Use of a small-scale primary screening method to predict effects of flumioxazin and carfentrazone-ethyl on native and invasive, submersed plants

When evaluating potential use patterns of new aquatic herbicides, it is important to determine effects on target as well as nontarget vegetation. Small-scale primary screens that provide data on the relative sensitivity of a species to a given herbicide or herbicide use rate can be used to enhance the design of more-costly and time-consuming, large-scale, growth-chamber and mesocosm studies. Flumioxazin (2-[7-fluoro-3,4-dihydro-3-oxo-4-(2-propynyl)-2H-1,4-benzoxazin-6-yl]-4,5,6,7-tetrahydro-1H-isoindole- 1,3(2H)-dione) and carfentrazone-ethyl (ethyl alpha,2-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4- triaol-1-yl]-4-fluorobenzenepropanoate) are protoporphyrinogen oxidase (protox)-inhibiting herbicides recently registered by the U.S. Environmental Protection Agency (USEPA) for use in aquatic sites. These protox inhibitors disrupt chlorophyll synthesis and result in formation of oxygen radicals that damage cell membranes, causing them to leak cell contents. This leakage of electrolytes was measured and used to determine herbicide injury following exposure to protox inhibitors. Electrolyte leakage was measured at 5 d posttreatment for 15 submersed aquatic plant species exposed to flumioxazin at 0, 200, and 400 mu g ai (0, 0.0000265, and 0.000053 ppb) and to carfentrazoneethyl at 0 and 200 mu g ai L-1 in small-scale laboratory assays. In these assays, flumioxazin significantly increased electrolyte leakage for coontail (Ceratophyllum demersum L.) and curlyleaf pondweed (Potamogeton crisps L.). Eurasian water-milfoil (Myriophyllum spicatum L.). fanwort (Cabomba caroliniana Gray), hydrilla verticillata (L. 11) Royle], longbeak buttercup (Ranunculus longirostris Godr.). springtape (Sagittaria kurziana Gluck), variable watermilfoil (Myriophyllum heterophyllum Michx.), and waterstargrass [Heteranthera (labia (Jacq.) MacM.]. Species that were not significantly affected by flumioxazin included American pondweed (Potamogeton nodosus Poir.), common elodea (Elodea canadensis Michx.), Illinois pondweed (Potamogeton illinoensis Morong), sago pondweed Stuckenia pectinatus (L.) Boerner], southern naiad [Najas guadalupensis (Spreng.) Magnus], and American eel-grass (Vallisneria americana Michx.). Of the species tested, carfentrazone-ethyl only increased electrolyte leakage of coontail. Eurasian watermilfoil, and variable watermilfoil.