J. Aquat. Plant Manage. 51: 77–83 Interactions of herbicides, surfactants, and the giant salvinia weevil (Cyrtobagous salviniae) for control of giant salvinia (Salvinia molesta) CHRISTOPHER R. MUDGE, NATHAN E. HARMS, AND JULIE G. NACHTRIEB* ABSTRACT INTRODUCTION Herbicides and the biological control agent Cyrtobagous Giant salvinia is a free-floating, mat-forming aquatic fern salviniae Calder and Sands (giant salvinia weevil, Coleoptera: native to southeastern Brazil (Forno and Harley 1979) that Curculionidae) are the most effective means to manage the has become problematic in water bodies throughout the floating aquatic invasive giant salvinia (Salvinia molesta southeastern United States, Puerto Rico, and Hawaii. This Mitchell) in North America. Limited efforts have been species dominates water bodies where dense infestations made to integrate these technologies and no information is disrupt transportation, hinder water uses, impact desirable available on the direct and indirect impacts of herbicides on native plant communities, and increase mosquito breeding giant salvinia weevils. Flumioxazin 2-[7-Fluoro-3,4-dihydro- habitat (Jacono 1999, Jacono and Pitman 2001, Nelson et al. 3-oxo-4-(2-propynyl)-2H-1,4-benzoxazin-6-yl]-4,5,6,7-tetra- 2001). It is estimated that under optimal growth conditions, hydro-1Hisoindole-1,3(2H)-dione and (penoxsulam) 2-(2,2- plants can double in coverage every 36 to 53 h (Cary and difluoroethoxy)-N-(5,8-dimethoxy[1,2,4]triazolo[1,5-c]pyri- Weerts 1983, Johnson et al. 2010). This plant has become midin-2-yl)-6-(trifluoromethyl)benzenesulfonamide, ap- especially problematic in Texas and Louisiana and, by 2004, plied directly to the weevils resulted in less than 5% had been reported in 4 reservoirs, 5 rivers (or streams) and weevil mortality, whereas the addition of an aquatic 20 ponds in Texas alone (Owens et al. 2004). Although an surfactant (nonionic and buffering agent [surfactant A]) estimate of current total acreage in Texas is not available, 17 alone and in combination with the herbicides resulted in 20 major water bodies are confirmed to be infested by giant to 47% giant salvinia weevil mortality. Alternate surfactants, salvinia (H. Elder, pers. comm.). In 1999, an initial including a methylated vegetable oil and organo-silicone infestation in Louisiana estimated to be 400 A expanded blend (surfactant B), a silicone-polyether copolymer (sur- to . 28,340 ha in 20 lakes, 7 bayous or rivers, the factant C), and a nonionic organo-silicone blend (surfactant Atchafalaya River basin, the Red River, and the coastal D) resulted in 22 to 23% mortality. In a mesocosm fresh water marsh from Lafitte to Morgan City (Johnson et experiment, all weevil and herbicide treatments (alone or al. 2010). combination) resulted in 52 to 97% reductions in giant Management of giant salvinia has been attempted via salvinia biomass by 4 wk after treatment (WAT). By the chemical, biological, mechanical, and physical control conclusion of the experiment (6 WAT), flumioxazin, methods (Madsen and Wersal 2009), with chemical and flumioxazin plus giant salvinia weevil, and penoxsulam plus biological methods being more widely used in the United giant salvinia weevil resulted in 98 to 100% plant control. States. When applied to smaller or less dense populations of Treatment with the giant salvinia weevil alone caused a giant salvinia, herbicide treatments can selectively and significant reduction in biomass (68%) and continued to precisely provide rapid control. Although herbicide control damage plant tissue at 6 WAT. The mesocosm experiment programs have increased over recent years, giant salvinia also provided evidence of the minimal indirect impacts infestations continue to expand (Sanders et al. 2010). In an herbicides and surfactants will have on the giant salvinia attempt at management, rearing, harvesting, and release of weevil. The experiment also demonstrated that giant the giant salvinia weevil (Cyrtobagous salviniae Calder and salvinia weevils were capable of surviving at least 4 wk on Sands), a biological control agent, has increased in recent plant material treated with foliar applications of flumiox- years (Harms et al. 2009, Sanders et al. 2010). The giant azin and penoxsulam. salvinia weevil, originally occurring in southeastern Brazil, Key words: biological control, chemical control, flumiox- Bolivia, Paraguay, and northern Argentina (Wibmer and azin, foliar-applied herbicides, integrated pest management, O’Brien 1986, Calder and Sands 1985), was first released in penoxsulam. the United States in 2001 at sites in Louisiana and Texas, with a subsequent reduction in plant populations observed *First and second authors: Research Biologists, U.S. Army Engineer at release sites (Tipping 2004, Tipping et al. 2008). Research and Development Center, 3909 Halls Ferry Rd., Vicksburg, MS Successful control of giant salvinia below problematic levels 39180; third author: Research Biologist, U.S. Army Engineer–Lewisville has also been achieved in Zimbabwe, South Africa, Senegal, Aquatic Ecosystem Research Facility, 201 East Jones St., Lewisville, TX 75057. Corresponding author’s E-mail: Christopher.R.Mudge@usace. Mauritania, and India, often within 2 yr after initial stocking army.mil. Received for publication and in revised form. Received for (Jayanth 1987, Cilliers 1991, Chikwenhere and Keswani publication November 6, 2012 and in revised form April 11, 2013. 