Selective Control of Eurasian Watermilfoil and Curlyleaf Pondweed in Noxon Rapids Reservoir, Montana: Herbicide Small-Plot Evaluations, 2010-2011

Abstract

Noxon Rapids Reservoir, Montana, is one of several large impoundments on the Lower Clark Fork River, stretching for over 48 km (30 miles) with a surface area of ~ 3,120 ha (7,700 acres). Management strategies were evaluated for their effectiveness in controlling invasive plant problems in the reservoir, specifically with Eurasian watermilfoil and curlyleaf pondweed. A 3-year field program was developed to evaluate the effectiveness of aquatic herbicides to selectively control the invasive plants.The herbicide endothall (Aquathol® K) was applied to four plots totaling 5.5 ha (13.6 acres) at 3000 µg ai/L (3 ppm); diquat (Reward®) was applied to four plots totaling 3.3 ha (8.1 acres) at 370 µg ai/L (0.37 ppm); and a combination of both products was applied to four plots totaling 4.7 ha (11.5 acres), with endothall at 1500 µg ai/L (1.5 ppm) and diquat at 190 µg ai/L (0.19 ppm). Herbicides were applied by boat using a variabledepth injection system. Aqueous herbicide dissipation was monitored in selected plots. Bulk water exchange processes were also measured. Quantitative surveys were conducted in each plot to assess the plant community at pretreatment, and at 6 weeks and 52 weeks post treatment.Treatments provided significant reductions in Eurasian watermilfoil (59-69%) and curlyleaf pondweed (40-60%), through 52 weeks post treatment. Both products provided some degree of selective control, with a variety of native plants surviving the treatments. DISCLAIMER:The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR.