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of $17.007 million and $15.126 mil- Evaluating the Effects of Acetic and lion in federal and state funds to d- on Four Aquatic Plants control aquatic plants in Florida’s public water bodies in fiscal year (FY) 2017–18 and FY 2018–19, respec- Lyn A. Gettys1, Kyle L. Thayer1, and Joseph W. Sigmon1 tively (FWC, 2018, 2019a). More than half of this funding ($10.01 million and $8.86 million in FY ADDITIONAL INDEX WORDS. broadleaf sagittaria, , Eichhornia crassipes, invasive , natural , organic herbicides, pickerelweed, Pistia 2017–18 and FY 2018–19, respec- stratiotes, Pontederia cordata, Sagittaria latifolia, vinegar, waterhyacinth, tively) was allocated for managing waterlettuce the submersed weed hydrilla (Hydrilla verticillata), whereas 25% of those SUMMARY. The foundation of most aquatic weed management programs in Florida is monies ($4.04 million in FY 2017–18 synthetic herbicides because many of these U.S. Environmental Protection Agency and $4.19 million in FY 2018–19) (USEPA)-registered products are effective, selective, and inexpensive compared with other strategies such as mechanical harvesting. However, stakeholders have was spent for floating plant control, expressed concern regarding their use and managers are interested in exploring al- which primarily comprise waterhya- ternative methods for aquatic weed control. To that end, we evaluated the efficacy, cinth (Eichhornia crassipes)and selectivity, and costs of the ‘‘natural’’ products acetic acid and d-limonene (alone and waterlettuce (Pistia stratiotes). Exces- in combination with each other and citric acid) on the invasive floating plants sive growth of floating plants causes waterhyacinth (Eichhornia crassipes) and waterlettuce (Pistia stratiotes), and the a number of problems in aquatic eco- native emergent plants broadleaf sagittaria (Sagittaria latifolia) and pickerelweed systems, including reducing the pene- (Pontederia cordata). These products, plus an industry-standard synthetic tration of and light into the (diquat dibromide), were applied once as foliar treatments to healthy plants, which water column by blocking the air– were grown out for 8 weeks after treatment to allow development of phytotoxicity water interface, creating monocultures symptoms. A 0.22% concentration of diquat dibromide eliminated all vegetation, but neither ‘‘natural’’ product alone provided acceptable (>80%) control of floating by outcompeting native plants, and weeds, even when applied at the maximum concentrations under evaluation (20% interfering with flood control opera- acetic acid, 30% d-limonene). Citric acid (5% or 10%) had no effect on the activity of tions by creating large, dense mats that acetic acid or d-limonene, but some combinations of acetic acid and d-limonene obstruct canals and water movement controlled floating weeds effectively without causing unacceptable damage to native structures (Gettys, 2019). Waterhya- plants. However, these treatments are much more expensive than the synthetic cinth, a Brazilian species, was intro- standard and managers would realize a 22- to 26-fold increase in product cost alone duced intentionally to Florida during without factoring in other expenses such as additional labor and application time. the late 1800s as a water garden orna- Combinations of acetic acid and d-limonene may have utility in some areas where mental and was released from cultiva- the use of synthetic herbicides is discouraged, but broad-scale deployment of this tion soon thereafter (Gettys, 2020a). strategy would likely be prohibitively expensive. The native range of waterlettuce is cryptic and may include the southeast- lorida’s resource managers are ern United States, but the species charged with keeping aquatic exhibits aggressive growth and is con- vegetation at maintenance levels sidered invasive in Florida regardless Received for publication 3 Dec. 2020. Accepted for F to facilitate navigation, flood control publication 17 Feb. 2021. of its true point of origin (Gettys, efforts, and other uses of state waters. 