Selective Control of Eurasian Watermilfoil and Curlyleaf Pondweed in Noxon Rapids Reservoir, Montana: Aquatic Herbicide Evaluations, 2009-2010
ENGINEER RESEARCH AND DEVELOPMENT CENTER VICKSBURG MS ENVIRONMENTAL LAB
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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 gL, triclopyr 1300 - 1850 gL, and endothall 1890 - 2500 gL. 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 environmentalhuman 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.
- Agricultural Chemistry