1997, Pieterse et al. 2003, Diop and Hill 2009). Despite the J. Aquat. Plant Manage. 51: 2013 77 reported success of this biocontrol agent in other parts of MATERIALS AND METHODS the world, limited distribution of the giant salvinia weevil and minimal large-scale releases in the United States have Direct impacts of herbicides and surfactants on giant likely hindered potential effectiveness (Mudge and Harms salvinia weevils 2012). In addition, severe winters can limit the increase and spread of giant salvinia weevil populations in the spring, Experiment 1. This experiment was conducted at the U.S. and maintenance of populations may be necessary (Tipping Army Engineer Research and Development Center (ERDC) et al. 2008). in Vicksburg, MS. Adult giant salvinia weevils were In contrast to a herbicide- or biocontrol-focused man- collected from cultures in aboveground rearing boxes (1.5 agement program, a more prudent approach may be to 3 3.0 3 0.6 m deep [4.9 3 9.8 3 2.0 ft]) at the U.S. Army combine technologies to achieve rapid biomass reduction Engineer Research and Development Center, Lewisville and long-term control of giant salvinia. Minimal research Aquatic Ecosystem Research Facility (LAERF), Lewisville, has been conducted to determine the potential of combin- TX. Weevils were reared on giant salvinia supplemented ing herbicides and biological agents (Center et al. 1999) or with nutrients approximately every month to maintain 10ppm mg LÀ1 nitrogen (Miracle-Grot1) and 3 mg LÀ1 mycoherbicides (Nelson et al. 1998) for the management of 2 aquatic plants. Possible candidates for evaluations should iron. Weevils were harvested for experimentation on 3 include the recently registered, reduced-risk aquatic herbi- separate occasions from May to June 2011. Collection cides. These newer chemistries are highly plant-specific occurred through Berlese funnel extraction (Harms et al. (minimal toxic impacts on animals), applied at very low use 2009) into glass jars containing moist paper towels, which rates (g ai haÀ1) and concentrations (lgaiLÀ1), and possess a were shipped overnight to ERDC. Weevils were immediately high degree of selectively against target plants, thereby transferred onto fresh giant salvinia contained in 1.056 minimizing damage to desirable vegetation (Koschnick et al. gallon plastic containers with nutrient (Miracle-Gro)- 2007, Mudge 2007, Glomski and Mudge 2009). amended water. Weevils were allowed to acclimate in a 8 In exploring the compatibility of these 2 technologies, walk-in growth chamber for 6 d at a temperature of 80.6 the direct and indirect impacts of herbicides on the giant and a 14-h : 10-h (light : dark) photoperiod. salvinia weevil should be considered, as impacts to giant Prior to herbicide exposure, 15 adult weevils per replication (n ¼ 3; 45 weevils per treatment) were removed salvinia weevil fitness may alter their long-term effective- from the giant salvinia plants with forceps and placed into ness. In addition to herbicides, surfactants typically used in petri dishes for exposure. Treatments were replicated 3 combination with foliar-applied herbicides should be times and randomly assigned. Solutions of flumioxazin, examined for their impact on the giant salvinia weevil. (quick-acting contact herbicide) and penoxsulam, (a slow- Surfactants, which are a type of adjuvant, improve the acting systemic herbicide) were prepared by diluting emulsifying, dispersing, spreading, and wetting, as well as formulation concentrates in distilled water equivalent to a increasing the spray coverage on the foliage to aid in diluent of 200 gallons. The aquatic herbicides flumioxazin3 herbicide uptake by the plant (Ferrell et al. 2008). (12 oz AÀ1) and penoxsulam4 (5.6 oz AÀ1) were used for the Surfactants enhance the herbicide application or efficacy experiment (Table 1). In addition, a surfactant (nonionic and do not necessarily provide control as stand-alone and buffering agent blend) at 0.25% v/v was evaluated alone treatments. The inclusion of certain ingredients in adjuvant and in combination with the herbicides to determine direct formulations is regulated by the