2020b). Published online 23 March 2021. This goal is most often achieved by As with all pesticides registered 1University of Florida, Institute of Food and Agricul- tural Sciences, Fort Lauderdale Research and Educa- using herbicides that have been ap- by the USEPA, aquatic herbicides are tion Center, 3205 College Avenue, Davie, FL 33314 proved by the U.S. Environmental only labeled for use if they ‘‘will not This research was supported by the Florida Agricul- Protection Agency (USEPA) for use generally cause unreasonable adverse tural Experiment Station and by the U.S. Department in aquatic systems, with statewide effects on the environment . . . taking of Agriculture, National Institute of Food and Agri- culture (HATCH project FLA-FTL-005682). Fund- oversight and coordination of treat- into account the economic, social, ing was also provided by the Florida Fish and Wildlife ments provided by the Florida Fish and environmental costs and benefits Conservation Commission. and Wildlife Conservation Commis- of the use of any pesticide’’ (USEPA, We thank Ian Markovich, Mohsen Tootoonchi, and sion (FWC, 2018, 2019a). For exam- 1996). However, public concerns re- Angie Diez for their invaluable contributions to this project. ple, the FWC oversaw the expenditure garding herbicide use in aquatic Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product and does not imply its approval to the exclusion of other products or vendors that also may Units be suitable. To convert U.S. to SI, To convert SI to U.S., This article is based on a presentation given at the multiply by U.S. unit SI unit multiply by 2020 Florida State Horticultural Society’s annual meeting in Oct. 2020. 3.7854 gal L 0.2642 9.3540 gal/acre LÁha–1 0.1069 L.A.G. is the corresponding author. E-mail: lgettys@ 2.54 inch(es) cm 0.3937 ufl.edu. 25.4 inch(es) mm 0.0394 This is an open access article distributed under the CC 0.4536 lb kg 2.2046 BY-NC-ND license (https://creativecommons.org/ 28.3495 oz g 0.0353 licenses/by-nc-nd/4.0/). 1 ppm mgÁL–1 1 https://doi.org/10.21273/HORTTECH04769-20 (F – 32) O 1.8 F C(C · 1.8) + 32

• April 2021 31(2) 225 systems have added to the challenges treatments state that target weeds regarding the efficacy of natural prod- faced by managers. The FWC should be sprayed to wet. Products ucts such as acetic acid and d-limo- ‘‘paused’’ chemical weed manage- that include broadcast instructions in- nene as weed control agents, either ment activities in early 2019 in re- dicate that 15 to 30 gal/acre of prod- alone or in combination. Domen- sponse to public outcry and to uct should be used. ghini (2020) reported that acetic acid provide an opportunity for stake- There are many other acetic acid applied at a concentration of 20% or holder voices to be heard. A primary products on the market, but most are 30% could be a viable alternative to message that arose from listening not specifically labeled for weed con- glyphosate, but that multiple applica- sessions during the pause was that trol. For example, a note on the tions would be necessary for pro- the public believes ‘‘chemical’’ (her- website for Bradfield Natural Horti- longed weed control. Webber et al. bicide) usage in aquatic systems cultural Vinegar (20% acetic acid) (2018) evaluated 5% and 20% acetic should be reduced drastically (FWC, states, ‘‘Although many folks, espe- acid solutions alone or with sweet 2019b). Although other aquatic cially in the organic culture, have ( sinensis) or canola weed management options such as historically used strong vinegars to (Brassica napus) oil; they found that mechanical harvesting do exist, most abate vegetation growth, be advised weed control increased as acetic acid have greatly increased costs and re- that Acetic of 8% or less when concentration increased and that duced efficacy compared with chem- characterized as an inert ingredient, there was little or no advantage to ical control tools. The need for in a mixture, are exempt from regis- adding either type of oil to the acetic ‘‘softer’’ products that can be used tration by the USEPA as a pesticide acid solutions. Evans and Bellinder for aquatic weed control is urgent, under USEPA ‘Minimum Risk Pesti- (2009) stated that broadcast applica- and exploration is needed to identify cide’ FIFRA 25B, List 4A. Thus this tions of 15%, 20%, and 30% acetic acid ways for managers to maintain water product (at 20% acidity) is not to be mixed with 1.7% or 3.4% clove oil resources effectively without the use labeled, marketed or characterized in could be useful for weed suppression of synthetic herbicides. any way as having any herbicidal in sweet corn, onion (Allium cepa), ‘‘Natural’’ herbicides are used virtues’’ (Bradfield Industries, 2019; and potato (Solanum