Cody J. Gray, J.D. Madsen, R.M. Wersal and K.D. Getsinger

Two invasive weed species, Eurasian watermilfoil (Myriophyllum spicatum L.) and parrotfeather [Myriophyllum aquaticum (Vell.) Verdc.], were grown in outdoor mesocosms to determine the efficacy of carfentrazone-ethyl (a,2-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]-4-fluorobenzenepropanoic acid, ethyl ester), alone and in combination with 2,4-D, a herbicide routinely used for watermilfoil control. Eurasian watermilfoil control was ≥ 98% when carfentrazone rates were applied alone at ≥ 150 μg ai L-1.
Carfentrazone alone initially controlled parrotfeather; however, tissue viability 3 weeks after treatment suggested plant recovery was likely. Both Eurasian watermilfoil and parrotfeather control was 100% when 2,4-D was applied at 1000 μg ae. L-1; however, when 2,4-D rate was reduced to 100 μg ae L-1 control declined to <50%. Herbicide applications containing carfentrazone with low rates of 2,4-D resulted in 100% death of both plant species. These results indicate that Eurasian watermilfoil control can be obtained using carfentrazone alone; but the addition of low levels of 2,4-D may be needed to achieve desired parrotfeather control.