Plant Community Response in Small Plots One Year after Treatment with Triclopyr and Endothall in Noxon Rapids Reservoir, MT, 2011

Aquatic plants are important to lake ecosystems. The introduction of nonnative plants into littoral zone habitats often alters the complex interactions occurring in these areas. Dense stands of non-native plants are often responsible for reduction in oxygen exchange, depletion of dissolved oxygen, increases in water temperatures, and internal nutrient loading. Eurasian watermilfoil (Myriophyllum spicatum L.) is a non-native invasive species that, when present, has been associated with declines in native plant species richness and diversity. Although the impacts form Eurasian watermilfoil are numerous, controlling this species is often difficult and unpredictable. Flowing water complicates use of herbicides as water flow will increase the dilution and dissipation of the herbicides. Herbicide applications in run of the river reservoirs are often subject to more extreme perturbations than those of natural lakes. The use of auxin mimicking herbicides such as 2,4-D and triclopyr, and the contact herbicide endothall have been used extensively for Eurasian watermilfoil control. Additionally, herbicide concentration exposure time (CET) relationships have been designed under controlled conditions to guide management decisions on choosing the correct herbicide concentration with respect to contact time. However, little data exists with respect to combining a contact herbicide with a systemic herbicide to reduce the exposure time requirements and maintain plant control. Objectives of this study were to: 1) Demonstrate at the field scale the effectiveness of combining triclopyr with endothall for control of Eurasian watermilfoil and curlyleaf pondweed in flowing systems. 2) Evaluate the aquatic plant community response to herbicide treatments one year after treatment.

A Report Submitted to Sanders County, MT, the Montana Weed Trust, and the U.S. Army Engineer Research and Development Center. Funding was provided by a grant under the American Recovery and Re-Investment Act and the Montana Weed Trust as a subcontract from Sanders County, Montana to Mississippi State University.