tuberosum) cul- extensively by home gardeners, or- USEPA, 2004b). Horticultural vine- tivation. Shrestha et al. (2012) ganic farmers, and others who wish to gar, with an acetic acid concentration reported that a single application of reduce their use of synthetic herbi- of 30%, is available, but is not listed 20% d-limonene provided up to 95% cides. Products used for natural weed specifically as a herbicide. Acetic acid weed control in organic almond (Pru- control include acids [i.e., acetic acid is corrosive and can damage applica- nus dulcis) orchards 1 week after (vinegar) and citric acid], oils [clove tion equipment when used in highly treatment, but that efficacy was re- (eugenol) (Syzygium aromaticum), concentrated form, but Evans et al. duced to 53% control 5 weeks after (Pinus sp.), (Mentha (2009) stated that lower concentra- treatment, necessitating repeat appli- ·piperita), and citronella (Cymbopo- tions can be used if application vol- cations every 5 to 6 weeks. gon sp.)], soaps, iron- or -based ume is increased to deliver the same Even less information is available herbicides, corn (Zea mays) gluten, total amount of acetic acid. For ex- in the scientific literature regarding and combinations of these products ample, 15% acetic acid applied at 68 the use of natural products for weed (Smith-Fiola and Gill, 2017). Acids gal/acre will reportedly yield similar control in aquatic ecosystems. These and oils may destroy cell membranes, results as 30% acetic acid at 34 gal/ products are not labeled for use as which can lead to cell leakage, plant acre (Quarles, 2010). herbicides in aquatic areas, so they desiccation, and plant death (Baker, Although citric acid is often have not been subjected to the many 1970; Webber et al., 2018). When a component in ‘‘natural’’ herbicide tests required before USEPA ap- used in upland (terrestrial) areas, mixes, products that rely only on proval, which include environmental these are nonselective contact foliar citric acid or citrus oil are less com- fate and ecological toxicity assess- sprays that kill most broad-leaved mon. Avenger Weed Killer Concen- ments (Stubbs and Layne, 2020), weeds. trate (Cutting Edge Formulations, and their effects on aquatic fauna have There are a number of commer- Buford, GA) contains 70% d-limo- not been well-characterized. How- cially available natural herbicides that nene. The label indicates the product ever, Saha et al. (2006) reported that list acetic acid as the active ingredient. should be diluted at 1:6 (10% d- acetic acid had a 96-h 50% lethal The most common acetic acid con- limonene for small annual weeds) to concentration value of 273 mgÁL–1 centration in single-ingredient prod- 1:3 (17.5% d-limonene) for hard-to- on tilapia (Oreochromis mossambicus) ucts is 20% [e.g., Maestro-Gro control weeds, then sprayed to wet. (i.e., tilapia populations exposed to Organic Vinegar (Maestro-Gro, Jus- GreenMatch Burndown Herbicide this concentration for 96 h would be tin, TX), Vinagreen Natural Non (Cutting Edge Formulations) is 55% expected to experience death of half Selective Herbicide (Fleischmann’s d-limonene, and label instructions the population), and that dissolved Vinegar Co., Cerritos, CA), Weed specify diluting at a 1:6 ratio (8% d- oxygen and plankton populations Pharm Fast Acting Weed and Grass limonene) for broadcast treatments were reduced after exposure to 17 Killer (Pharm Solutions, Port Town- or a 1:4 ratio (11% d-limonene) for mgÁL–1 acetic acid. The USEPA send, WA)]. Products containing 20% spot treatments. (2004a) stated that technical-grade acetic acid are typically labeled as Despite widespread interest in and formulated d-limonene is ‘‘prac- ready to use (although some can be reducing synthetic herbicide use, tically nontoxic or slightly toxic to diluted to a concentration of 10%), there is a dearth of information avail- birds, fish and invertebrates,’’ but and label instructions regarding spot able in the scientific literature Kim et al. (2013) reported that

