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Aggressive treatment planned for Eurasian watermilfoil found in the Iowa Great Lakes

  • 9/13/2022 12:22:00 PM
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SPIRIT LAKE - The Iowa Department of Natural Resources (DNR) will start treating East Okoboji, Upper Gar, Lake Minnewashta, and Lower Gar this week with the approved aquatic herbicide, Sonar A.S. to eliminate Eurasian watermoilfoil.

Sonar A.S. has successfully eradicated Eurasian watermilfoil from other lakes in Iowa by preventing the plant from producing a pigment needed for photosynthesis, starving the plant over several weeks. The herbicide has no restrictions for swimming, fishing, irrigation, or drinking water at the planned dose. 

Treatments will continue through next spring.  The Iowa DNR will test the water throughout the Iowa Great Lakes chain every two weeks to measure concentration of the herbicide and prescribe “bumps” of herbicide to maintain the herbicide concentration. These bumps can even be done through the ice.

The herbicide is effective at very low concentrations, explains Jason Euchner, aquatic vegetation management biologist for the Iowa DNR. “We will treat the lake at five parts per billion.  Only sixty-three gallons of the herbicide will be needed to reach the desired concentration across all four lakes.”

Eurasian watermilfoil was discovered in the East Okoboji, Upper Gar, Lake Minnewashta, and Lower Gar in early August. This aggressive invasive plant is known to outcompete native plants and take over areas of a lake.

A group of representatives from the Iowa Great Lakes has been working with the Iowa DNR to formulate a management plan that could eliminate the plant from these lakes. Fast action is required since the plant is quickly expanding and could infest other lakes and wetlands in the chain making eradication nearly impossible.

Along with funding from the DNR, several local groups and organizations are helping to pay for this aggressive treatment plan, with partners committing over $335,000 in only a couple of weeks. 

Terry Wilts, with the East Okoboji Lakes Improvement Corporation, says, “This is a threat to all the lakes in the chain and to other surrounding lakes in the region.  We are in this together and if there is a chance to take out this plant, we’d regret not taking that opportunity.”

Eurasian watermilfoil and curly-leaf pondweed, another invasive species in the Iowa Great Lakes, are very susceptible to Sonar A.S.  The treatment should only impact these two species. Since Eurasian watermilfoil does not produce seeds, a single treatment can eliminate the plant. Curlyleaf pondweed will be drastically reduced because of the treatment, but will likely rebound since it produces seed-like structures that help it repopulate.

The DNR and local partners are starting to build a long-term management plan for the chain of lakes. If the project is successful, prevention will continue to be the best way to guard against future re-introduction. If the treatment is not successful, the group will discuss long-term management strategies. 

“Keeping the plant out of the lakes over the past 30 years has allowed time for better tools to be developed for managing this plant,” said Mike Hawkins, district fisheries biologist with the Iowa DNR. “I’m confident we can work together locally to manage it long-term. In the meantime, we plan to take our best shot at eliminating it.”