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Home > Selective Control of Eurasian Watermilfoil on Tomahawk Lake

Selective Control of Eurasian Watermilfoil on Tomahawk Lake

2008 - 2010

Selective Control of Eurasian Watermilfoil on Tomahawk Lake, Bayfield County, WI Using Early Spring Applications of 2,4-D

Selective Control of Eurasian Watermilfoil on Tomahawk Lake, Bayfield County, WI Using Early Spring Applications of 2,4-D

John Skogerboe1 and Tim Asplund, Jen Hauxwell, Frank Koshere, Carroll Schaal, and Pamela Toshner2

1USAERDC, Waterways Experiment Station, Eau Galle Ecology Research Facility,

W500 Eau Galle Dam Rd, Spring Valley, (651) 325- 8181

2 Wisconsin Department of Natural Resources


The US Army Engineer Research and Development Center (ERDC) is currently developing cooperative research with the Minnesota Department of Natural Resources (MDNR), the Wisconsin Department of Natural Resources (WDNR), and the US Forest Service to demonstrate aggressive whole lake management of invasive plants to restore and protect native aquatic plant communities. In 2007 the Town of Barnes, Bayfield County, Wisconsin, agreed to participate in this research partnership. One management strategy currently being evaluated is early season application of aquatic herbicides to selectively control Eurasian watermilfoil (Myriophyllum spicatum L.). Eurasian watermilfoil begins to actively grow in early spring soon after ice out, while many native plants are dormant.

Herbicide applied in early spring can allow a window of application that minimizes exposure to native plants, and thus reduces likelihood of plant injury (Skogerboe and Getsinger 2006). Additional advantages of early spring applications include younger, smaller target plants that are more susceptible to aquatic herbicides, less biomass eliminating concerns for dissolved oxygen depletion, slower herbicide degradation resulting in longer contact times, and minimal lake use causing less disturbance to the treatment. The herbicide 2,4-D is selective for dicotyledon (dicots) plants and has been frequently used to selectively control Eurasian watermilfoil (Parsons et al. 2001) where native plant communities are dominated by monocotyledons (monocots). Early spring applications may improve herbicide performance and increase selectivity particularly when native dicots are present. The herbicide 2,4-D can be applied either as a liquid (2,4-D amine) or as a granular formulation (2,4-D ester).

Tomahawk Lake is a 134 acre oligo-mesotrophic seepage lake located in Bayfield County in the Town of Barnes, Wisconsin (Figure 1). Eurasian watermilfoil was first discovered in Tomahawk Lake by WDNR in August 2004. There has been no active management of Eurasian watermilfoil since this discovery. The native aquatic plant community in Tomahawk Lake was quantitatively evaluated in June 2006 and July 2007 using the point intercept method and the Wisconsin Floristic Quality Assessment (WFQA) system ( Griffin and Bernthal 2003). Tomahawk Lake had a 2007 sampled species richness of 24 species in addition to 3 species that were observed but not directly sampled (Table 1). An uncontrolled Eurasian watermilfoil infestation could have significant adverse impacts on a native plant community sensitive to disturbance and could act as a source for infestation of numerous nearby lakes, including the surrounding Potowotomi lakes and the Eau Claire Chain of Lakes.


The objective of this study is to evaluate early spring applications of 2,4-D conducted over three consecutive years to selectively control Eurasian watermilfoil and protect a valuable and diverse native plant community.


Treatment and Reference Lake

Active management with herbicides will occur in Tomahawk Lake. Sand Bar Lake is directly adjacent to Tomahawk Lake and will serve as a reference lake with active management limited to small-scale manual removal around dock areas. Sand Bar Lake is a 118 acre oligo-mesotrophic seepage lake with a July 2007 aquatic plant species richness of 22, including visuals (Table 1). Data collection will be similar in both lakes.

Herbicide Applications and Residual Monitoring

The initial herbicide application will include an application of 2,4-D amine applied in early spring 2008 to all of Tomahawk Lake. Final determination of areas infested with Eurasian watermilfoil will be determined based on a comprehensive plant survey in April 2008 including a point intercept survey combined with hydro acoustics and underwater video camera. The herbicide will be applied in spring 2008 when water temperatures are approximately 12C (55oF), which generally occurs in early May. The herbicide will be applied as liquid amine 2,4-d at 0.5 mg/L throughout the eastern and western basins.