226 • April 2021 31(2) metabolites of d-limonene may cause cordata) and broadleaf sagittaria (Sag- pots) and all plants were then sub- skin irritation in humans and other ittaria latifolia). Plants were treated jected to treatment. animals. As stated earlier, acids and in pairs of one invasive floating spe- Treatments were applied as sin- oils can destroy cell membranes cies and one native emergent spe- gle spot ‘‘spray to wet’’ foliar applica- (Baker, 1970; Webber et al., 2018) cies. Run 1 focused on waterhyacinth tions to above-water foliage, and all and thus should be considered non- and broadleaf sagittaria, whereas treatments included 1% v/v of a non- selective and likely to cause damage to run 2 focused on waterlettuce and ionic surfactant (Induce; Helena off-target flora that comes into con- pickerelweed. Agri-Enterprises, Collierville, TN) to tact with these products. Anderson Target species were field-col- aid in penetration and emulsification. (2007) reported that rhizomes of the lected or pulled from cultures main- Nine single-product treatments (5%, emergent aquatic weed smooth cord- tained at the University of Florida 7.5%, 10%, 15%, and 20% acetic acid; grass (Spartina alterniflora) had 90% Fort Lauderdale Research and Edu- and 10%, 15%, 20%, and 30% d-lim- reductions in shoot number and plant cation Center (FLREC) in Davie, onene), 30 combination treatments height 9 months after exposure to FL, and were moved to 18-gal plas- (all combinations of single acetic acid acetic acid. Acetic acid reduces tic tubs filled with well water. Run 1 and d-limonene treatments; all con- hydrilla regrowth from root crowns tubs (waterhyacinth) were amended centrations of acetic acid plus 5% and (Spencer and Ksander, 1995), and with10geachofcrushed15N– 10% citric acid), three synthetic stan- inhibits viability and sprouting of 3.9P–10K controlled-release fertil- dard-practice treatments (0.22%, hydrilla and sago pondweed (Stuck- izer formulated for 6-month release 0.45%, and 0.89% diquat dibromide), enia pectinata) tubers (Spencer and in Florida (Osmocote Plus; ICL Spe- and an untreated control were evalu- Ksander, 1997, 1999). However, us- cialty Fertilizers, Dublin, OH) and ated, with four replicates of each ing acetic acid to suppress submersed 7N–0P–0K iron chelate micronu- treatment. materials were 30% weed growth would only be practical trient (Sprint 330; BASF Corp., acetic acid (Green Gobbler 30% Vin- when employed in conjunction with Research Triangle Park, NC). Run egar Home and Garden; EcoClean dewatering, because achieving an ad- 2 tubs (waterlettuce) were amended Solutions, Copiague, NY), 100% d- equate concentration of acetic acid in with 3.4 g of 24N–3.5P–13.3K wa- limonene (100% Pure Technical the entire water column is virtually ter-soluble fertilizer (Miracle-Gro Grade D-Limonene, EcoClean Solu- impossible and could cause significant Water Soluble All Purpose Plant tions), 37.3% diquat dibromide (Tri- off-target damage to other flora and Food; Scotts Miracle-Gro Products, bune Herbicide; Syngenta Crop fauna in the system. For these rea- Protection, Greensboro, NC), and Marysville, OH) and 1.2 g of 7N– sons, the scope of this research is 100% citric acid (Milliard Citric Acid; 0P–0K iron chelate micronutrient. limited to testing the effects of con- Milliard Brands, Lakewood, NJ). Each tub was initially ‘‘seeded’’ with tact products on above-water vegeta- Treatments were applied to run 1 10 plants of a target species, which tion (i.e., floating and emergent plant and run 2 plants on 12 Nov. 2019 were grown out for 4 to 6 weeks to material). and 15 Jan. 2020, respectively. Plants The primary goals of this project allow development of more than 80% were monitored weekly for 8 weeks were to evaluate the effects of acetic surface coverage. after treatment and then the project acid and d-limonene (alone and in Nontarget species were pur- lead assigned a numerical value of combination with each other) on two chased from an aquatic nursery 0 through 10 to describe visual qual- floating invasive target species and (Aquatic Plants of Florida, Myakka ity (0 = dead; 5 = fair quality, accept- two emergent desirable nontarget City, FL) and transported to a green- able, somewhat desirable form and species, and to compare the costs of house at FLREC, where individual color, little to no chlorosis or necro- using these products vs. a synthetic plants were transplanted into 2-L sis; 10 = excellent quality, perfect USEPA-approved aquatic herbicide. plastic pots without holes that were condition, healthy and robust, excel- According to the FWC National Pol- filledwithsand[graindiameter lent color and form). Although some lution Discharge Elimination System 0.25–0.5 mm (Multi-Purpose Sand; authors (e.g., Cutelle et al., 2013; report for calendar year 2018, diquat