Chemical Control

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Chemical Control

QLIA Board Assesses Efficacy of June 20, 2017 Herbicide Treatment

On September 14, 2017 QLIA volunteers sampled aquatic plants in five areas previously infested by Hybrid Eurasian Water Milfoil (HEWM) in Lost Land Lake prior to treatment with Reward herbicide on June 20, 2017. QLIA President Dave Neuswanger, Vice-President Norm Bratteig, and Director-at-Large Steve Fiala sought to evaluate the extent to which invasive HEWM and desirable native plants may have returned to these areas since the chemical treatment. Reward herbicide (Diquat dibromide at 2 gallons per surface acre) had been applied on June 20 under contract with Cason & Associates, LLC at a total cost of $6,023 ($607/acre in 9.9 treated acres).

QLA Vice-President Norm Bratteig looks on as President Dave Neuswanger examines a rake sample of Fern pondweed in Ole Creek Bay of Lost Land Lake, 9/14/17. PHOTO: Steve Fiala

Using a garden rake to sample plants at depths from 4 to 7 feet, we found no purebred Eurasian Water Milfoil (Myriophyllum spicatum) anywhere. In three of five areas previously infested with Hybrid Eurasian Water Milfoil (sample sites in Northern Landing Camp Bay, South Shore Ole Creek Bay, and South Central Wilson Bay), we found no HEWM. Near the mouth of Wilson Creek, we found only one stem of HEWM in one of three samples, though we observed a few more scattered stems in the vicinity that were at least a foot below the water surface. In the most heavily infested area prior to treatment (Northwestern Wilson Bay), we found only a few isolated stems of low-growing HEWM mixed with other low-growing native macrophytes. We were pleasantly surprised at the extent to which HEWM had been minimized and several species of desirable native plants had returned. Click on the PDF file links below to view a map of our sample sites and review all site-specific data.

HEWM Sample Sites on Lost Land Lake 9-14-2017

Lost Land Lake 9-14-17 Qualitative HEWM Survey Table

Based on product labeling, we expected Reward herbicide to quickly kill the exposed stems and leaves of all submersed plants without killing their root systems. We further expected viable root systems to shoot new stems and leaves into the water column, but we did not know which species would predominate in the post-treatment environment. Reward herbicide is not advertised as a product that selectively controls HEWM, so our biggest concern was that HEWM would return to previous levels of dominance at the continued expense of more desirable native plants in the treatment areas, and that it might again “top out” (grow tall enough to lay over the water surface) before the end of the boating season, as it did in 2016 when no control efforts were made. Our most optimistic scenario was for this presumptively one-time chemical treatment to keep HEWM from topping out in 2017, giving us time to raise funds to buy a mechanical Eco-Harvester as the preferred method of HEWM control in the long term.

Fortunately, even our most optimistic expectations were exceeded. HEWM was either eradicated or reduced to trace levels everywhere, and several desirable native species assumed dominance as vegetative growth redeveloped in post-treatment sample areas. Flat-stem pondweed was well-represented in 4 of the 5 post-treatment areas, having grown to heights of 2-3 feet during the 56-day post-treatment period. Clasping-leaf pondweed (excellent fish habitat, often called “small cabbage” by anglers) was sampled in 2 of the 5 areas, but it was observed in most post-treatment areas, and it was the most visible returning plant due to its height, which in many areas came to within less than one foot of the water surface. Another top-rated plant for its fish habitat value, Large-leaf pondweed (often called “large cabbage” by anglers), was sampled in 3 of the 5 sample areas. Our native species of Northern milfoil was sampled in 3 of the 5 areas where no specimens of HEWM were collected by rake or otherwise observed.

Dave Neuswanger with a healthy specimen of Clasping-leaf pondweed — a favored habitat of walleye and other fish species. PHOTO: Steve Fiala
Norm Bratteig examines a healthy stem of native Northern water milfoil collected in an area previously dominated by HEWM. PHOTO: Steve Fiala
Dave Neuswanger displays a healthy specimen of Large-leaf pondweed — the most valuable provider of fish habitat in Lost Land Lake. PHOTO: Steve Fiala

We were also impressed by the total number of species in these redeveloping plant communities, which included specimens of Coontail, Bladderwort, Wild celery (a favorite food of waterfowl), Fern pondweed (in the shallowest sample area in Ole Creek Bay), and Sago pondweed and Nitella (both growing low to the sediments in areas of formerly high-density HEWM). Altogether, we documented nine native species redeveloping, with some highly desirable species reclaiming dominance, in chemical treatment areas previously dominated by HEWM. We are not expert aquatic plant taxonomists, so it is likely that we failed to identify one or more additional species in our rake samples.

Despite all this good news, it is important to note that 2017 has been an unusual year. Spring was cooler than normal, and summer air temperatures rarely exceeded 85F. As a result, rooted aquatic plants developed slowly. Throughout our lakes, these plants did not achieve the density and height that long-time residents are accustomed to seeing. Our chemical treatment of HEWM occurred a month later than anticipated to enable applicators to see the tops of plants from the water surface in areas of infestation. This raises the following questions: Did the unusually cool weather somehow favor the redevelopment of native plants over HEWM? Is it reasonable to expect such a positive and selective response to Reward herbicide if applied under typically warmer seasonal conditions? The QLIA Board will discuss and adjust our strategy, as warranted.

MORE ABOUT REWARD HERBICIDE

Click on the PDF file links below to learn more about Reward herbicide.

2002 Technical Bulletin on Reward Herbicide (PDF)

2012 WDNR Chemical Fact Sheet on Diquat in Reward Herbicide (PDF)

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