Management Implications
Eichhornia crassipes (waterhyacinth) management relies heavily on herbicides; however, herbicide application techniques have not changed in decades. Current application techniques largely consist of high-volume applications that can appear “heavy-handed” to public stakeholders. Furthermore, these high-volume applications provide excellent spray coverage but poor overall spray retention. Consequently, we tested the effect of four application methods at three carrier volumes on 2,4-D, diquat, and glyphosate efficacy on E. crassipes in mesocosms. Reducing carrier volume did not reduce control in any experiment. Conversely, E. crassipes control increased with reduced carrier volume in 2,4-D and glyphosate. These data support field evaluation of reduced carrier volume techniques for E. crassipes management.
Introduction
Waterhyacinth [Eichhornia crassipes (Mart.) Solms] is a rapidly growing invasive plant that has been a management challenge in Florida for more than 130 yr (Center and Spencer Reference Center and Spencer1981; Penfound and Earle Reference Penfound and Earle1948; Pieterse Reference Pieterse1978). In addition to impeding navigation, irrigation, and recreation, E. crassipes can also reduce water quality (dissolved oxygen and pH) and shelter mosquito species responsible for the spread of harmful human diseases (Holm et al. Reference Holm, Plucknett, Pancho and Herberger1977; Owens and Madsen Reference Owens and Madsen1995; Penfound and Earle Reference Penfound and Earle1948; Schreiner Reference Schreiner1980; Seabrook Reference Seabrook1962; Ultsch Reference Ultsch1973). Therefore, managing E. crassipes populations at the lowest feasible level with herbicides has become compulsory for protecting public waterways and human health (Joyce Reference Joyce1985; University of Florida 2011).
Control of E. crassipes has for decades largely relied on diquat and 2,4-D; however, glyphosate, imazapyr, triclopyr, and penoxsulam can also provide effective control (Enloe et al. Reference Enloe, Netherland, Haller and Langeland2018; Wersal and Madsen Reference Wersal and Madsen2010). Diquat is often favored due to rapid development of symptoms and the fact that it provides simultaneous control of waterlettuce (Pistia stratiotes L.), which is commonly found with E. crassipes in mixed stands (Mudge and Netherland Reference Mudge and Netherland2014). Rapid symptom development is important in aquatic plant management, as it provides a visual cue to the applicator to delineate which plants have been treated. This is essential, as few landmarks exist on open water to keep tabs on the spray track. Though 2,4-D does not control P. stratiotes, it is more selective than diquat on a number of desirable native plants and still produces herbicide symptoms within 1 to 3 d of treatment. Glyphosate and other slow-acting enzyme inhibitors are not as widely used for E. crassipes control, because the delayed activity makes it difficult to distinguish treated from nontreated areas (Wersal and Madsen Reference Wersal and Madsen2010). Because these plants are free-floating and move within the lake from day to day, the delay in herbicide symptoms can result in wasted time and herbicide product, as the same plants may be sprayed multiple times as they float to different areas.
Foliar application techniques for aquatic herbicides have not changed after decades of use despite improvements in application technology in terrestrial systems. Traditionally, foliar aquatic herbicides have been applied in high carrier volumes (average of 935 L ha−1) using handguns consisting of a single nozzle (Haller Reference Haller2020). This application technique resembles spot-treatment methods with percent solutions in which plants are “sprayed-to-wet” or “sprayed to runoff.” This procedure has proven successful with multiple herbicides and plant species over decades of use. Furthermore, this application technique is easily taught to applicators compared with standard sprayer calibration. While high carrier volumes in any application can provide excellent spray coverage, spray droplet retention and spray solution concentration are often low (Enloe et al. Reference Enloe, Leary, Prince, Sperry and Lauer2020). Additionally, applications at 935 L ha−1 are often conducted on public waters in full view of recreational users. As such, this water volume is highly visible and appears excessive to the public.
