Background on the Aquatic Herbicides Registered for Use in Florida
Herbicides Described Below
There are seventeen herbicide active ingredients (chemical compounds) approved for use in Florida waters as of 2018. These active ingredients may be formulated and sold under various trade names such as Aquathol, Rodeo and Sonar. More than 100 different herbicide trade names were available among the 17 herbicide compounds for use in the FWC aquatic plant management program. They are applied directly to the target plant or dispersed within the water for the purpose of treating invasive weeds. These formulations are purchased and used by state, federal and local government agencies responsible for environmental management and weed abatement. They are also used by environmental contractors, farmers and, in some cases, homeowners. Permission in the form of a state permit may be legally required prior to herbicide application depending on type of water body, ownership, aquatic species present, proximity and hydrologic connection to other water bodies, amount of herbicides to be used, and other variables. Contact your regional Florida Fish and Wildlife Conservation Commission (FWC) office prior to herbicide application to ensure compliance with regulations and to gain helpful advice for your treatment.
According to the plant species being treated, the location, time of year, weather, water-oxygen levels, and a host of other variables, aquatic herbicides may be applied directly to the plant, directly to the water, or to the plant and water at the same time. On some occasions, environmental conditions may dictate that certain herbicides are not permissible. It is up to the user to follow the herbicide use label that accompanies the herbicide container and to follow it exactly. The herbicide’s label is a legal document that provides important information about the specific herbicide product that you are using.
A registered pesticide is one that has been researched and approved by the EPA for uses that are specified on the label and labeling. Look for the EPA registration number that must appear on all pesticide labels that confirms EPA registration.
The label explains in detail:
- which type of site may be legally treated with the herbicide (ex: lakes, flowing waters, canals, reservoirs);
- how to use the herbicide;
- which rates to use for specific plants and environmental situations;
- precautionary statements relative to the herbicide's possible hazards;
- specific first aid.
A separate document, the herbicide’s Material Safety Data Sheet (MSDS), presents specific technical information that is useful in the event of a spill or mistake. It includes hazardous ingredients, spill or leak procedures, data on physical components, fire and explosion, reactivity, health hazards, and special precautions.
Useful Terminology for Herbicide Use and Invasive Plant Management
The following terminology is often used to describe herbicides, their proper use and expected outcomes. This information can be obtained by reading the herbicide label, the MSDS and technical fact sheets and supplements provided by the herbicide manufacturer.
- The lethal plant dose is the amount of active ingredient required to kill a plant, often measured in parts per million (ppm) or parts per billion (ppb).
- A contact herbicide causes injury to plant tissue where contact occurs; contact herbicides control plants relatively quickly.
- A systemic or translocated herbicide is absorbed into the plant through the leaves, stems or roots and translocated throughout the plant to kill it from the inside. Systemic herbicides control plants less quickly than contact herbicides.
- Selectivity refers to the ability of a herbicide to kill certain types of plants without causing significant injury to others. Herbicides can be selective (narrow-spectrum) or non-selective (broad spectrum), based on the range of plants affected. Selectivity is also influenced by factors including application rate, time, method of application, environmental conditions and stage of plant growth.
- Liquid formulations contain active ingredients that are suspended in liquid. Liquid formulations may be best for certain species of plants or certain management situations, such as areas where water movement is slow, or where there are deep sediments.
- Dry formulations contain active ingredients that are mixed into dry, slow-dissolving pellets, granules, or wettable powders. Pellets or granules make it easier to "time-release" herbicide into the water or soil at the lethal dose rate. This is important especially in areas where water movement might dilute liquid formulations.
- “Mechanism of action” is the actual biochemical site of herbicide; generally, an enzyme or co-factor factor. In some cases, the actual site is unknown. See table below for mechanisms of action for the 17 herbicide compounds registered for use in Florida waters. See the Weed Science Society of America (WSSA) Summary of Herbicide Mechanisms of Action.
