MESOCOSM EVALUATION OF THE SPECIES-SELECTIVE POTENTIAL OF FLURIDONE

Fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl) phenyl]-4(1H)-pyri dinone} was evaluated at rates of 0, 5, 10, and 20 mu g/L in 6,700-L o utdoor mesocosm tanks for selective control of the exotic species Eura sian watermilfoil (Myriophyllum spicatum L). Non-target species includ ed elodea (Elodea canadensis Michaux), American pondweed (Potamogeton nodosus Poiret), sage pondweed (Potamogeton pectinatus L), and vallisn eria (Vallisneria americana Michaux). Throughout the study, untreated mesocosms were dominated by Eurasian watermilfoil and elodea with limi ted growth of vallisneria and the two pondweed species. Fluridone trea tments were conducted on April 23 and May 23 1995 to determine if plan t growth stage affected species selectivity. Fluridone residues were m onitored in water and target concentrations were achieved with a measu red fluridone half-life of 33 days. The 5 mu g/L treatments in April ( 90 days of exposure) and May (60 days of exposure) reduced biomass of Eurasian watermilfoil by > 90% in late July while biomass of non-targe t species generally exceeded the levels of untreated reference plants. Treatments of 10 and 20 mu g/L also reduced Eurasian watermilfoil bio mass by > 90%; however, these application rates also reduced growth se veral non-target species by > 90%. Chara (Chara spp.) and southern nai ad (Najas guadalupensis (Sprengel) Magnus) propagules sprouted and att ained high levels of biomass in mesocosms treated at 10 and 20 mu g/L. With the notable exception of vallisneria, plants exposed to 10 and 2 0 mu g/L in April (90 day exposure) did not show potential for recover y when placed in untreated water for 40 days. In contrast, when transf erred to untreated water all species (with the exception of sage pondw eed) exposed to 5, 10, and 20 mu g/L in May (60 day exposure) recovere d significantly in comparison to plants that remained exposed to low c oncentrations of fluridone (1 to 4 mu g/L). Results suggest that fluri done can selectively control Eurasian watermilfoil; however, initial t reatment rate, length of exposure, and initial biomass of the plants a re key factors. The range of fluridone concentrations that provide sel ective control of Eurasian watermilfoil in a mixed plant community may be quite narrow.