Previous research regarding herbicide carrier volume in aquatic plant management is limited to glyphosate on phragmites [Phragmites australis (Cav.) Trin. ex Steud.], E. crassipes, and giant salvinia (Salvinia molesta Mitchell) (Nelson et al. Reference Nelson, Glomski and Gladwin2007; Riemer Reference Riemer1976; Van et al. Reference Van, Vandiver and Conant1986). For both P. australis and E. crassipes, control was improved when carrier volumes were decreased. This was not surprising, considering the vast amount of carrier volume data published for terrestrial systems (Ambach and Ashford Reference Ambach and Ashford1982; Creech et al. Reference Creech, Henry, Werle, Sandell, Hewitt and Kruger2015; Knoche Reference Knoche1994; O’Sullivan et al. Reference O’Sullivan, O’Donovan and Hamman1981; Ramsdale and Messersmith Reference Ramsdale and Messersmith2001; Sandberg et al. Reference Sandberg, Meggit and Penner1978; Shaw et al. Reference Shaw, Morris, Webster and Smith2000; Stahlman and Phillips Reference Stahlman and Phillips1979; Wolf et al. Reference Wolf, Caldwell, Mcintyre and Hsiao1992). Given that herbicide uptake in plants has been shown to be primarily driven by concentration gradient, it stands to reason that decreasing carrier volume (increasing herbicide concentration in each droplet) should result in greater herbicide penetration and absorption into the leaf (Devine et al. Reference Devine, Duke and Fedtke1992). Therefore, reducing carrier volume is an ideal and efficient way to maximize efficacy of systemic herbicides. Unfortunately, efficacy of contact herbicides such as diquat generally perform worse as carrier volume is reduced (Douglas Reference Douglas1968; Knoche Reference Knoche1994). This is because contact herbicides require a balance between droplet concentration and adequate leaf coverage to be consistently effective. However, there is a tipping point between high levels of spray coverage and reductions in spray retention. Sandberg et al. (Reference Sandberg, Meggit and Penner1978) reported that as much as 50% to 75% of the spray solution ran off leaf surfaces in 375 and 750 L ha−1 applications. Therefore, there appears to be opportunity to maintain diquat efficacy by reducing carrier volumes if application techniques can be adopted that will increase spray retention on the leaf.
In Florida, more than 72,843 hectares of floating plants received diquat in 2019 for management, while the remaining ~40,468 hectares received other herbicides or combinations (FFWCC 2019). Therefore, aquatic applicators commonly calibrate for 935 L ha−1, because diquat is their primary herbicide and this methodology has proven successful for decades. The application technique is accomplished with high-pressure handguns that will commonly deliver sufficient volume for plant control at distances of more than 10 m. Unfortunately, these sprayers are prone to produce driftable fines while appearing excessive and imprecise to stakeholders. This perception has resulted in a lack of public support for aquatic plant management. It is important to determine whether advancements can be made to our current floating-plant management strategies that maintain efficacy while increasing precision. The objectives of this research are: (1) determine whether reduced carrier volumes of 2,4-D, diquat, and glyphosate alter their efficacy on E. crassipes; and (2) document how different spray patterns interact with carrier volume for overall efficacy.
Materials and Methods
Three separate, yet concurrent experiments documenting the efficacy of glyphosate, 2,4-D, and diquat on E. crassipes as a function of carrier volume and application type were conducted at the University of Florida Center for Aquatic and Invasive Plants in Gainesville, FL (29.721542°N, 82.417300°W) in 2020. Methods were identical across all three experiments, and each was repeated twice with treatment dates of March 21 and May 18, 2020. In each experiment, herbicide rate was held constant while a factorial arrangement of treatments consisted of carrier volume (935, 467, and 187 L ha−1) and application method (conventional stream, conventional cone, adjustable cone, and a drizzle stream). Herbicides rates for glyphosate (Roundup® Custom, Bayer CropScience LLC, Research Triangle Park, NC, USA) and 2,4-D (Alligare 2,4-D Amine, Alligare LLC, Opelika, AL, USA) were both 2.2 kg ae ha−1, while diquat (Tribune™, Syngenta Crop Protection LLC, Greensboro, NC, USA) was applied at 1.1 kg cation ha−1. Conventional stream and conventional cone treatments were applied using a CO2-pressurized industry standard AA43 GunJet® (TeeJet® Technologies, Spraying Systems, Wheaton, IL, USA) equipped with a D4 orifice disk (TeeJet® Technologies) calibrated to deliver 1.7 L min−1 at 276 kPa. Conventional stream treatments were achieved on the AA43 GunJet® by utilizing setting “C” by depressing the trigger fully. Likewise, conventional cone applications were achieved on the GunJet utilizing setting “A” by depressing the trigger 50% to maintain a wide-angle cone. Adjustable cone treatments were applied using a CO2-pressurized FIMCO Deluxe Pistol Grip Handgun (FIMCO Industries, North Sioux City, SD, USA) equipped with an adjustable cone nozzle (FIMCO Industries) calibrated to deliver 0.53 L min−1 at 138 kPa. Drizzle applications were made with a CO2-pressurized JD9-C High Pressure Spray Gun (H.D. Hudson Manufacturing, Lowell, MI, USA) equipped with a J9 (H.D. Hudson Manufacturing) nozzle calibrated to deliver 3.4 L min−1 at 69 kPa. To ensure that each experimental unit received the desired carrier volume without changing the spray droplet spectrum, travel speed was the only parameter manipulated.
