Purple Loosestrife

 By
Bryan Dallolio, Ada County Noxious Weed Control

Purple Loosestrife flower
Purple Loosestrife flower

Purple Loosestrife (Lythrum Salicaria) is an aggressive invader that displaces our native vegetation in riparian areas and waterways, where it becomes a dense monoculture, and wreaks havoc with our biological diversity. Purple Loosestrife is a threat to wetlands everywhere, and has been declared Public Enemy Number One on federal wetlands by the United States Fish and Wildlife Services. In Idaho, this weed has been declared noxious, and quickly spreads into our waterways and wetlands.

What Does Purple Loosestrife Do?

This perennial plant produces vegetation up to eight feet tall, from rootstocks, two to three feet in diameter. From its multiple stems, it produces seeds, as many as 2 million from a single mature plant. These seeds, about the size of a grain of sand, blow and/or float to new areas where they absorb water, germinate, and grow. Prolific seed production, and the long life of the root crown, allows Purple Loosestrife to become a very dense monoculture. Purple Loosestrife provides no nutritive value to native insects, animals, or fish. Purple Loosestrife causes bird, fish and amphibian populations to decline when their native food species and nesting sites are eliminated by the presence of this plant. As time progresses, Purple Loosestrife effects the flow, temperature, and nutrient loads of the water, continuing to damage the necessary survival components of the flora and fauna in our wetlands. Recreation, agriculture, and public uses of our waterways suffer when this plant blocks access and interferes with equipment needs at the waters edge.

Galerucella Beetle is used as a biocontrol agent for Purple Loosestrife
Galerucella Beetle is used as a biocontrol agent for Purple Loosestrife

What Has Been Done?

Idaho has declared this a noxious weed; therefore, the Idaho State Department of Agriculture prohibits the introduction of Purple Loosestrife as an ornamental plant, as well as assists in appropriating funds for its eradication. Ada County Noxious Weed Control has been involved in several collections and releases of two species of loosestrife feeding beetles. The two agents are varieties of the Galerucella Beetle, Pusilla and Calmariensis, or more commonly, the Golden and Black Margined Loosestrife beetles. Each year, we collect and redistribute them in new areas of Purple Loosestrife invasion. We have seen a substantial improvement where these insects have been used. These beetles feed on the leaves and stems of these plants. The beetles damage the plants enough, that seed production is greatly hindered. Originally, these two beetles were acquired at great difficulty and expense through the National Biological Control Institute/ USDA APHIS program in Bozeman, Montana. Many states contributed money to arrange for the development and testing of these insects. The first collection for Ada County was made from the Moses Lake area in Washington State.

Professor Mark Schwarzlaender, U of Idaho, Plant Soils and Entomological Services gets ready to release Galerucella Beetles on Purple Loosestrife in Ada County.
Professor Mark Schwarzlaender, U of Idaho, Plant Soils and Entomological Services gets ready to release Galerucella Beetles on Purple Loosestrife in Ada County.

What Needs to Be Done?

We must stop the growth, and spread of Purple Loosestrife seeds. We urge you to do the same. Depending on if simple mechanical removal is all that is needed, or in more widespread areas, herbicide spraying, we hope that you will take the time and effort to remove this plant from your properties. If you are unsure what to do about an infestation of Purple Loosestrife, contact your local extension agent or county weed control. They can help you control this plant, and are the first step in arranging for a release of the biological control agents available, if they are appropriate for your situation.

Successes with Purple Loosestrife Control

Ada County has had excellent results using the two biological control agents commonly available to us. With the help of the University of Idaho Plant, Soils, and Entomological Services department, we have hosted several workshops, where the insects have been distributed to Purple Loosestrife infested areas. Depending on conditions at the release sites, we are seeing excellent control in as little as four years.

Rewards of Purple Loosestrife Control!

  • Saving our native or desirable plants and animals from harmful competition by this invader
  • Protecting our wetlands from what some refer to as a "biological pollution."
  • Preventing the inconvenience of plugged irrigation and boating structures.
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