- “Mode of action” describes the symptoms that occur after herbicide application leading to plant after herbicide application leading to plant death
- An algaecide is a substance used for the control of algae. There are several EPA-registered algaecides approved for use in Florida waters.
- Half-life is the time it takes for the concentration of a compound such as a herbicide to be reduced by half because of breakdown or deactivation of the molecule.
- Breakdown is the chemical transformation of a herbicide active ingredient into non-toxic compounds. This can happen through hydrolysis (breakdown due to contact with water), microbial breakdown (degradation from the action of microbes) and photolysis (breakdown from the absorption of energy from sunlight).
General Information on Herbicides Registered for Use in Florida Waters
The following table lists the chemical name for the 17 classes of herbicides registered for use in Florida waters by the EPA and Florida Department of Agriculture Consumer Services (DACS). The table lists each herbicide class, the types of plants (submersed, emergent, floating) most often controlled by that herbicide in FWC management programs, the year the herbicide was registered by EPA and DACS for use in Florida waters, the herbicide mechanism of action (MOA), and the closely related Weed Science Society of America (WSSA) resistance management grouping.
The mechanism of action (MOA) describes the biological processes that are disrupted by the herbicide. These biochemical pathways control the growth and development of plants. When herbicides are applied, these processes cannot be carried out and plant injury and death will occur. Classifying an herbicide’s MOA and WSSA Grouping provides crucial information on the possibility of a plant population developing resistance to an herbicide MOA within a particular grouping after repeated use. For example: the over-reliance (in acres and time) on one MOA for weed control in an agricultural system can increase the probability of selecting for an herbicide-resistant population. With repeated applications, susceptible individuals of a target weed species will die off while the numbers of resistant plants will continue to expand. In time, the MOA will no longer control that species in that location. To prevent/mitigate herbicide resistance, it is advised to rotate or combine herbicide MOAs to reduce the selective pressure applied by any one product. There may be multiple products within an herbicide class (e.g. sixteen different glyphosate products were applied in FWC-funded control programs in 2017) and there are several herbicide classes with the same MOA (e.g. four ALS inhibitors) registered for use in Florida waters. Applying different products or different herbicide classes within the same MOA does not constitute resistance management. See Section 4 for a more detailed look into herbicide resistance.
|Herbicide||Application Site||Year Registered||Mechanism of Action||WSSA Group|
|2,4-D||Submersed, Emergent, Floating||1959||Synthetic auxin||4|
|Endothall||Submersed||1960||Protein phosphatase inhibitor – PP1||Unknown|
|Diquat||Submersed, Emergent, Floating||1962||Photosystem I inhibitor||22|
|Glyphosate||Emergent||1977||Enolpyruvyl shikimate-3-phosphate synthase Inhibitor – EPSP||9|
|Fluridone||Submersed||1986||Phytoene desaturase enzyme inhibitor – PDS||12|
|Peroxides||Submersed – algicide||2002||Oxidize cell membrane|
|Triclopyr||Submersed, Emergent||2002||Synthetic auxin||4|
|Imazapyr||Emergent||2003||Acetolactate synthase enzyme inhibitor – ALS||2|
|Carfentrazone||Submersed, Emergent, Floating||2004||Protoporphyrinogen oxidase enzyme inhibitor – PPO (Protox)||14|
|Penoxsulam||Submersed, Floating||2007||Acetolactate synthase enzyme inhibitor – ALS||2|
|Imazamox||Submersed, Emergent, Floating||2008||Acetolactate synthase enzyme inhibitor – ALS||2|
|Flumioxazin||Submersed, Emergent, Floating||2011||Protoporphyrinogen oxidase enzyme inhibitor – PPO (Protox)||14|
|Bispyribac||Submersed, Floating||2012||Acetolactate synthase enzyme inhibitor – ALS||2|
|Topramezone||Submersed||2013||p-Hydroxyphenyl pyruvate dioxygenase enzyme inhibitor – HPPD||27|
|Sethoxydim||Emergent – grass specific||2017||Acetyl CoA carboxylase enzyme inhibitor – ACCase||1|
|Florpyrauxifen||Submersed, Floating, Emergent||2018||Synthetic auxin||4|
General information follows on each of the 17 classes of herbicides registered for use in Florida waters. Specific use patterns and considerations for applying these compounds in FWC aquatic plant management programs are detailed in Section 4 of this web site.
