Comparison of Imazapyr and Imazamox for Control of Parrotfeather ( Myriophyllum aquaticum (Vell.) Verdc.)

Parrotfeather ( Myriophyllum aquaticum (Vell. Verdc) is an invasive aquatic plant to the United States that is native to South America. Myriophyllum aquaticum is described as “stout, stems moderately elongate, partially submersed but with portions of leafy branches emersed (Godfrey and Wooten 1981). Emergent leaves are whorled, stiff, usually with 20 or more linear filiform divisions, appearing feather-like and grayish green. Submersed shoots are comprised of whorls of four to six filamentous, pectinate, red or orange, leaves arising from each node. Flowers are all pistillate, borne in the axils of unreduced leaves (Godfrey and Wooten 1981). Myriophyllum aquaticum is dioecious, however only pistillate plants are found outside of its native range (Sutton 1985). In fact, staminate plants are rare even in native populations of South America (Orchard 1981). For this reason, seed production is not known to occur and reproduction is exclusively vegetative through fragmentation of emergent shoots, submersed shoots, and stolons (Aiken 1981, Orchard 1981). Additionally, M. aquaticum lacks structures for storage, dispersal, and perennation (e.g., tubers, turions, and winter buds) and therefore stolons are believed to serve all these functions (Sytsma and Anderson 1993). Myriophyllum aquaticum grows rapidly and can persist as a submersed plant or more commonly grows as an emergent creeping stoloniferous perennial. The creeping growth has caused major problems in water-bodies in the United States, where such infestations of have reduced access, use, and runoff in ditches, streams, ponds, and shallow lakes (Sutton 1985). The creeping growth of M. aquaticum enables it to cover large areas of a water-body in a short period of time which impedes navigation, stream flow, and runoff to such an extent that flooding of adjacent lands occurs (Sutton 1985). Myriophyllum aquaticum also provides mosquito larvae a refuge from predation and can indirectly aid in the spread of insect born diseases (Orr and Resh 1989). The problems posed by M. aquaticum are often perpetuated as this species is widely cultivated and sold in the United States via the water garden industry (Aiken 1981, Sutton 1985). Myriophyllum aquaticum is difficult to control and once established, it is capable of thriving in a variety of environmental conditions in addition to the deployment of management techniques (Moreira et al. 1999). To date, chemical control has been the most effective method for controlling infestations of M. aquaticum . Contact herbicides such as diquat (6,7-dihydrodipyrido (1,2-a:2’,1’-c) pyrazinedium dibromide) and endothall (7-oxabicyclo (2.2.1) heptane-2,3-dicarboxylic acid) have been evaluated with mixed results (Moreira et al. 1999, Westerdahl and Getsinger 1988). Contact herbicides are typically effective for short-term control, but significant regrowth of M. aquaticum typically occurs and multiple applications are necessary (Moreira et al. 1999). Therefore, the use of a systemic herbicide may be more effective in controlling this species. Imazapyr (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)5-oxo1H-imazol-2-yl]-3-pyridinecarboxylic acid) is a systemic herbicide that has been labeled for aquatic use and has shown promise in controlling smooth cordgrass ( Spartina alterniflora Loisel) (Patten 2003) and torpedograss ( Panicum repens L.) (Hanlon and Langeland 2000). Imazamox, 2-(4,5-dihydro4-methyl-4-(1-methylethyl)-5-oxo-1H imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid), is a new herbicide currently being evaluated for use in aquatic systems under an experimental use permit (EUP) from the United States Environmental Protection Agency. Both imazapyr and imazamox belong to the imidazolinone herbicide family. The site of action of the imidazolinone herbicides is the acetohydroxyacid synthase (AHAS) enzyme, the first step in the biosynthesis of the essential branched chain amino acids isoleucine, leucine and valine (WSSA 2002). Imidazolinone herbicides inhibit the production of AHAS enzymes resulting in a lethal decrease in protein synthesis (Shaner and Mallipudi 1991). Both imazapyr and imazamox are rapidly absorbed into the foliage and translocated through the target plants via phloem and xylem tissues (Shaner and Mallipudi 1991, WSSA 2002). Both herbicides inhibit plant growth within the first 24 hours after application; however, with visual symptoms typically appear at least one week after treatment (WSSA 2002). The meristematic regions of the plant are targeted with symptoms of slow foliar chlorosis and necrosis (WSSA 2002). To date, there have been no published data on the use of imazapyr for control of M. aquaticum and no published data on the use of imazamox for the control of any aquatic plants. The objective of this study was to evaluate the efficacy of various rates of imazapyr and imazamox for the control of M. aquaticum . The systemic properties of these herbicides may provide control of the entire M. aquaticum plant, not just the emergent portion that is typically affected by application of contact herbicides. 1 GeoResources Institute, Mississippi State University, Box 9652 Mississippi State, MS 39762