Public Release: 

Weevil Shows Potential To Control Eurasian Watermilfoil

National Sea Grant College Program

DULUTH, MN. University of Minnesota Sea Grant researchers report that a tiny freshwater weevil (Euhrychiopsis lecontei) shows promise as a possible control for Eurasian watermilfoil, an exotic plant that has infested North American waters since the late 1940s.

Susan Solarz and Ray Newman conducted experiments at the University of Minnesota with a native weevil that normally eats northern watermilfoil, an usually benign native relative of the Eurasian type. Solarz and Newman found that weevils introduced to Eurasian watermilfoil in a lab setting prefer to lay eggs on the Eurasian variety over native varieties. The weevil lays its eggs on the tips of the milfoil plant. Once they hatch, the young burrow down the stem, eating their way through the plant, which slows down the growth of milfoil. Under the right environmental conditions, this could provide a chemical-free control method.

Eurasian watermilfoil continues to slowly spread throughout North American freshwater lake and river systems, usually due to transport by boaters. It currently is found in 40 states and three Canadian provinces. It can form dense mats of vegetation and crowd out native aquatic plants, clog boat propellers and make water transport and recreation difficult.

Currently milfoil is controlled through harvesting and chemical treatments, both labor intensive and costly. Estimates are that chemical treatments can run between $200-$300 per acre of water treated, while combinations of harvesting and chemicals can run considerably more.

A frequently cited example of the high control costs is Lake Minnetonka, a large multi-bay lake just west of Minneapolis that is a popular boating and fishing lake for Twin Cities residents. Control costs there have run as high as $250,000 a year. In Minnesota the plant currently impacts 75 lakes, primarily in the Twin Cities area, and four rivers or streams. Nationally the U.S. Army Corps of Engineers estimates millions of dollars are spent annually to control Eurasian watermilfoil infestations.

"Their results show that the weevils are definitely worth looking into as a control method and that additional research is necessary," said Chip Welling, coordinator of the Minnesota Department of Natural Resources' Eurasian watermilfoil program.

Newman and colleagues are currently monitoring experimental use of the weevil in Minnesota to seek answers as to where and where not it will work and what are the optimum conditions for its successful impact. Other researchers in Wisconsin, Vermont and Washington state are also investigating the use of the weevil.

"Finding a natural way to inhibit Eurasian watermilfoil is important," said Newman. "Although it is unlikely the weevils will eradicate Eurasian watermilfoil infestations, under certain conditions, which we are still investigating, they can reduce the amount of the plant that spreads across the water's surface, which can provide major benefits, especially for boaters."

"While chemical control may still play a role in specific settings -- such as near crowded lake accesses, and to clear navigational channels -- natural controls have advantages," said Newman. "First, these weevils are already here; there isn't the danger of adding a new exotic pest. The weevil specifically targets Eurasian watermilfoil, reducing the risk that native plants will be harmed in the process. Second, effective biological controls may result in long-term declines at a relatively low cost. This reduces the need for repeated treatments usually required with chemical and mechanical controls."

Solarz and Newman also discovered that once weevils are reared on the exotic plant in the lab, they will spend more time looking for it if the Eurasian variety is removed, instead of simply switching to the native species. They can eventually switch, but the weevils have long coexisted with the native variety.

Solarz and Newman's results were recently published in the international peer-review journal, Oecologia.

Contact for More Information:

Ray Newman, Minnesota Sea Grant Researcher 612-625-5704, E-Mail:

Marie Sales, Minnesota Sea Grant Communications Director 218-726-7677, E-Mail:

Note: an image of the Euhrychiopsis lecontei weevil is available on Minnesota Sea Grant's web site:

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