Comparison of Subsurface and Foliar Herbicide Applications for Control of Parrotfeather (Myriophyllum aquaticum)

Parrotfeather is an invasive, aquatic plant in the United States that is native to South America. It has impaired the use of water bodies throughout the United States and is difficult to control, despite using a variety of management techniques. Our objectives were to examine the efficacy of subsurface applications of seven herbicides labeled for aquatic use and to compare those applications to herbicides that can also be applied to emergent foliage. A replicated mesocosm study was conducted in 378-L (100-gal) tanks beginning in August 2007 and repeated during the same period in 2008. The maximum and half-maximum labeled rates of copper chelate, diquat, endothall, fluridone, triclopyr, and carfentrazone-ethyl were applied to the water column in designated mesocosms. The maximum labeled rate for foliar applications of diquat, triclopyr, and 2,4-D were used to compare treatment methods. Six weeks after treatment (WAT), copper, endothall, fluridone, and carfentrazone-ethyl did not achieve 90% control; in fact, control was less than 50% for each herbicide, and therefore, the herbicides were not considered efficacious for controlling parrotfeather. Diquat at all rates and application methods resulted in 70 to 90% biomass reduction. Triclopyr, with both the highest aqueous concentration and foliar application, resulted in an 84 and 86%, respectively, reduction in biomass at 6 WAT. The foliar application of 2,4-D was the only herbicide and application method that resulted in >= 90% biomass reduction of parrotfeather. In these studies, regrowth occurred in all tanks regardless of herbicide or treatment method, indicating multiple applications would be necessary to provide longer-term plant control. Future research should identify possible herbicide combinations or timing of applications to maximize treatment efficacy.