Chip Welling; Coordinator, Eurasian Watermilfoil Program; Minnesota Department of Natural Resources; 500 Lafayette Road; Saint Paul, MN 55155-4025; 651-297-8021.

Eurasian watermilfoil, or simply, milfoil, is a non-native, submersed aquatic plant that may cause problems in lakes when it becomes abundant. In the January-February 2004 issue of this newsletter, I provided some background on the use of herbicides to control milfoil. This issue will focus on the use of fluridone for milfoil control.

Fluridone is the active ingredient in Sonar or Avast! herbicides.

The principal difference between fluridone and other herbicides is that fluridone cannot be used effectively for spot-treatments, but must be applied to whole bays or lakes. The DNR has been making whole-lake treatments with fluridone since the early 1990s and monitoring their effects. The purpose of these studies was to determine whether this herbicide can be used to selectively control milfoil. Treatment in 1994 at a target rate of 10 ppb fluridone reduced native vegetation lake-wide, and was followed by a decrease in clarity of the water in one of two treated lakes in the Twin Cites area. This damage was considered to be significant. As a result, the DNR decided to allow use of fluridone only in lakes that have high potential to become a source of spread of milfoil in an area of Minnesota without milfoil.

Such a situation arose in 1999 when milfoil was discovered in McKinney and Ice Lakes in Grand Rapids. At the time, no other lakes in the area were known to have the exotic. Consequently, the DNR treated both lakes with fluridone in an attempt to limit further spread in this part of Minnesota.

In 2000, new information from Michigan suggested that application of fluridone at low rates of about five ppb might provide more selective control than had previously been observed in Minnesota. In an attempt to reproduce the Michigan results, the DNR treated three Minnesota lakes at about five ppb in 2002. Treatment reduced the frequency of Eurasian watermilfoil to zero, but, unfortunately, also reduced the biomass of native submersed plants by an average of 94 percent. Following treatment with fluridone, water clarity decreased in two of the three treated lakes. Unfortunately, this damage appears to outweigh the benefits of controlling milfoil.

The lakes treated in 2002 had low water clarities of 2.5 to 5.5 feet. Two of the lakes had communities of submersed plants dominated by milfoil and coontail with few native species. By comparison, some of the treated Michigan lakes as well as one other lake in the Twin Cities, Lac Lavon, had higher water clarity and more native plants before treatment.

Following treatment of Lac Lavon, the distribution of milfoil decreased dramatically, and native submersed species increased. Four years after treatment, the milfoil had returned to pre-treatment levels of abundance and the native species had decreased. After another treatment, the milfoil decreased again and the native plants began to increase. This result means that long-term control of milfoil will require repeated whole-lake treatments with fluridone, as is already the practice in Michigan.

As the DNR’s evaluation of fluridone continues, we expect to further investigate the relationship between lake condition, specifically water clarity and composition of the vegetation, versus the effects of whole-lake treatment with fluridone.

Posted in Lakes |