Effect of glyphosate rate and spray volume on control of giant Salvinia

Giant salvinia ( Salvinia molesta (D. S. Mitchell)) is a freefloating, aquatic fern native to Brazil that has established and become a nuisance in many lakes, rivers, irrigation canals, and reservoirs of the United States (Jacono 1999, Jacono and Pitman 2001, McFarland et al. 2004). Outside its native range, giant salvinia is considered one of the world’s worst weeds due to its prolific growth habit, effective means of dispersal, tolerance to adverse growing conditions, and difficulty of control (Oliver 1993, Jacono and Pitman 2001, McFarland et al. 2004). Heavy infestations not only limit human use of water resources (fishing, boating, irrigation) but can also negatively impact the ecology of aquatic systems by restricting light penetration, degrading water quality (decreased dissolved oxygen and pH), and reducing valuable native plant and animal habitat and biodiversity (Thomas and Room 1986, Oliver 1993, McFarland et al. 2004). Since its discovery in South Carolina in 1995 (Johnson 1995), giant salvinia has spread to more than 90 locations in 41 freshwater drainages of 12 states (Jacono and Richerson 2005, 2003). According to recent estimates by Jacono and Richerson (2003), giant salvinia can be expected to establish wherever waterhyacinth ( Eichhornia crassipes (Mart.) Solms.) persists. This would include aquatic habitats throughout the Atlantic Coastal Plain, from southeastern Virginia through Florida, west across the Gulf Coast states to Arizona, and north through central California. Although giant salvinia is listed as a Federal Noxious Weed by the U.S. Department of Agriculture (http:www.aphis.usda.gov/ppq/weeds/), expansion will likely continue since quarantines and the sale and distribution of this plant by the nursery industry have been difficult to enforce nationwide. In a recent survey of mail-order catalogs and on-line commercial websites, Kay and Hoyle (2001) found that giant salvinia was among the many noxious aquatic plants readily available for purchase over the Internet. The systemic herbicide, glyphosate ( N -(phosphonomethyl)glycine), is an effective chemical treatment for managing giant salvinia. Results of outdoor herbicide trials showed that 8.97 kg ae ha -1 glyphosate mixed with a nonionic surfactant, controlled 99% of giant salvinia 42 days after treatment (Nelson et al. 2001). In laboratory studies, Fairchild et al. (2002) reported significant control of giant salvinia following glyphosate application over a broad range of rates (0.45 to 3.60% solution mixed v:v). Currently, the efficacy of low-rate glyphosate applications has not been evaluated on giant salvinia grown under outdoor conditions. Reducing the rate of application would be more economical and reduce chemical inputs to the aquatic environment. Since giant salvinia frond surfaces are covered with numerous trichomes which can impede herbicide deposition and penetration, it has been suggested that adjuvant selection and ample wetting of frond surfaces could be critical factors for maximizing herbicide efficacy. Observations of outdoor tank experiments, demonstrated that adjuvant type (non-ionic, organosilicone, silicone blends, and combinations of these products) did not affect glyphosate performance against giant salvinia (L. Nelson, unpublished data). Fairchild et al. (2002) reported that glyphosate, with or without adjuvant addition, exhibited activity against giant salvinia. However, complete plant mortality using low glyphosate rates (0.45% solution) was achieved only when mixed with Optima® (Helena Chemical Company, Collierville, TN), a specialized adjuvant blend of wetting and buffering agents. This data suggests that adjuvant choice may be critical for enhancing glyphosate performance when low application rates are used. Variability among surfactant effects on glyphosate efficacy has been reported for other plant species (O’Sullivan et al. 1981, Nalewaja and Matysiak 1992, Riechers et al. 1995). Spray volume also affects herbicide deposition on leaf surfaces and can impact herbicide performance. Previous research has demonstrated that post-emergent herbicides, such as glyphosate, control weeds better when applied in low volumes of water than when applied in high volumes (Buhler and Burnside 1983, 1987). Low spray volumes are preferred by applicators because of savings in time when filling sprayer tanks and traveling to and from treatment sites. Excluding aerial applications, aquatic herbicides are typically delivered in total spray volumes between 468 to 1870 L water ha -1 (50 to 200 gal water A -1 ). Information concerning the effect of spray volume on glyphosate performance in relation to floating aquatic plants like giant salvinia, should be investigated. Therefore, the objectives of these studies were to determine the effect of glyphosate rate and spray volume on control of giant salvinia grown under outdoor conditions.