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Effects Of 2,4-D Herbicide Studied On Three Northwoods Lakes

Wisconsin DNR

BOULDER JUNCTION – A research project regarding the application of 2,4-D herbicides to control Eurasian water milfoil showed no statistically significant effects on fish or zooplankton in three Northwoods lakes, according to the principal investigator of the study.

Dr. Dan Isermann, of the Wisconsin Cooperative Fishery Research Unit, College of Natural Resources, UW-Stevens Point, led the three-year project. Nick Rydell, a graduate student, and numerous undergraduate technicians, assisted him. Isermann presented the results of the team’s research July 23 at the Trout Lake Research Station, Boulder Junction. About two dozen people attended.

The Wisconsin Department of Natural Resources funded the research. The agency uses such research to identify best practices for when and how to control species that impact the environment and lake users. Data were collected on the lakes in 2015 (no treatment), 2016 (year of treatment) and 2017 (follow-up survey).

The herbicide was applied on Kathan, Silver, and Manson lakes. Upper Gresham, Little Bearskin, and Brandy did not get treated and served as reference lakes.

Eurasian watermilfoil Myriophyllum spicatum (EWM) is one of the most problematic aquatic invasive species in North America because it can outcompete native macrophytes (water plants). It adversely affects fish and wildlife, and when it grows out of control, forms a mat on the surface that can clog outboard motors and prevent swimming and waterskiing activities. It affects the biodiversity and frequency of native aquatic plant species, and lowers the dissolved oxygen and nutrient cycling in the littoral zone.

The state and lake associations have turned to herbicides and hand-pulling of the invader in attempts to eradicate or control its spread. Applied early, the 2,4-D herbicide targets and kills the invasive species. Specifically, 2,4-dichlorophenoxy acetic acid (2, 4-D) based herbicides are widely used for controlling EWM in Wisconsin and other states, but little is known regarding the effects of these herbicide treatments on fish and zooplankton communities outside of a laboratory setting.

Isermann and his team set out to determine if 2,4-D herbicide treatments used to control EWM would affect: – abundance, diversity, and size structure of fishes at different life history stages; – feeding, survival, and growth of larval fishes, and – diversity, abundance, and size of zooplankton.

In Wisconsin, 2,4-D is used at low doses and for short exposure times on Eurasian watermilfoil. At those levels, the biodegradable herbicide dissipates rapidly as UV light and microorganisms living in the water and sediments convert the herbicide to carbon dioxide, water and chlorine, according to research literature. Water movement also adds to the biodegradable rate.

The DNR says the half-life of 2,4-D (the time it takes for half of the active ingredient to degrade) ranges from 12.9 to 40 days depending on water conditions. As an aside, Isermann noted that 2,4-D – a popular residential and farming herbicide – is used widely and is a primary ingredient in many products readily available to the public. Initial surveying of the lakes took place in 2015, with application of 2,4-D in 2016. In Manson Lake, he explained, the herbicide substantially knocked down the Eurasian water milfoil following the application. But the invasive species rebounded somewhat the following year. “It’s not a long term eradication (solution), he said. It’s more of a “control scenario.”

The herbicide was applied in May and June, but not quite at the target concentration. Nevertheless, it was effective in reducing the Eurasian water milfoil. After 62 days, the herbicide was not detectable in the water column. As to the effect of 2,4-D on zooplankton and fish, he said: “We had no meaningful treatment level effects.” Zooplankton trends were not consistent among all the lakes, he added. Also Daphnia (planktonic crustaceans -- commonly known as water fleas) numbers were up in some lakes; down in others. The growth rate of young yellow perch in the treatment area was “basically” similar to that of the non-treatment lakes.

Similar conclusions were drawn for the larval bluegills... “(We) did not detect anything significant with this data.” However, as he cautioned in his report, “...(L)ack of significant responses to 2,4-D observed in this evaluation does not necessarily mean that herbicide application has no effects on these or other metrics.” “There was a substantial natural variability from day to day, from year to year,” he continued. He said that his team’s research “does not refute” those results of a laboratory study by another scientist that showed adverse effects on fathead minnows from 2,4-D.

Too, no conclusions on long-term effects of repeated application of 2,4-D on fish and zooplankton should be drawn from their study, he said. The other thing to keep in mind, he explained, is the extraordinary high morality rate of larval fish – “often 99 percent of them are dying (naturally through predation and other sources) every day.” That left him with 1 percent to study. He noted that fluctuations of yellow perch populations in some treatment lakes could be attributed to the removal of the Eurasian water milfoil, which yellow perch use for spawning and feeding, as well as for cover from predators.

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