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4 Summer 2015 Managing Eurasian Watermilfoil in Our Lakes By Meg Rattei, Barr Engineering If you’ve been swimming, boating, or fishing in the city’s major recreational lakes – Lake Olson, Lake DeMontreville, Lake Jane, and Lake Elmo –you’ve likely had an encounter with Eurasian watermilfoil (EWM). This aquatic invasive species is not native to the United States. And, once in a lake, it becomes a bully. It reproduces quickly, spreads easily, and flour- ishes to the detriment of native plants, fish, and wa- terfowl – making it difficult to swim, boat, or fish. Although persistent, this bully can be stopped by effec- tive management. During the fall of 2014, representatives from Valley Branch Watershed District (VBWD), the City of Lake Elmo, the Lake DeMontreville/Olson Association, the Lake Jane Association, and the Lake Elmo Association collaborated to determine an effective EWM manage- ment strategy in the city’s major recreational lakes. The first step in tackling the problem was preparation of Lake Vegetation Management Plans (LVMPs) for the four lakes, presenting the plans to the public, and submit- ting the plans to the Minnesota Department of Natural Resources (MDNR) for approval. LVMP plan approval is a prerequisite for a MDNR permit needed to com- plete the large scale herbicide treatments necessary to effectively manage the large infestations of EWM in the lakes. Extensive discussion preceded a decision by the MDNR that the LVMPs would not be approved. For Lake Elmo, a natural environment lake in which the use of herbicides is not allowed without a variance to MN rule, MDNR suggested mechanical harvesting as an alternative to the use of herbicides to manage Eurasian watermilfoil and has awarded grant monies to help fund the harvesting. The Lake Elmo Association is in the process of finalizing EWM harvesting plans. MDNR allowed herbicide treatment on the other lakes, but restricted treatment to less than 9% of their areas and less than 25% of the EWM-infested areas. MDNR said “If these approaches are not meeting objectives, we can re-evaluate management efforts in the future.” MDNR and Washington County grants have helped fund the treatments and VBWD has provided the need- ed technical assistance for the project. Plant surveys of the lakes were completed on May 9 and 10 to deter- mine where the EWM was growing and to select her- bicide treatment areas. MDNR issued permits and the herbicide 2,4-D was applied to Lake Olson and Lake DeMontreville on May 19 and to Lake Jane on May 20. Wave action in the lakes quickly mixed the herbi- cide in the small treatment areas with untreated wa- ters, diluting the herbicide below the concentration needed to kill both the EWM plant and its root crown. Herbicide residue samples collected from the lakes documented that concentrations were below the le- thal concentration of 0.3 ppm on 3 days after treat- ment - ranging from 0.1 ppm to 0.25 ppm. Hence, long-term control of EWM is not expected. Herbicide concentrations not only decline because of dilution from untreated waters, but also from bac- teria breaking 2,4-D down into carbon dioxide and water, harmless natural components of the lakes. A final 2,4-D sample will be collected on 30 days after treatment in each lake to verify that 2,4-D is no longer detectable. VBWD will complete plant surveys on the lakes about a month after treatment. Once the survey results be- come available, representatives from VBWD, the City of Lake Elmo, the Lake DeMontreville/Olson Associa- tion, the Lake Jane Association, and the Lake Elmo Association intend to meet with MDNR staff to dis- cuss the results and determine a long-term strategy that will result in effective EWM management in the city’s major recreational lakes. For more information on Eurasian Milfoil or other aquatic invasive plant species please contact the Val- ley Branch Watershed District at 952-832-2622 or the Minnesota Department of Natural Resources.