Selective Effects of Aquatic Herbicides on Sago Pondweed

Three aquatic herbicides effective on the exotic weed Eurasian watermilfoil (Myriophyllum spicatum L.) - 2,4-D ((2,4-dichlorophenoxy)acetic acid), endothall (7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic acid) and triclopyr ([(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid) - were evaluated in the laboratory for selective effect or efficacy on the native submersed species, sago pondweed (Potamogeton pectinatus L.). For each herbicide, three concentrations in ranges associated with Eurasian watermilfoil or sago pondweed control were applied in static exposures of 24 hr, and plants were monitored for 35 d. Endothall at 0.5, 1 and 2 mg L super(-1) significantly reduced final biomass, by greater than or equal to 72%, confirming that this herbicide will not maintain populations of sago pondweed where it is used to manage Eurasian watermilfoil. Application of the growth regulator-type systemic herbicides at 1, 1.5 and 2 mg L super(-1) resulted in no significant reduction in biomass from 2,4-D, but up to 24% reduction with triclopyr. The more selective activity of these compounds towards sago pondweed supports their use for controlling Eurasian watermilfoil in plant communities where it is desirable to maintain the native species. However, in areas where sago pondweed is itself a nuisance plant, endothall gives effective chemical control.