Eichhornia crassipes plants were collected from the Rodman Reservoir in Florida (29.515923°N, 81.877916°W) in the summer of 2019 and transferred to 1,000-L concrete vaults. Vaults were amended with soluble fertilizer (24-8-16, Miracle-Gro® All Purpose Plant Food, Scotts Company, Marysville, OH, USA) at 0.2 g L−1 and chelated iron (Grow More Iron Chelate 10%, Grow More, Gardena, CA, USA) at 0.02 g L−1 to ensure sufficient growth of ramets. Likewise, plants were treated as needed with zeta-cypermethrin (GardenTech Sevin Insect Killer Concentrate, TechPac, Atlanta, GA, USA) for insect control. Experiments were set up in a completely randomized design with four replications per experimental run. Experimental units were established as 20 individual plants of similar size in 95-L plastic tubs maintained with the same fertilizer and insect control regime described for concrete vaults. Plants were left to acclimate in tubs for 2 wk before treatment initiation.
In addition to herbicides in each experiment, all treatment solutions contained a nonionic surfactant (Induce®, Helena Agri-Enterprises, Collierville, TN, USA) at 0.25% v/v. Likewise, all solutions contained rhodamine WT tracer dye (Rhodamine WT Liquid, Keystone Aniline, Chicago, IL, USA) at 0.25% v/v to stain spray cards for coverage analysis.
On the day of treatment, pretreatment biomass from four mesocosms per experiment was harvested, dried in a forced-air oven at 60 C for 5 d, and weighed for biomass. Also, two photo cards (Kromekote® Photo Paper, CTI Paper USA, Sun Prairie, WI, USA) per experimental unit were set at canopy height on ring stands set adjacent to experimental units (Ferguson et al. Reference Ferguson, Cheschetto, Hewitt, Chauhan, Adkins, Kruger and O’Donnell2016; Hewitt and Meganasa Reference Hewitt and Meganasa1993; Higgins Reference Higgins1967; Roten et al. Reference Roten, Connell, Hewitt and Woodward2015). After cards were dry (˜15 min), spray cards were collected into envelopes. Visual estimates of percent control were conducted at 7, 14, 21, and 28 d after treatment (DAT) on a 0% to 100% scale, with zero being similar to the nontreated control (NTC) and 100% being complete plant death. At 6 wk after treatment (WAT), all viable biomass in each mesocosm was harvested, dried in a forced-air oven at 60 C until constant moisture level, and weighed for biomass.
Spray cards were scanned into JPEG files, converted to 8-bit format, threshold adjusted, and analyzed for percent coverage using ImageJ software (Schneider et al. Reference Schneider, Rasband and Eliceiri2012). All percent data were arcsine-square-root transformed to improved homogeneity of variance before analysis to meet model assumptions; however, back-transformed means are presented for clarity. All data were subject to mixed-model ANOVA under the lme4 package in R (v. 3.6.1; Bates et al. Reference Bates, Maechler, Bolker and Walker2015; Mendiburu Reference Mendiburu2019; R Core Team 2019), where carrier volume and application method were considered fixed effects and experimental run and replicate (nested in experimental run) were considered random effects (Blouin et al. Reference Blouin, Webster and Bond2011). Where significant effects were detected, means were separated using Fisher’s LSD test (α = 0.05) under the emmeans package in R (Lenth Reference Lenth2020). Additionally, a t-test was conducted at α = 0.05 to determine significant differences among treated and nontreated biomass.