Sakrete, Charlotte, NC)] amended Koschnick et al., 2005; Mudge dibromide, which is nonselective, was with 4 g of the same controlled- et al., 2007) report visual injury or the most commonly used herbicide release fertilizer used in run 1 tubs. damage resulting from herbicide for floating weed management, with Plants were grown out on green- treatments, we recorded visual qual- a total of 7871.77 gal of formulation house benches and irrigated twice ity, which has also been used to de- (37.3% diquat dibromide) applied per day (10:00 AM and 4:00 PM)with scribe plant response to differing (Clark and Dew, 2019), so this prod- the equivalent of 0.5 inch of water culture conditions (e.g., Gettys and uct will serve as the synthetic ‘‘stan- per irrigation before being used in Moore, 2018, 2019), herbicides dard practice’’ treatment in these experiments. New shoots were cut (e.g., Gettys and Haller, 2009, experiments. back during this culture period to 2010, 2012; Smith et al., 2014), salt ensure that each 2-L pot contained stress (e.g., Tootoonchi et al., 2020), a single nontarget plant. When target and other experimental factors. After Materials and methods plant coverage reached more than visual scoring, a destructive harvest EFFICACY STUDIES. Target (weed) 80% of the surface of the water, one was conducted to collect all live bio- species were waterhyacinth and water- potted nontarget plant was intro- mass of floating species and all live lettuce, whereas nontarget (desirable) duced to each tub (water depth, aboveground shoots of emergent spe- species were pickerelweed (Pontederia 20 cm above the surface of the cies. Harvested materials were placed

• April 2021 31(2) 227 Visual data were arcsine-trans- formed before analysis to normalize distribution. Data within each spe- cies were evaluated using a general- ized linear model (SAS version 9.4; SAS Institute, Cary, NC) to deter- mine whether treatment means dif- fered from those of untreated plants at P = 0.05. Treatment means of visual values and dried biomass were then compared with untreated con- trols. Haller and Gettys (2013) reported that an ideal herbicide treatment should cause a >90% re- duction in these parameters in tar- get weeds and a <50% reduction in nontarget native plants. Therefore, we used these values as benchmarks for efficacy on the floating weeds waterhyacinth and waterlettuce, and selectivity on the emergent native plants pickerelweed and broadleaf sagittaria. COST COMPARISONS. A total of four diquat dibromide products, all formulated as 37.3% diquat dibro- mide, were used by the FWC in FY 2018–19 [67 gal Alligare Diquat (Alligare, Opelika, AL), 368 gal Di- quat SPC 2L (Nufarm Americas, Burr Ridge, IL), 34 gal Reward (Syngenta Crop Protection), and 7407.77 gal Tribune (Syngenta Crop Protection)]. The majority (>94%) of diquat dibromide was ap- plied as Tribune, which was pur- chased at the FWC’s negotiated contract price of $35.50/gal (ven- dor, Helena Agri-Enterprises; size, 2.5-gal jug) (Clark and Dew, 2019; Cleary and McNiel, 2019). As such, cost comparisons will assume a pur- chase price of $35.50/gal for all di- quat dibromide products. Small volumes (e.g., 1 gal) of 30% acetic acid and technical grade d-limonene are $24.99/gal and $59.99/gal, respectively (Factory Di- rect Chemicals, 2019a, 2019b). When purchased in bulk, 30% acetic acid is $8.00/gal (275-gal tote) and technical grade d-limonene is $31.82/gal (4 · 55-gal drums) (Fac- Fig. 1. Biomass of (A) waterhyacinth, (B) waterlettuce, and (C) pickerelweed 8 weeks tory Direct Chemicals, 2019a, after single-product treatment. Bars are the mean of four replicates and error bars represent 1 SD from the mean. Treatments coded with the same letter are not different 2019b). If acetic acid and d-limonene at P = 0.05. The upper bold horizontal rule indicates the mean of untreated control were to be used for aquatic weed (UTC) plants, whereas the central and lower bold horizontal rules indicate 50% and control in Florida, it is likely they 90% reductions compared with UTC plants; 1 g = 0.0353 oz. would be purchased in bulk, so cost comparisons will assume a purchase in paper bags and moved to a forced- evaluations and destructive harvests price of $8.00/gal for 30% acetic acid air oven maintained at 65 C for 2 occurred on 7–9 Jan. 2020 (run 1) and $31.82/gal for technical grade d- weeks before being weighed. Visual and 11–13 Mar. 2020 (run 2). limonene.

