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Weevil Shows Potential to Control Eurasian Watermilfoil 
  October 24, 1996
  For more information, please contact: Marie Zhuikov: 218.726.7677

Euhrychiopsis lecontei Weevil

University of Minnesota Sea Grant researchers think a tiny freshwater weevil(Euhrychiopsis lecontei) shows promise as a possible control for Eurasian watermilfoil. Watermilfoil continues to slowly spread in Minnesota, usually due to accidental transport by boaters.

Susan Solarz and Ray Newman conducted experiments at the University of Minnesota with a native weevil that normally eats northern watermilfoil--a 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.

"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.

"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.

Eurasian watermilfoil is an exotic plant that has infested North American waters since the late 1940s. If can form dense mats of vegetation and crowd out native aquatic plants, clog boat propellers and make water recreation difficult. Eurasian watermilfoil has spread to 75 lakes, primarily in the Twin Cities area, and four rivers or streams. The most recent finding was in Rice Lake (the northwest Metro area) this summer.

Solarz and Newman's results were recently published in Oecologia. A reprint of this journal article, "Oviposition Specificity and Behavior of the Watermilfoil Specialist, Euhrychiopsis lecontei," is available from Sea Grant by calling 218.726.6191.


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