Beetles battle Minneapolis’ purple loosestrife

For centuries, people have intentionally introduced plants for agricultural, medicinal and ornamental purposes. Non-native plants have also been introduced accidentally, spread by seeds attached to clothing and animals or dumped from the ballast water of ships.

Non-native plants are strong competitors in our native ecosystems. Free from natural enemies (predators, diseases, parasites and competitors), non-native plants can crowd out native species and harm ecologically diverse native plant communities.

In the U.S., invasive plants infest an estimated 100 million-plus acres and are expanding their range annually by 8 to 20 percent. Garlic mustard, Eurasian watermilfoil, buckthorn and purple loosestrife are some of the non-native invasive species that the Minneapolis Park and Recreation Board is actively trying to control in Minneapolis' parklands.

Purple loosestrife is an invasive, perennial Eurasian plant that the Park Board is trying to control with biological control (biocontrol) methods, such as insects. Purple loosestrife invades wetland and lakeshore habitats, displacing native plant communities and destroying habitat and food sources for waterfowl and other wetland wildlife.

The plant came to North America as a contaminant in the ballast of European ships and was also introduced as a medicinal and ornamental plant.

Purple loosestrife became established on the East Coast in the 1830s and now occurs in all the Canadian provinces and throughout the contiguous United States, except Florida and Hawaii. In Minnesota, records show that in the 1920s several garden clubs actually introduced purple loosestrife into wetlands for "beautification." Purple loosestrife is now found in 68 of Minnesota's 87 counties and has infested approximately 40,000 acres.

Producing half a million seeds per square meter allows purple loosestrife to easily re-establish itself.

Mowing, cutting and pulling proved to be costly and largely ineffective control methods, requiring continual maintenance. Herbicide treatments were used with some success but they also kill desirable native plant species.

In the mid-1980s, the U.S. Department of Agriculture and European researchers began identifying insects to control purple loosestrife. Their requirements for a suitable insect were:

  • must eat only purple loosestrife

  • no threat to native plants

  • causes significant damage to the plant or reduces seeding.

    Of the 120 insect species studied, five were selected as the most promising. In the early 1990s, U.S. researchers released the first loosestrife-eating insects. Since 1999, the Minneapolis Park and Recreation Board has collaborated with the Minnesota Department of Natural Resources to introduce leaf-feeding beetles as a local biocontrol for purple loosestrife.

    Using biocontrol methods for controlling invasive plants is not a quick method. It can take several years for an insect population to become established and effectively feed on purple loosestrife. Extremely cold winters, wet spring/summers and seasonally flooded areas do not favor establishment of leaf-feeding beetles.

    This past winter, with its warmer temperatures, has definitely favored the beetles. Of the 20 sites where beetles were released, 15 have substantial beetle populations this summer. Although we may have increased beetle numbers this year, effective loosestrife control is still a ways off. Temperature fluctuations and flooding may reduce future beetle populations. However, biocontrol of purple loosestrife looks like a feasible, if not quick, method to restore wetland habitats.