Tomahawk Lake Association, Inc. Wisconsin

By John Rybski, Board Member, Environmental/Education Committee Chair

During the summer of 2009, we saw a resurgence of Eurasian Water Milfoil (EWM). The resurgence was particularly disappointing because it followed on the heels of our first truly successful year. Between the fall of 2007 and the fall of 2008, we saw a 55 percent decrease in the total acreage of EWM. The question we are all asking now is, “Why did we experience an increase in 2009?”

There are a number of factors which can, singly or in combination, impact the effect of chemical treatment. Factors include water temperature, pH, the growth stage of the EWM and the number of daylight hours. Consider:

• EWM that is not actively growing doesn’t absorb the herbicide. If the herbicide is applied before the submerged plants are actively growing, it is not absorbed and has no effect. 
• Treatment effectiveness depends on the number of hours the herbicide is present in a sufficient concentration to affect the target plant species.
• When the appropriate quantity of pelletized herbicide is applied, critical concentration levels may not be maintained for an appropriate period of time. One reason this can happen is that wind driven currents can reduce concentration levels. Another reason is that dense beds of EWM can intercept chemical pellets as they sink, preventing the chemical from reaching the required concentration.

So, how do we prevent even more growth in 2010, while simultaneously determining what inhibited a successful chemical treatment in 2009? Our answer is threefold.

First, we will continue treatment with pelletized 2, 4 D in some areas and, with DNR approval, begin using liquid 2, 4 D in environments where it can be more effective. Chemical treatment will be supplemented with diver-based manual removal.

Second, during chemical application, staff and volunteers will collect data on surface and bottom temperature, pH, currents, and EWM growth state. They will visually verify application rates and collect water samples to determine chemical concentration levels during the seventy-two hours following chemical application. The data collected will be analyzed to identify correlations between environmental conditions, application processes and outcomes. If the post treatment survey in July shows poor control, we will seek approval from the DNR to immediately spot treat those areas. The extent of such spot treatment is severely limited under current DNR policy, but any follow up treatment is better than none.

Third, we will continue our education of property owners and visiting boaters.

Certainly Mother Nature will not take a year off while we seek answers. We are dealing with an aggressive biological invader in a complex and sensitive environment. Our three strategies will provide us with more data to continue improving our treatment methodology. And as we move forward each year, best practices, research and diligence must continue to guide us in meeting this ongoing challenge.