228 • April 2021 31(2) mixes of acetic and citric acids, some combinations of acetic acid and d- limonene had good efficacy on both floating weeds (waterhyacinth and waterlettuce biomass and visual qual- ity, P < 0.01). The most promising combinations on waterhyacinth were 15% acetic acid plus 15%, 20%, or 30% d-limonene and 20% acetic acid with any concentration of d-limonene; these treatments reduced dry biomass by >80% (Fig. 3A) and visual quality by >60% (Fig. 4A). Most combina- tions of acetic acid and d-limonene had good efficacy on waterlettuce; dry weight (Fig. 3B) and visual quality ratings (Fig. 4B) were reduced com- pared with untreated control plants in all but a single treatment (5% acetic acid plus 10% d-limonene). Only 5 of the 20 treatment combinations failed Fig. 2. Visual quality of waterlettuce 8 weeks after single-product treatment. A to reduce biomass by >90% compared numerical scale of 0 through 10 is used to describe visual quality, where 0 is dead; with untreated control plants, and 5 is fair quality, acceptable, somewhat desirable form and color, little to no two combinations failed to reduce chlorosis or necrosis; and 10 is excellent quality, perfect condition, healthy and visual quality by >50%. Pickerelweed robust, excellent color and form. Bars are the mean of four replicates and error biomass and visual quality were affected bars represent 1 SD from the mean. Treatments coded with the same letter are not by combinations of acetic acid and d- different at P = 0.05. The upper bold horizontal rule indicates the mean of untreated control (UTC) plants, whereas the central and lower bold horizontal limonene. Biomass was reduced in rules indicate 50% and 90% reductions compared with UTC plants. plants treated with any combination of acetic acid and d-limonene compared with untreated control plants [P < 0.01 Results >90% and >75%, respectively, after (Fig. 3C)], and visual quality was re- treatment with 20% or 30% d-limo- ducedin13ofthe20treatments[P = SINGLE PRODUCTS. The only sin- nene, but no other single-product 0.03 (Fig. 4C)]. Broadleaf sagittaria gle-product treatments that provided treatments were different from un- biomass was unaffected by mixes of good control of both waterhyacinth treated control plants. Pickerelweed acetic acid and d-limonene (P =0.34). and waterlettuce were diquat dibro- dry weight was reduced by most Visual quality of sagittaria was affected mide at 0.22%, 0.45%, and 0.89%, single-product treatments compared [P = 0.04 (Fig. 4D)], but treated plants with all three concentrations com- with untreated control plants [P = were not different from untreated plants pletely eliminating both floating 0.01 (Fig. 1C)], but none reduced and all differences occurred among weeds. Unfortunately, all nontarget weight by >80%, and visual quality treatment combinations. native plants were eliminated by these was unaffected (P = 0.16). Single These results suggest that some treatments as well. Although a goal of natural products had no effect on combination treatments of acetic acid these experiments was to compare the broadleaf sagittaria dry weight (P = and d-limonene may be useful for efficacy of natural products to the 0.48) or visual quality (P = 0.68). managing populations of invasive synthetic herbicide diquat dibromide, ACETIC ACID AND CITRIC ACID waterhyacinth and waterlettuce while it became clear that most natural MIXES. Combinations of 5% to 20% providing a level of selectivity with treatments were much less effective acetic acid and 5% or 10% citric acid reduced damage to the native plants than diquat dibromide, and compar- did not reduce biomass or visual broadleaf sagittaria and pickerelweed. isons between natural treatments and quality by at least 50% in any of the COST ANALYSIS OF ACETIC ACID untreated controls would be more species evaluated in these experi- AND D-LIMONENE MIXES FOR FLOATING informative. Thus, diquat dibromide ments. Dry weight and visual quality WEED MANAGEMENT. The synthetic treatments were removed from data of treated and untreated plants were standard-practice