating monarch butterfly larvae. Biological control is also better for the environment, because it reduces the use of herbicides.

Beetle to help control the Purple Loosestrife.

If you would like more information about biological control or invasive species, please feel free to contact me at the DRM. During summer, stop by and visit the greenhouse insect-rearing facility. We’d be happy to give you a tour!

Posted in Plants |