2-17498 p56-58 APMdj

Parrotfeather ( Myriophyllum aquaticum Vell. Verdc.) and Eurasian watermilfoil ( Myriophyllum spicatum L.) are non-native invasive species that are often difficult to control. Once established, these species thrive in a variety of environmental conditions and have shown resiliency to control techniques. To date, chemical control has been the most effective method for managing infestations of these species. Contact herbicides such as diquat (6,7-dihydrodipyrido (1,2-a:2’,1’-c) pyrazinedium dibromide) and carfentrazone-ethyl (a,2dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo1 H -1,2,4-triazol-1-yl]-4-fluorobenzenepropanoic acid, ethyl ester) are used to rapidly kill standing biomass (Westerdahl and Getsinger 1988, Moreira et al. 1999). These rapid-acting contact herbicides often provide only short-term control and significant regrowth of non-impacted plant tissues is common. Diquat is a photosynthesis inhibitor that interferes with electron flow by accepting electrons from photosystem I. The interference of electron flow leads to the production of superoxide radicals that ultimately results in the peroxidation of cell membranes (Hess 2000, Senseman 2007). Diquat symptoms appear within hours of application in full sunlight, with complete foliar necrosis by 1 to 3 days after application (Senseman 2007). Diquat has shown excellent efficacy on Eurasian watermilfoil where 90 to 100% control was achieved under short half life scenarios (Skogerboe et al. 2006); however, there are few published accounts of its efficacy on parrotfeather. Carfentrazone-ethyl is a protoporphyrinogen oxidase inhibitor approved for use in aquatic systems in 2004. Carfentrazone-ethyl competes with protoporphyrinogen for binding sites on the protoporphyrinogen oxidase enzyme (Devine et al. 1993, Hess 2000). This competition causes protoporphyrinogen to leak into the cytoplasm and, in the presence of light, to be converted to protoporphyrin IX. Protoporphyrin IX reacts with oxygen in the presence of light to form singlet oxygen radicals that cause the oxidation of lipid membranes, resulting in cell death (Devine et al. 1993, Hess 2000). Foliar desiccation is rapid, and susceptible plants become necrotic and die within a few days after treatment (Senseman 2007). Carfentrazone-ethyl has shown variable control of parrotfeather where ratings ranged from 29 to 70% (Glomski et al. 2006, Gray et al. 2007). Eurasian watermilfoil control was ≤ 70% in those same studies. Both diquat and carfentrazone-ethyl require sunlight for activity, so when applied during daylight hours, herbicide symptoms appear rapidly at the point of contact. The rapid destruction of plant tissues by these herbicides results in selflimited translocation of the herbicide in the plant and subsequently limits damage to the point of contact (Slade and Bell 1966, Funderburk and Bozarth 1967, Senseman 2007). A possible method to increase herbicide uptake and translocation into target plants may be to apply these herbicides at night to allow greater herbicide movement into target plants prior to light activation and significant tissue damage. Baldwin (1963) reported that a period of darkness following diquat applications resulted in considerable amounts of herbicide being transported away from the point of contact. This relationship was also noted with paraquat ( N , N ’-dimethyl-4,4’-bipyridinium dichloride) (Slade and Bell 1966). The objective of this study was to compare the effectiveness of subsurface applications of diquat and carfentrazone-ethyl applied to parrotfeather and Eurasian watermilfoil under light and dark conditions. This study is a first account of diquat efficacy on parrotfeather, especially as a subsurface application.