treatments in these sets before further statistical analyses not different (P = 0.06 to P = 0.89) in experiments used diquat dibromide. were conducted. Waterhyacinth bio- most cases. The sole exception was These experiments evaluated foliar mass was affected by single-product dry weight of waterlettuce (P = 0.02); treatments only and we envision that natural treatments [P < 0.01 (Fig. treated plants were not different field treatments for floating weed 1A)], but no treatment reduced bio- from untreated plants, but differences management would be applied as spot mass by >50% or affected visual qual- were detected among treatment treatments as opposed to broadcast ity (P = 0.66). These treatments had combinations. treatments. Diquat dibromide can be an effect on waterlettuce biomass [P = ACETIC ACID AND D-LIMONENE applied at a concentration of up to 2% 0.03 (Fig. 1B)] and visual quality [P < MIXES. In contrast to single-product formulated product (equivalent to 0.01 (Fig. 2)], which were reduced by natural herbicide treatments and 0.89% diquat dibromide) for floating

• April 2021 31(2) 229 0.89%) of diquat dibromide, but the lowest concentration completely elimi- nated all plant material, so calculations are based on spot treatments using a 0.22% solution. As mentioned in the Materials and Methods, the FWC ap- plied a total of 7871.77 gal of 37.3% diquat dibromide to floating plants in 2018 (Clark and Dew, 2019). Assuming all diquat field treatments were mixed to a concentration of 0.22% diquat dibromide, a total of 1,574,354 gal of ready-to-use (RTU) mix was made from the 7871.77 gal of concentrate pur- chased. The FWC’s contract price for 37.3% diquat dibromide was $35.50/ gal, resulting in a total cost of $283,383.72; after dilution to a 0.22% concentration, the final cost is $0.1775/ gal RTU, or $0.18/gal RTU. As mentioned in the Materials and Methods, these calculations are based on purchase prices of $8.00/ gal for 30% acetic acid and $31.82/ gal for technical grade d-limonene. Neither acetic acid nor d-limonene resulted in acceptable control of waterhyacinth when applied alone, but some combinations of the two provided good control (>80% reduc- tion in biomass) of waterhyacinth. These were 15% acetic acid plus 15%, 20%, or 30% d-limonene and 20% acetic acid with any concentra- tion of d-limonene. The material costs to make RTU 15% or 20% acetic acid are $1.20/gal or $1.60/gal, re- spectively. The material costs for RTU d-limonene are $3.18/gal (10%), $4.77/gal (15%), $6.36/gal (20%), and $9.55/gal (30%). Thus, the least expensive efficacious natural treatment (20% acetic acid + 10% d- limonene; $1.60 + $3.18) for water- hyacinth is $4.78/gal RTU, or nearly 26· more expensive than the synthetic standard (0.22% diquat dibromide) treatment ($0.18/gal RTU). Assuming other application parameters (e.g., sur- factant cost, labor) remain unchanged between the treatment types, and re- membering that a total of 1,574,354 gal of RTU mix was used in 2018, switching Fig. 3. Biomass of (A) waterhyacinth, (B) waterlettuce, and (C) pickerelweed 8 weeks completely from synthetic to natural after treatment with combinations of acetic acid and d-limonene. Bars are the mean of products for waterhyacinth management four replicates and error bars are 1 SD from the mean. Treatments coded with the same would increase single-year product costs letter are not different at P = 0.05. The upper bold horizontal rule is the mean of from $283,383.72 (0.22% diquat dibro- untreated control (UTC) plants, whereas the central and lower bold horizontal rules mide) to $7,525,412.12 (20% acetic acid indicate 50% and 90% reductions compared with UTC plants; 1 g = 0.0353 oz. + 10% d-limonene). In contrast to waterhyacinth, and marginal weeds, whereas spot treat- to 0.22% diquat dibromide) (Syngenta there were many treatments that re- ments should use a concentration of Crop Protection, 2011). We evaluated duced waterlettuce biomass by at least 0.5% formulated product (equivalent three concentrations (0.22%, 0.45%, and 90% compared with untreated control

