Selective Control of Eurasian Watermilfoil and Curlyleaf Pondweed in Noxon Rapids Reservoir, Montana: Aquatic Herbicide Evaluations, 2009-2010

A field demonstration was developed linking herbicide application methods with site-specific water exchange patterns to selectively control infestations of Eurasian watermilfoil (EWM) and curlyleaf pondweed (CLP) in Noxon Rapids Reservoir, MT. Objectives of this work are to evaluate species-selective control of these invasive plants employing innovative herbicide application techniques; and to provide recommendations for invasive plant management in the reservoir, and similar impoundments in the Pacific Northwest. Bulk water exchange patterns occurring in plant stands selected for herbicide applications were determined using rhodamine WT (RWT) tracer dye. These site-specific patterns were matched with appropriate herbicide application rates required to selectively control target plants. Treatments were conducted using a variable-depth injection system, simultaneously applying RWT and herbicides to provide maximum chemical contact time around plants stands. In late July 2009, two plots (8.2-11.5 ha) were treated using combinations of RWT (10 g/L), triclopyr (1300 - 1850 g/L), and endothall (1890 - 2500 g/L). Dye (in situ) and herbicide residues (via enzyme-linked immunosorbent assay) were measured through the water column, inside and outside of the plots. Applications were conducted to coincide with the minimum reservoir discharge patterns. Whole plot water exchange half-lives ranged from 16 to 33 hr. Herbicide residues were highest around plants growing in the lower half of the water column (19-48 hr). External herbicide dissipation patterns were below levels of environmental/human health concerns. Treatments provided selective control of EWM for two years ( 85%) and CLP for one year ( 75%). Native plant species richness and dissolved oxygen levels were unchanged in treatment plots during the study period. Prepared in cooperation with the Geosystems Research Institute, Mississippi State University, Starkville, the Crop Science Department, North Carolina State University, Raleigh, and the School of Aquatic and Fishery Science, University of Washington, Seattle.