Experimental early-season aquatic herbicide treatments at Stone and Atwood lakes in LaGrange County appear to be reducing the population of invasive Eurasian watermilfoil.

Eurasian watermilfoil is present in most Indiana natural lakes. It often becomes abundant, creating a nuisance for boating and fishing. The plant begins growing early in the season and can form dense canopies that out-compete native species.

In previous years, Eurasian watermilfoil occupied as much as 17 percent of surface area at Atwood Lake, but now has become difficult to find. Similar results have been documented at Stone Lake, where Eurasian watermilfoil once occupied 28 acres of the 116-acre lake.

Between 2012 and 2014, the watermilfoil population at both lakes was treated annually in late April as water temperatures warmed between 50 and 60 degrees. By comparison most treatments designed to control Eurasian watermilfoil selectively take place in late May or early June. While each lake was treated with a different chemical, both treatments are designed to be selective for watermilfoil and benefit native species.

Both lakes have a diverse native plant population with as many as 10 native species. Both lakes include a species listed as rare or threatened in Indiana, according to Rod Edgell, aquatic biologist with the DNR Lake and River Enhancement Program (LARE).

“We believe the timing of these treatments is critical to the selective control of Eurasian watermilfoil and possibly the reduction in the population over time,” Edgell said. “The plant is often present and growing before most native species. So, by treating it early we can control milfoil and give the native plants an advantage throughout the growing season.”

Similar results also have been documented at other Indiana lakes receiving early-season treatments. Nonetheless, this approach may not work at every lake and treating early in the growing season comes with risks. If good control is not obtained with the initial treatment, retreatment may be needed later in the summer and would result in additional costs.

Funding for treatments at both lakes has been provided by the Indiana Department of Natural Resources LARE Program in combination with the local lake associations.

For more information on other aquatic invasive species and how to prevent their spread see InvasiveSpecies.IN.gov.