230 • April 2021 31(2) plants. Most efficacious treatments used combinations of acetic acid and d-limonene, but applications of 20% and 30% d-limonene alone also re- duced waterlettuce biomass by 90%. The least expensive efficacious natural treatment (10% acetic acid + 10% d- limonene; $0.80 + $3.18) for water- lettuce is $3.98/gal RTU, or 22· more expensive than the synthetic stan- dard treatment ($0.18/gal RTU). As with the caveats just described for waterhyacinth, switching completely from synthetic to natural products for waterlettuce management would in- crease single-year product costs from $283,383.72 (0.22% diquat dibro- mide) to $6,265,928.92 (10% acetic acid + 10% d-limonene). Thefigurescalculatedheredo not account for the likelihood that applications using natural products would take longer, resulting from the need to transport very large volumes of base material. For example, consider a spray boat with a 100-gal tank that uses diluent water drawn from the system being treated. Filling the tank once for a synthetic treatment would require the transport of 0.5 gal of 37.3% diquat dibromide, but filling the tank once with the least expensive efficacious natural treatment for waterhyacinth management would re- quire the transport of 67 gal of 30% acetic acid and 10 gal of technical grade d-limonene, or 630 lb of materials (without factoring in the weight of the containers used to trans- port the materials). Rather than trans- porting base materials and adding them to the tank at the treatment site, appli- cators would likely add the natural treat- ment components to the tank at the ramp and would thus have to return to shore for reloading after applying 100 gal of RTU natural mix. In contrast, an applicator with a single 2.5-gal jug of 37.3% diquat dibromide would have enough base material to make 500 gal of RTU synthetic mix without returning to the ramp. As a result, replacing synthetic herbicides with natural products would Fig. 4. Visual quality of (A) waterhyacinth, (B) waterlettuce, (C) pickerelweed, not only greatly increase material costs and (D) broadleaf sagittaria 8 weeks after treatment with combinations of acetic but would also decrease productivity (as acid and d-limonene. A numerical scale of 0 through 10 is used to describe visual measured in acres treated per day). quality, where 0 is dead; 5 is fair quality, acceptable, somewhat desirable form and color, little to no chlorosis or necrosis; and 10 is excellent quality, perfect Discussion condition, healthy and robust, excellent color and form. Bars are the mean of four The natural products evaluated in replicates and error bars represent 1 SD from the mean. Treatments coded with the thesestudiesmayhavesomeutilityfor same letter are not different at P = 0.05. The upper bold horizontal rule indicates managing floating weeds such as water- the mean of untreated control (UTC) plants, whereas the central and lower bold hyacinth and waterlettuce selectively horizontal rules indicate 50% and 90% reductions compared with UTC plants. without causing unacceptable levels of

• April 2021 31(2) 231 damage to desirable native plants such as annual report of pollutant discharges to 2018–2019. 10 June 2020. . (>50% reduction in biomass and visual through 31 Dec. 2018. 20 Mar. 2019. < Florida Fish and Wildlife Conservation quality) damage to either weed species, https://myfwc.com/media/19111/ npdes-2018.pdf>. Commission. 2019b. FWC implementing whereas d-limonene alone caused >90% enhancements to aquatic plant manage- reduction in biomass of waterlettuce Cleary, R. and D. McNiel. 2019. The ment program. 10 Jan. 2021. . org/CDN/floridainvasives/Herbicide_ acid to acetic acid had no effect on acetic Gettys, L.A. 2019. Breaking bad: Na- Bank_Handbook2020.pdf>. acid efficacy, so further investigations tive aquatic plants gone rogue and the with citric acid are not recommended. Cutelle, M.A., G.R. Armel, J.T. Brosnan, invasive species that inspire them. Some combinations of acetic acid and d- D.A. Kopsell, W.E. Klingeman, P.C. 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