Results and Discussion
2,4-D
All visual evaluations of E. crassipes control and biomass reductions from 2,4-D treatments were affected independently by the main effects of carrier volume and application method (Table 1). As the interaction between carrier volume and application methods was not significant, only the main effects will be shown. Eichhornia crassipes control at 7 and 14 DAT increased with decreasing carrier volume, and by 21 and 28 DAT applications at 467 and 935 L ha−1 performed similarly. However, reducing carrier volume to 187 L ha−1 resulted in 14% and 10% greater E. crassipes control compared with treatment at 935 L ha−1 at 21 and 28 DAT. Biomass reduction improved by 8% when reducing carrier volume from 935 to 187 L ha−1. Conventional cone and adjustable cone treatments resulted in greater E. crassipes control (97%) than conventional stream and drizzle-stream methods (83% to 85% control) regardless of carrier volume across all evaluation intervals. These control estimations translated into a similar trend in biomass reduction, with conventional cone and adjustable cone methods reducing biomass 8% to 9% more than either stream method. Additionally, all treatments reduced biomass compared with the NTC.
Table 1. Eichhornia crassipes control and biomass reduction as affected by the main effects of carrier volume and application method from 2,4-D (2.2 kg ae ha−1) treatments in mesocosms.a
a Means within a column and main effect (carrier volume or application method) followed by the same letter are not different according to Fisher’s LSD test (α = 0.05).
b DAT, days after treatment.
c Pretreatment dry biomass was 1,776 and 1,344 kg ha−1 in experimental runs 1 and 2, respectively. Mean dry biomass of nontreated control was 3,548 and 3,860 kg ha−1 in experimental runs 1 and 2, respectively. Asterisks signify significant difference compared with the nontreated control according to t-test (α = 0.05).
The stream application methods are commonly desired, because these spray patterns largely minimize production of fine spray droplets with high drift potential. Without the nozzle atomizing spray solution, the stream either fractures as it falls through the air or shatters into droplets upon impact with plant leaves. The stream technologies tested in this small-scale experiment study did not provide adequate coverage to achieve optimum activity with 2,4-D. This limitation could potentially be avoided if the herbicide possessed in-water activity at potential concentrations, thus providing two routes of entry into the plant (roots and leaves). Though 2,4-D is used as an in-water treatment of Eurasian watermilfoil (Myriophyllum spicatum L.) (Elliston and Steward Reference Elliston and Steward1972; Getsinger et al. Reference Getsinger, Davis and Brinson1982; Green and Westerdahl Reference Green and Westerdahl1990), these data lead us to suggest that in-water activity of 2,4-D on E. crassipes is limited at the resultant concentrations in the current study. However, additional research should investigate the dose–response relationship between E. crassipes and in-water 2,4-D exposure.
Diquat
Eichhornia crassipes response to diquat was not affected by application method at any evaluation timing, but control ranged from 93% to 96% at 7 DAT and quickly approached 99% to 100% control by 14 DAT (data not shown). However, early E. crassipes control evaluations (7 and 14 DAT) revealed that decreased carrier volume to 187 L ha−1 increased control 3% to 9% (Table 2). By 28 DAT, all treatments resulted in 100% control, and no viable plant tissue was present for biomass harvest.
Table 2. Eichhornia crassipes control and biomass reduction as affected by the main effect of carrier volume from diquat (1.1 kg cation ha−1) treatments in mesocosms.a
a Means within a column followed by the same letter are not different according to Fisher’s LSD test (α = 0.05).
b DAT, days after treatment.
c Pretreatment dry biomass was 1,652 and 1,227 kg ha−1 in experimental runs 1 and 2, respectively. Mean dry biomass of nontreated control was 3,353 and 3,270 kg ha−1 in experimental runs 1 and 2, respectively. Asterisks signify significant difference compared with the nontreated control according to t-test (α = 0.05).