Bispyribac has been labeled and used for weed control in rice for many years. It was registered by the U.S. EPA and FDACS for aquatic plant control in 2012 and is sold under the trade name Tradewind. Bispyribac’s mechanism of action is an ALS enzyme inhibitor and is in the WSSA Resistance Management Grouping #2. It is a systemic herbicide that is absorbed into and moves within the affected plant. Bispyribac accumulates in the growth regions (meristems) of plants where it inhibits a plant-specific enzyme called acetolactate synthase (ALS). When this enzyme is blocked, the production of certain amino acids necessary for plant growth and development is stopped. The amino acid synthesis that is blocked by bispyribac is specific to plants; this biochemical pathway does not occur in animals. The plant stops growing and eventually dies over a long period of time – generally weeks to months. Like most systemic aquatic herbicides, control is highly dependent on contact time or exposure of the plant to the herbicide. The primary degradation pathway for bispyribac is microbial metabolism. The half-life in water is about 30 days.
Bispyribac is most often applied in FWC management programs to control hydrilla (Hydrilla verticillata). It is applied alone via subsurface injection at 30-45 ppb and requires 60-90 days of exposure to achieve control. With a half-life in water of about 30 days, monitoring and reapplication to sustain the original prescribed dose and exposure period is required. Combining bispyribac at 30 ppb with 1.0 ppm potassium endothall not only reduces the exposure time required to control hydrilla, but also provides a measure of resistance management by applying two different mechanisms of action. Research with bispyribac is still underway in many areas, but the herbicide appears to offer selective control of submersed and a few floating aquatic weed species.
Carfentrazone was registered for aquatic plant control in Florida waters in 2004. Its mechanism of action is classified as a protoporphyrinogen oxidase (PPO) enzyme inhibitor and is listed along with flumioxazin in the WSSA Resistance Management Grouping #14. Carfentrazone is a contact herbicide. It does not move within the plant. It is absorbed through the leaves and inhibits the protoporphyrinogen oxidase enzyme that is important in chlorophyll synthesis. Carfentrazone needs 1-2 hours of contact for good herbicidal activity. It is fairly slow acting once inside the leaves, causing symptoms in 2-5 days and plant necrosis in about 3-4 weeks. It provides good selectivity in that it will not control comingled non-target plants like pickerelweed or grasses that may be mixed with water lettuce. Carfentrazone breaks down both through microbial action in soil and through hydrolysis with a half-life of 3-5 days in water. It has very low toxicity to fish and waterfowl.
Carfentrazone has limited use in FWC management programs. It is primarily applied in combination with imazamox directly to the leaves to control Uruguayan primrose complex (Ludwigia grandiflora / hexapetala). While this combination may provide faster control, it is not confirmed if carfentrazone adds long-term efficacy above imazamox alone. Carfentrazone is occasionally applied via foliar applications to control water lettuce and sometimes water hyacinth.
Copper is a fast-acting, broad-spectrum, contact herbicide that kills a wide range of algae and aquatic plants. Although copper is a micro-nutrient required by living plants and animals in small amounts, too much copper kills plants by interfering with plant enzymes, enzyme co-factors, and plant metabolism in general. The mechanism of action is undefined; however, copper-based products are believed to target specific physiological processes such as electron transport in photosystem I, cell division and nitrogen fixation. Copper is not classified in the WSSA Resistance Management Grouping. Several environmental factors influence bioavailability of copper in aquatic systems including: pH, alkalinity, hardness, ionic strength, organic matter, and redox potential. Copper chelates are broken down by hydrolysis and rapidly decline to ambient concentrations with a half-life reported from 2-8 days depending on conditions.
