MESOCOSM EVALUATION OF BENSULFURON METHYL ACTIVITY ON EURASIAN WATERMILFOIL, VALLISNERIA, AND AMERICAN PONDWEED

Concentration/exposure time relationships for the compound bensulfuron methyl (methyl yl)amino]-carbonyl]amino]sulfonyl]methyl]benzoate) aga inst Eurasian watermilfoil (Myriophyllum spicatum L.), vallisneria (Va llisneria americana Michx.), and American pondweed (Potamogeton nodosu s Poiret) were evaluated in a large outdoor mesocosm system. Initial t reatment rates ranged from 25 to 100 mug/L, and plants were exposed fo r a 12-week period. Estimates of plant control were based on weekly me asurements of shoot height, biomass harvested at 6 and 12 weeks posttr eatment, and regrowth of root crowns following removal from herbicide- treated conditions. Biomass of Eurasian watermilfoil averaged approxim ately 50% less than that of untreated references after 6 weeks exposur e to all treatments. At 12 weeks exposure, Eurasian watermilfoil bioma ss was reduced 96 to 98% compared to untreated references at all chemi cal rates tested. Shoot height of untreated Eurasian watermilfoil had reached the water surface (100 cm), but averaged less than 20 cm in he ight in all bensulfuron methyl treatments. Vallisneria and American po ndweed exhibited greater than 95% reduction in biomass at all chemical rates tested compared to untreated references at 12 weeks posttreatme nt. Shoot heights of vallisneria and American pondweed were reduced gr eater than 90% at all bensulfuron methyl treatment rates compared to u ntreated references. When removed from herbicide-treated conditions, s ome root crowns from all species initiated regrowth. After 12 weeks ex posure, results show that bensulfuron methyl contact time may be more important than rate of application for controlling the species tested.