Local volunteers will collect water samples for residual herbicide analysis from 5 sites in Tomahawk Lake and 3 sites in Sand Bar Lake before the herbicide application and 1, 5, 7, 14, 21, 28, 35 and 42 days after treatment. ERDC will provide dark sample bottles, coolers, FedEx shipping labels, and a sampling device. The samples will be refrigerated after collection, shipped in 2 or 3 batches, and incrementally analyzed by ERDC (i.e. Dr. Mike Netherland). This will allow for field-verification of previously developed laboratory- and mesocosm-derived concentration and exposure time relationships (Green and Westerdahl 1990) and to develop data on degradation rates in cold water.

The project will entail an adaptive management approach by evaluating results and adjusting actions on the basis of what has been learned. As a part of the adaptive management framework, additional herbicide applications, SCUBA removal, or alternative management techniques may be conducted in 2009 and 2010 depending on monitoring results.

Aquatic Plant Community Evaluations

Plant community assessments will be conducted to quantify the effect of large-scale management strategies applied in early spring on both target (Eurasian watermilfoil) and non target species (native species) in both lakes. Pretreatment plant community assessments were conducted in July of 2007 using the point intercept method, and will provide background data to evaluate the effects of future herbicide treatments on native plant diversity. Plant evaluations in following years will be conducted in April, prior to herbicide application, and again in June and August through at least 2011.

Plant abundance will be evaluated using a rake device (Crowell et al 1994) to collect biomass samples. One fourth of the sample points used to evaluate species occurrence will be randomly selected to evaluate plant species abundance. The sample will be bagged and returned to the Eau Galle Aquatic Ecology Laboratory (EGAEL), where species will be separated and oven dried to a constant weight and recorded. Changes in invasive and native plant biomass will be analyzed using Analysis of Variance (ANOVA), transforming data to fit a normal distribution if needed.

The plant community structure will also be evaluated using Biosonics hydro acoustic equipment to measure plant community density and structure. Both the MDNR and WDNR are currently evaluating this method as a means of quantifying the effects of invasive plants and management techniques on native plant communities and fisheries habitat.


Water Quality

Local volunteers will monitor water clarity by collecting secchi disk readings every 2 weeks from ice out to the end of September. Water quality including pH, alkalinity, DO, and temperature will also be evaluated with a water quality meter, provided by WDNR or ERDC.



The US Army Engineer Research and Development Center (ERDC) will be responsible for scheduling data collection and analyses, and for scheduling application of the herbicides. A certified aquatic herbicide applicator will be responsible for providing the herbicide product and herbicide applications for 2008, 2009, and 2010. ERDC will also provide hydro acoustic data collection, spring pre-treatment plant monitoring, and analyses of herbicide residues. WDNR will be responsible for summer aquatic plant surveys. WDNR and volunteers will be responsible for water quality data and herbicide residue sample collection. The Town of Barnes will be responsible for grant administration, recreational use ordinance(s), and contracting with the herbicide applicator.



  • Conduct pretreatment evaluations of plants Apr, 2008-2010
  • Initiate herbicide applications Apr-May, 2008-2010
  • Conduct post treatment evaluations of plants Jun-Aug, 2008-2011



  • Interim Data Reports Feb 2008-2010
  • Draft Final Report Mar 2011



Crowell, W.J., N. Troelstrup, L. Queen, and J. Perry. 1994. Effects of harvesting on plant communities dominated by Eurasian watermilfoil Lake Minnetonka, MN. Journal of Aquatic Plant Management 32:56-60.

Green, W.R. and H.E. Westerdahl. 1990. Response of Eurasian watermilfoil to 2,4-D concentrations and exposure times. J. Aquat. Plant Manage. 28: 27-32.

Griffin, M.A., and Bernthal, T.W. 2003. Development of a Floristic Quality Assessment Methodology for Wisconsin. Final Report to USEPA Region V, Wetland Grant #CD975115-01-0. Madison, WI.

Madsen. 1999. Point Intercept and Line Intercept Methods for Aquatic Plant

Management. APCRP Technical Notes Collection (TN APCRP-MI-02).

U.S. Army Engineer Research and Development Center, Vicksburg, MS.

Skogerboe, J.G., A.G. Poovey, K.D. Getsinger, and G.K. Kudray. 2003. Invasion of Eurasian Watermilfoil in Lakes of the Western Upper Peninsula, Michigan. APCRP Technical Report (ERDC/EL TR-03-10). U.S. Army Engineer Research and Development Center, Vicksburg, MS.

Skogerboe, J.G., and K.D. Getsinger. 2006. Selective control of Eurasian watermilfoil and curly-leaf pondweed using low doses of endothall combined with 2,4-D. APCRP Technical Notes Collection (ERDC/TN APCRP-CC-05). U.S. Army Engineer Research and Development Center, Vicksburg, MS.

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