Copper has long been used in natural and industrial waters for algae control, often applied directly to water as blue copper sulfate crystals. Today in Florida, chelated copper is most often used for aquatic plant and algae management. Chelate is a chemistry term meaning combining a metal ion, in this case, copper, with an organic molecule, triethanolamine or ethylenediamine. Chelated liquid copper products reportedly remain in solution longer than copper salts (when applied to hard water). Copper that is in solution (suspended in the water) for a longer time has greater effect on the aquatic plants and algae. All copper formulations are considered highly toxic to mollusks and can be toxic to some species of fish at relatively low doses, especially if the water has less than 50 ppm of carbonate hardness (soft water). Levels of 1-5 ppm are toxic to fish, so copper is usually applied at concentrations 1 ppm or less. Toxicity generally decreases as water hardness increases. Chelated copper is also less toxic than copper salts to non-target organisms.
Several different brands of copper are available for aquatic plant and algae control in Florida; however, the chemistry and mechanism of action of each is similar. Because copper is an element, it will accumulate in the sediments regardless of its bioavailability. The FWC only permits or funds the use of copper herbicides in waters when no alternative management options are available. Most notably, FWC authorizes the use of copper herbicides to control aquatic plants near potable water intakes where use of other herbicide compounds may be restricted. There are no drinking water restrictions for copper herbicides when applied at label rates.
Diquat is a fast-acting contact herbicide. It was first formulated in the mid-1950s and used in aquatic plant control beginning in 1962. Its mechanism of action is a Photosystem 1 inhibitor and is classified in the WSSA Resistance Management Grouping #22. Diquat is rapidly absorbed by plant leaves and interferes with cell respiration, the process by which plants take in oxygen. Diquat interferes with photosynthesis by forming highly reactive and toxic free-radicals, such as peroxide and super oxide, in plant cells. Plant tissues are killed too quickly to allow translocation to other parts of the plant. Diquat kills the aerial portions of plants in 24-36 hours. Diquat is water soluble and diffuses rapidly through the water, quickly absorbing into plant tissue. However, diquat is also strongly cationic and has a positive charge, so it quickly adsorbs to and is tightly held by the negative charges of clay and peat. For this reason, diquat is ineffective in muddy waters where it will bind to the suspended particles. Diquat becomes inactivated in the water and breaks down slowly in sediments. Half-life in water is 1-7 days.
In FWC-funded programs, diquat is mainly used to control invasive floating plants including water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), giant salvinia (Salvinia molesta) and feathered mosquito fern (Azolla pinnata). Diquat is also applied alone or in combination with 2,4-D to control Cuban club rush (Cyperus blepharoleptos). Diquat is relatively ineffective when used alone to control hydrilla (Hydrilla verticillata). However, combining diquat with chelated copper provides quick, albeit temporary hydrilla control. A synergism has been reported between these two compounds such that hydrilla takes up more diquat and copper when they are applied together than when equal concentrations are applied separately. This is a good option for quick control to eradicate or contain small pioneer populations, e.g., around boat ramps. Diquat and copper are not often considered for large-scale hydrilla control because repetitive treatments increase the amount of copper in the system. Diquat is used in combination with potassium endothall or flumioxazin herbicides for small to moderate-scale hydrilla control. In each case, the addition of diquat provides more rapid and perhaps more thorough hydrilla control while introducing a second mode of action for herbicide resistance management. Resistance has been confirmed in Landoltia spp. (a duckweed species) in Florida after repeated diquat use.
Endothall acid, first available as an aquatic herbicide in the 1960s, was originally used in agriculture as a plant desiccant. Products that contain endothall are used primarily for control of submersed weeds as well as filamentous algae. The active ingredient is relatively fast-acting and is formulated into two compounds for aquatic use: a potassium salt (potassium endothall) and an alkylamine salt (amine endothall). Both compounds are available in liquid and granular formulations. The mechanism of action for endothall herbicides is categorized as protein phosphatase inhibitor. Endothall is listed in the WSSA Resistance Management Group “Unknown”. Endothall resistance in hydrilla (Hydrilla verticillata) has been reported in two Florida public waters after repeated use of the potassium salt formulation.
