Evaluation of flumioxazin on seven submersed macrophytes in New Zealand

Several invasive submersed aquatic plant species have established in many waterways within New Zealand, causing substantial economic, recreational, and ecological impacts. The herbicides currently registered for use for the management of submersed aquatic plants do not control all aquatic weed species under field conditions, and additional control methods are sought. This study evaluates the effectiveness of flumioxazin against four target submersed species (Ceratophyllum demersum, Egeria densa, Lagarosiphon major, and Elodea canadensis) and three nontarget native species (Myriophyllum triphyllum, Potamogeton ochreatus, and Nitella sp. aff. cristata). Single applications of flumioxazin required high concentrations (400 mu g ai L-1) to reduce C. demersum and L. major biomass by at least 50% at pH 8.4, E. densa was not controlled effectively at any rate (>= pH 8.4). However, low-rate applications (25, 50, 75, 100 mu g ai L-1) followed by high-rate applications (100, 200, 300, 400 mu g ai L-1, respectively) provided additional control of all species exposed to flumioxazin. M. triphyllum and P. ochreatus biomass was reduced with increasing concentration, N. sp. aff. cristata showed no symptoms from the application of flumioxazin. The potential use of flumioxazin in New Zealand is restricted, because susceptible aquatic weeds are required to be growing in low pH (less than 8.5) waters to achieve effective control. To overcome this potential restriction a second application of flumioxazin within a short period (ca. 1 mo) following initial application to less dense vegetation could substantially improve efficacy under these conditions.