Diquat is a fast-acting contact herbicide that is highly effective on E. crassipes and has in-water activity (Langeland et al. Reference Langeland, Hill, Koschnick and Haller2002). Consequently, the rate used in this study may have been too high to observe the application technique effects. Additionally, these data in combination with herbicide use records in Florida suggest that diquat is highly versatile and can be forgiving when application technique is not optimal (FFWCC 2019). Future work will evaluate these application effects at lower rates.
Glyphosate
Eichhornia crassipes control at 7 and 14 DAT with glyphosate was affected by an interaction between application method and carrier volume (Table 3). At 7 DAT, decreasing carrier volume resulted in greater E. crassipes control in every application method except for the drizzle stream. In the drizzle-stream treatments, carrier volumes of 935 and 467 L ha−1 performed similarly; however, when carrier volume was reduced to 187 L ha−1, control increased 27% to 37%. Additionally, E. crassipes control was greatest, independent of application method, at 7 DAT when a carrier volume of 187 L ha−1 was used. This suggests that reduced carrier volume applications of glyphosate may compensate for spray coverage deficiencies of certain application methods. By 14 DAT, all reductions in carrier volume resulted in increased E. crassipes control, except in the conventional stream treatments. In the conventional stream treatments, carrier volumes of 467 and 187 L ha−1 resulted in similar E. crassipes control; however, carrier volumes of 935 L ha−1 decreased control 39% to 47%. Despite the conventional stream at 935 L h−1 treatment resulting in the lowest E. crassipes control of the conventional stream treatments, this treatment resulted in the greatest E. crassipes control of all 935 L ha−1 treatments.
Table 3. Eichhornia crassipes control at 7 and 14 d after treatment (DAT) with glyphosate (2.2 kg ae ha−1) as affected by the interaction between application method and carrier volume in mesocosm experiments.
a Means within a column followed by the same letter are not different according to Fisher’s LSD test (α = 0.05).
Eichhornia crassipes control from glyphosate at 21 and 28 DAT and biomass reduction data were independently affected by the main effects of application method and carrier volume, and no interactions were detected (Table 4). At 21 DAT, the conventional stream method provided 8% and 22% greater E. crassipes control compared with the adjustable cone and drizzle-stream methods, respectively. By 28 DAT, conventional stream and cone methods performed similarly, yet resulted in 9% to 15% greater E. crassipes control compared with adjustable cone or drizzle-stream methods. Biomass reductions ranged from 72% to 81%, with no differences observed among application methods. Reducing carrier volume in glyphosate treatments resulted in greater E. crassipes control at 21 and 28 DAT and greater biomass reduction. Likewise, biomass reduction was 42% greater when glyphosate was applied at 187 L ha−1 compared with 935 L ha−1.
Table 4. Eichhornia crassipes control at 21 and 28 d after treatment (DAT) and biomass reduction as affected by the main effects of carrier volume and application method from glyphosate (2.2 kg ae ha−1) treatments in mesocosms.a
a Means within a column and main effect (carrier volume or application method) followed by the same letter are not different according to Fisher’s LSD test (α = 0.05).
b Pretreatment dry biomass was 2,136 and 1,073 kg ha−1 in experimental runs 1 and 2, respectively. Mean dry biomass of nontreated control was 3,719 and 3,377 kg ha−1 in experimental runs 1 and 2, respectively. Asterisks signify significant difference compared with the nontreated control according to t-test (α = 0.05).
Reducing carrier volumes in glyphosate applications has been shown to increase efficacy on several terrestrial species (Ambach and Ashford Reference Ambach and Ashford1982; O’Sullivan et al. Reference O’Sullivan, O’Donovan and Hamman1981; Sandberg et al. Reference Sandberg, Meggit and Penner1978). Similar to the current study, Van et al. (Reference Van, Vandiver and Conant1986) observed greater E. crassipes control with 1.7 kg ae ha−1 glyphosate when treatment was applied at 187 L ha−1 compared with 468 or 935 L ha−1. However, Van et al. (Reference Van, Vandiver and Conant1986) also reported that increasing glyphosate rate eliminated any increased activity from lower carrier volumes. Despite the same rate of herbicide being applied to a given surface area (e.g., 1 kg ai ha−1), glyphosate efficacy can be increased by manipulating application technique. While we did not test the concept directly in the current study, these data suggest that the same level of E. crassipes control may be obtained with a reduced herbicide rate simply by reducing carrier volume and promoting greater absorption and translocation in the target plant. However, further research is needed to test this hypothesis.
