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Aquatic Herbicide Trials for Selective Control of Eurasian Watermilfoil and Curlyleaf Pondweed in Noxon Rapids Reservoir: 2009-2010

Getsinger, K. D., Madsen, J. D., Wersal, R. M., Skogerboe, J. G., Nawrocki, J., & Richardson, R. J. (2011). Aquatic Herbicide Trials for Selective Control of Eurasian Watermilfoil and Curlyleaf Pondweed in Noxon Rapids Reservoir: 2009-2010. Western Aquatic Plant Management Society Annual Meeting. Denver, CO.

Abstract

While management of invasive submersed aquatic plants in quiescent water is now generally predictable and consistent, managing these species in moving waters continues to present a challenge. We developed a study on Noxon Rapids Reservoir near Thompson Falls, MT, to selectively manage Eurasian watermilfoil and curlyleaf pondweed in a run of the river reservoir environment. Noxon Rapids Reservoir has seasonal and daily fluctuating flow rates dependent upon water discharge patterns related to hydropower production. Eurasian watermilfoil was found in 13% of the reservoir’s littoral zone, or 323 acres, in 2009. Herbicide treatments were made with combinations of endothall and triclopyr in two plots (plots 1 and 3, 20.2 acres and 18.9 acres, respectively), with two plots (plots 2 and 4 – acres?) as untreated reference plots. Submersed plant communities were surveyed in all plots by the point intercept method before treatment, five and 52 wk after treatment (WAT) using from 30 to 38 points per plot. Dye and herbicide half lives were 33 h in plot 1 and16 h in plot 3. In plot 1, Eurasian watermilfoil frequency was 66% before treatment, 8% by 5 WAT, and 14% by 52 WAT, yielding an80% reduction. Plot 3 Eurasian watermilfoil frequency was 50% before treatment, 10% by 5 WAT and 3% by 52 WAT, for a 94% reduction by one year after treatment. Native plants decreased slightly in plot 1 by 5 WAT increased in plot 1 by 52 WAT, and were less affected at both 5 and 52 WAT in plot 3 after treatment. While some level of native plant injury was measured in the year of treatment, by 52 WAT native plant diversity and frequency increased, and treatments were effective in reducing Eurasian watermilfoil distribution.


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