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Purple loosestrife
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     Monitoring Protocol | Ecology | Distribution & Spread | Problem | Previous Control | Biological Control

Purple Loosestrife


  • Lythrum salicaria L. (Lythraceae)


  • L. salicaria, a plant of European origin, has spread and degraded temperate North American wetlands since the early nineteenth century. The plant was introduced both as a contaminant of European ship ballast and as medicinal herb for treatment of diarrhea, dysentery, bleeding, wounds, ulcers and sores.


  • L. salicaria is a herbaceous, wetland perennial that grows in a wide range of habitats. Established plants can reach heights of 2m with 30-50 stems forming wide-topped crowns that dominate the herbaceous canopy. One mature plant can produce more than 2 million seeds annually. Seeds are easily dispersed by water and in mud adhered to aquatic wildlife, livestock and people. High temperatures (>20C) and open, moist soils are required for successful germination and seedling densities can approach 10,000-20,000 plants/m2. A woody rootstock serves as a storage organ, providing resources for growth in spring and regrowth if the above-ground shoots are cut or damaged.
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Distribution and Spread:

  • By the 1830's, L. salicaria was well established along the New England seaboard. The construction of inland canals and waterways in the 1880's favored the expansion of the plant into interior New York and the St. Lawrence River Valley. The continued expansion of L. salicaria coincided with increased development and use of road systems, commercial distribution of the plant for horticultural purposes, and regional propagation of seed for bee forage. As of 1996, L. salicaria is found in all contiguous states (except Florida) and all Canadian provinces.


  • Invasion of L. salicaria into a wetland can result in the suppression of the resident plant community and the eventual alteration of the wetland's structure and function. Large monotypic stands of L. salicaria jeopardize various threatened and endangered native wetland plants and wildlife by eliminating natural foods and cover. Dense plant establishments in irrigation systems has impeded the flow of water.

Previous Control Methods:

  • No effective method is available to control L. salicaria, except where it occurs in small localized stands and can be intensively managed. In such isolated areas, uprooting the plant by hand and ensuring the removal of all vegetative parts can eliminate L. salicaria. Other control techniques include water-level manipulation, mowing or cutting, burning, and herbicide application. These control methods are costly, require continued long-term maintenance and, in the case of herbicides, are non-selective and environmentally degrading.
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Biological Control:

  • Ideally, natural enemies, as well as competition with other plants, prevent many plants from expanding their distributions. In turn, the abundance of the plant (acting as a host) influences the abundance of its natural enemies. L. salicaria was introduced to North America without its natural enemies and the objective of our program is to restore the self-regulatory potential of this plant-insect interaction by using biological weed control. Four host specific insect species approved by USDA-APHIS have been released in the US. These species are Hylobius transversovittatus, a root-mining weevil, Galerucella calmariensis and Galerucella pusilla, two leaf-eating beetles, and Nanophyes marmoratus a flower-feeding weevil. Nanophyes brevis, a seed feeding weevil, has been approved for introduction, however, European specimens are infested with a nematode, and this infection has prevented its introduction. Although infested adults of N. brevis do not show reduced life-spans or increased mortality and females lay fertile eggs, the potential for harmful effects of the nematode to indigenous North American insects exists. At present we have stopped any attempts to introduce N. brevis. In the biocontrol program targeting L. salicaria, our strategy is to achieve long-term control, not complete eradication, through provision of a simple, yet diverse, collection of natural enemies. Historically, introduction strategies for biological control agents have ranged from single-species to multiple-species releases. Through evaluation of single- and multiple-species releases, basic investigations of the biology and ecology of target plant and control agents, and follow-up monitoring, we hope to refine current methodologies for selecting natural enemies for the biological control of invasive, non-indigenous plants. These studies should allow to improve the scientific basis of biological weed control and help to establish this technique as a viable and environmentally friendly alternative to conventional control measures for the next century.
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Bernd Blossey