Weed Control Successful at Dewart Lake

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Dewart LakeAn aquatic herbicide application two years ago to control Eurasian water milfoil in Dewart Lake has nearly eliminated the noxious non-native weed from the 551-acre Kosciusko County lake.

During sampling late this month, milfoil was found at only three of 90 locations checked by DNR Division of Fish and Wildlife biologists. Last year, biologists were unable to find milfoil at any of the 90 locations in May or July.

Prior to 2006, Eurasian water milfoil covered nearly 60 percent of the shallow water area of  the lake and 80 percent of the area between 5 and 15 feet deep. In some spots, the plant reached the surface and was so thick that boaters could not get through it.

This invasive weed, which interferes with boating and fishing, was the target of a state-approved, lake-wide control program in May 2006. Funds for the treatment came from the DNR Lake and River Enhancement (LARE) program and local lake residents.

A licensed contractor was hired to apply approximately 28 gallons of Sonar® to the lake. The herbicide contains the chemical fluridone, which, when used properly at low dosages, can selectively kill milfoil with little harm to other plants.

“The herbicide treatment was very effective and knocked milfoil down to a very low level,” said Jed Pearson, DNR fisheries biologist. “We expect it to come back but it should be easier to control with spot treatments.”
Although the fluridone application caused some damage to coontail and other native plants in the lake, Pearson said he expects them to recover.

“Coontail covered 43 percent of the lake’s shallow-water area before the treatment and dropped to 12 percent after,” Pearson said. “Hopefully, coontail will come back to that same level. Overall, aquatic plants still cover more than 80 percent and include a good mix of native species.”

Biologists will re-check Dewart Lake in July and monitor the plant community for several years to study long-term benefits of the treatment.

Success of the milfoil control program at Dewart is similar to results at other lakes in northern Indiana, including Heaton Lake in Elkhart County, Lake-of-the-Woods in Marshall County, and Wall Lake in LaGrange County. 

“The treatments reduced milfoil in all cases, but we won’t know how long the control will last or the full effects for many years,” said Rod Edgell, a DNR biologist who has examined the impacts of fluridone on water quality and food habits of fish at Dewart and Heaton lakes.

No changes in largemouth bass diets were detected at either lake after fluridone treatments.

“Although we’re content with results so far, we want to be cautious when approving fluridone use at other lakes,” Edgell said.