As many of us in the Northwoods know, our lakes are a precious resource and we do our best to take care of them in any way we can. Most of us also know the dangers of an invasion from an aquatic invasive species, in this case Eurasian water-milfoil and curly-leaf pondweed.
One of the most popular methods of treating an invasion from one of these pesky plants is to use herbicide in our lakes in hopes to keep the invading plant under control and prevent it from becoming any more of a nuisance. This method of control begs the questions: How much herbicide are we using in our lakes? Is it effective? What are the repercussions?
In my pursuit to answer these questions, I collected data from DNR permits and lake management plans. I collected information from 2006-2011 from Vilas, Oneida, Lincoln, Langlade, Forest and Florence counties. The most common chemical used is a granular form of Navigate (2,4-D) and in more recent years the next most common is the liquid form of Navigate. Both of these are largely used to treat Eurasian water-milfoil. To a far lesser extent, the chemical Aquathol (endothall) has been used to treat curly-leaf pondweed.
In 2006 and 2007, chemical application occurred at the lowest recommended concentration based on the chemical’s label. After treatments occurred, a consultant would take a survey of the remaining plant life and determine how effective the treatment was. At the lowest-possible dose, it was clear that treatment was not as effective as many would have liked. So starting in 2008, treatment concentrations went up and every chemical permit in this six-county region required a pre-treatment and post-treatment survey to assess both effectiveness of reducing the invading plant and damage done to native plants.
Also, around this time, more lake surveys were being done on other lakes and more invasions were found. Due to finding more invasions and learning that treating at the lowest dose was not effective, there was a fairly dramatic increase in chemical treatments.
In 2006, approximately 7,700 pounds of granular 2,4-D were used to treat 105 acres of Eurasian water milfoil throughout the six-county area. In 2007, approximately 20,015 pounds of granular 2,4-D were used to treat 309 acres. Then in 2008, a major increase in both pounds of chemical and acres treated occurred for the reasons stated above. In that year, approximately 74,800 pounds were used to treat 648 acres. This increase in finding new invasions and using more chemical held on for a few years. In 2009, approximately 104,690 pounds of 2,4-D were used to treat 1,043 acres of Eurasian water-milfoil. And again in 2010, approximately 102,670 pounds were used to treat 1,428 acres.
Then in 2011, both the pounds of chemicals used and the acres treated dropped substantially. Approximately 66,035 pounds of granular 2,4-D were used to treat 541 acres of Eurasian water milfoil. Seeing these drops in numbers, it looks like a few years of treatment may prove to be effective in controlling invasive aquatic plants. It will be interesting to see if this reduction in treatment will be sustained through the 2012 season.
Certainly from lake to lake, efficacy of chemical treatment varies. There have been a handful of lakes in the Northwoods that have been treated annually for the last five or six years and they all have a different story to tell. While some seem to have great success, others vary greatly from year to year. A myriad of factors may play a role here; another invasive species, such as zebra mussels, may be present, the amount of chemical herbicide used may vary, the stratification of warm and cold water layers in the lake may affect treatment, the existing population of the invasive species may be very small in one lake and very large in another. However, it is undeniable that some treatments seem to be very effective while others may not show much a difference.
What are some of the repercussions from using chemical herbicides in our lakes? The most common and most known impact is the loss of native vegetation. Science has shown that using an herbicide to treat one specific plant can most certainly kill other plants. The loss of native vegetation leads to a decrease in fish habitat and possible increase in algae blooms. It could also open up space for an invasive plant to move in.
Accumulation of chemicals in the lake sediments is also a potential concern. Currently the DNR is conducting research to figure out whether or not our lake sediments will, in fact, accumulate chemicals. They expect to see some results from their study within the next six months.
Another concern is the concentration of the chemical directly following treatment. Many of the chemicals come with warning labels and restrictions for water use following a treatment. For instance, application of 2,4-D requires a 24-hour waiting period for swimming. Irrigation restrictions may apply for up to 30 days.
The Army Corp of Engineers and the DNR have partnered to study the duration of high concentrations of chemicals in the lake directly following a treatment. They’re looking for a balance between having the chemical concentration high enough to be effective in treating the invasive plant and also being low enough that it will mitigate undesirable effects, such as taking down a native plant population.
At the end of the day, chemicals have proven to be effective in many circumstances and not as much in other circumstances. Research is ongoing to learn more about long-term effects of these chemicals. At this point, chemical herbicides are one of the tools that we can employ to manage aquatic invasive species.
Carol Warden is an aquatic invasive species (AIS) specialist with the University of Wisconsin Center for Limnology at their Trout Lake research station. She also works directly with the DNR on many AIS research topics.