Invasive Species

Purple Loosestrife

Purple loosestrife, native to Europe and Asia, has spread throughout much of the United States and Canada. Because the flower is so attractive, purple loosestrife has been sold as an ornamental plant. Recently, many states including Michigan have banned its sale.

Purple loosestrife has the following characteristics:

• Plants grow an impressive four to seven feet tall.
• Blooms are long spikes of beautiful purple flowers.
• It grows prolifically in wetlands and other moist areas.
• Each mature plant produces 30 or more flowering stems and can produce over 2.5 million seeds per year.

A fierce competitor, purple loosestrife eventually overtakes native vegetation, forming nearly impenetrable stands of this single species. As the native plants are reduced, so too are the wildlife species that depend on them. Ecologists are concerned about the loss of habitat for endangered plants and animals, and declines in ducks, muskrats, mink, and some amphibians. Dense strands of purple loosestrife also impair recreational use of wetlands and rivers and impede water flow in drainage ditches.

Control by conventional means (water level management, burning, herbicides, direct digging) has proven to be extremely difficult, impractical, and ineffective on a large scale. Sometimes referred to as "the purple plague," purple loosestrife is a serious problem throughout the Great Lakes bioregion.

As often occurs with introduced exotic species, purple loosestrife arrived in North America without its natural enemies. Three species of plant-feeding beetles (Galerucella calmariensis, Galerucella pusilla, and Hylobius transversovittatus) show particular promise as biological controls for purple loosestrife. These insects have undergone extensive testing to determine their safety, host specificity and effectiveness, receiving USDA approval in 1992.

Galerucella beetles have been established in several Michigan locations by the Michigan Department of Natural Resources and the Michigan State University's Purple Loosestrife Project scientist. Galerucella beetles feed on buds, leaves, and stem tissue; causing defoliation and prevention of flowering and seed production, eventually leading to plant death.

Those who love to see purple loosestrife's beautiful display need not fear. Experts agree that these natural enemies cannot eliminate purple loosestrife, only reduce its density and harmful effects. It is anticipated that the combination of these natural enemies may be capable of reducing the density of purple loosestrife by up to 90% over most of its current range. Remaining plants will serve to maintain a resident population of beetles for the future. Reducing purple loosestrife density will allow the reestablishment of native vegetation.

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