Although the speed of herbicide activity was not statistically evaluated in these experiments, trends in early control evaluations (7 DAT) suggest that reduced carrier volume may decrease time to observable symptoms. Therefore, applicators and resource managers may be more open to glyphosate use for E. crassipes if lower carrier volumes are utilized.
Eichhornia crassipes Spray Deposition
For each herbicide experiment, an interaction between carrier volume and sprayer type was detected in spray coverage data (Table 5). For glyphosate experiments, all 935 and 467 L ha−1 treatments resulted in spray coverage >90% regardless of sprayer type, except for the 467 L ha−1 conventional stream treatment. Conventional cone and adjustable cone 187 L ha−1 treatments resulted in spray coverage of 92% and 99%, respectively. However, the conventional stream and drizzle spray types resulted in 75% and 40% spray coverage when carrier volume was reduced to 187 L ha−1. In 2,4-D and diquat treatments, spray coverage consistently decreased with decreasing carrier volume in the conventional stream and drizzle sprayers only. Conversely, conventional cone and adjustable cone sprayers produced excellent spray coverage (>97%) across all carrier volumes, likely due to atomization of finer droplets.
Table 5. Quantified spray coverage from herbicide applications made to Eichhornia crassipes as affected by the interaction between carrier volume and application method.
a Means within a column followed by the same letter are not different according to Fisher’s LSD test (α = 0.05).
The systemic herbicides tested in these experiments provided greater E. crassipes control when carrier volume was reduced from 935 L ha−1. However, little difference was observed among diquat treatments. This suggests that the diquat rate tested was likely too high and could have prevented observation of the treatment effects. However, for the given rate tested, these data indicate that diquat does not require as much spray coverage as many applicators believe. Alternatively, foliar applications of diquat may simply be more forgiving due to its in-water activity under nonturbid conditions (Hofstra et al. Reference Hofstra, Clayton and Getsinger2001). Sandberg et al. (Reference Sandberg, Meggit and Penner1978) reported that 50% to 75% of spray solution ran off leaf surfaces in 375 and 750 L ha−1 spray applications. Therefore, the efficacy of diquat on E. crassipes may require a combination of foliar and in-water absorption, as the E. crassipes meristem is usually found just below the water surface (Penfound and Earle Reference Penfound and Earle1948). However, in-water activity of diquat under field conditions may be reduced by turbidity of both source water and site water around E. crassipes plants. Conversely, glyphosate is largely inactive in water due to adsorption and rapid microbial degradation (Borggaard and Gimsing Reference Borggaard and Gimsing2008; Zaranyika and Nyandoro Reference Zaranyika and Nyandoro1993). Likewise, 2,4-D is applied in-water to control some submersed species such as M. spicatum; however, it has not exhibited in-water activity on E. crassipes in preliminary studies (Getsinger et al. Reference Getsinger, Davis and Brinson1982; unpublished data). Therefore, the greater increases in 2,4-D and glyphosate efficacy from reduced carrier volumes compared with diquat in the current study may be related to in-water activity. However, further research is needed to test these hypotheses.
Overall, these data suggest that lower carrier volume herbicide applications made to E. crassipes provided high levels of efficacy with reduced spray coverage. In fact, there were several accounts of increased efficacy and rate of symptom development from reduced carrier volumes. Likewise, these data also support the evaluation of alternative application equipment and techniques that may be more discrete. While spray coverage was reduced by some application types, these data indicate that complete coverage is not a requirement for optimal E. crassipes control. Future research will investigate alternative application techniques with other herbicides and plant species as well as field verification of these techniques.
Acknowledgments
The authors extend their appreciation to the Florida Fish and Wildlife Conservation Commission for partial funding of the research. Additionally, we thank J. P. Keller and Jake Myers for assistance in conducting this work. No conflicts of interest have been declared.