Endothall interferes with plant respiration and photosynthesis by disrupting plant cell membranes. Endothall breaks down in water microbially and the half-life is approximately 5-10 days. Because endothall is fast-acting, it was long considered to function as a contact herbicide, but it is somewhat mobile in plant tissues. Endothall is absorbed by submersed plants in lethal concentrations in 12-36 hours depending on the concentration applied. Endothall acid works by interfering with plant respiration, affecting protein and lipid biosynthesis and disrupting plant cell membranes. It causes cellular breakdown of plants within 2-5 days. Symptoms of plant damage, including defoliation and brown shriveled tissues, will become apparent within a week of herbicide application. Plants will fall out of the water column within 3-4 weeks after application.
Potassium endothall, normally used at rates of 2-3 ppm, is not toxic to adult fish, eggs or fry at rates of 100-800 ppm depending on fish species. Potassium endothall is used extensively in Florida’s hydrilla management program since hydrilla’s increasing tolerance to fluridone herbicide was confirmed in 2000. FWC sponsored extensive research and operational monitoring with the University of Florida and U.S. Army Corps of Engineers to develop new use patterns with potassium endothall alone and in combination with other herbicide active ingredients, including bispyribac, diquat, fluridone, penoxsulam and topramezone. Objectives include improving cost-effectiveness and resistance management, increasing selectivity, and reducing overall herbicide use. These objectives are more achievable when applying potassium endothall to large areas in cooler months (November – April) when hydrilla is actively growing but native submersed plants may be dormant. Cooler water slows microbial degradation of potassium endothall allowing lower rates (1.5 – 2.0 ppm) and providing a more thorough control of the hydrilla standing crop. Killing the stems and root crowns slows hydrilla’s ability to recover, forcing regrowth to come from tubers, turions and fragments. Good results have been achieved for large-scale hydrilla control using potassium endothall in combination with bispyribac and penoxsulam, and for smaller scale control when applied in combination with diquat.
In contrast to potassium endothall, amine endothall is 2-3 times more active on plants, but 200-400 times more toxic to fish than the potassium salt. Laboratory studies have shown the monoamine salts are toxic to fish at dosages above 0.3 ppm. The liquid formulation will readily kill fish present in a treatment site. EPA approved label rates for plant control range from 0.05 to 2.5 ppm. In recognition of the extreme toxicity of the alkylamine salt, amine endothall is rarely used for aquatic plant control in FWC aquatic plant management programs. When used, it is applied, usually to small areas and at very low rates, with potassium endothall for hydrilla and crested floating heart (Nymphoides cristata) control to increase efficacy and to introduce a second active ingredient for herbicide resistance management.
Florpyrauxifen-benzyl was registered by the U.S. EPA and DACS for aquatic use in 2018. Florpyrauxifen-benzyl represents a new mode of action for hydrilla management and offers unique properties for aquatic plant control as a member of the arylpicolinate herbicide chemical family. It has been assigned reduced risk status from U.S. EPA. It is practically non-toxic to fish and birds and has no drinking, fishing or swimming restrictions. Florpyrauxifen-benzyl is a systemic herbicide with an affinity for aquatic vegetation. It is classified in the WSSA Resistance Grouping #4; an auxin mimic. It absorbs through leaves and shoots via foliar treatments or underwater tissues during in-water applications. Following rapid uptake, the herbicide translocates in the xylem and phloem and accumulates in meristematic tissues where it bonds with a specific target receptor (AFB5-Aux/IAA co-receptor). It mimics the effect of a persistent high dose of the natural plant hormone auxin, causing over-stimulation of specific auxin-regulated genes, which results in the disruption of several growth processes in susceptible plants. Tissues which are undergoing active cell division and growth are particularly sensitive. Susceptible plants become brittle and shatter within a few days after exposure. Plants exhibit reduced growth, and ultimately turn chlorotic and necrotic, reaching a level of control within 1-3 weeks after treatment.
The primary breakdown pathway for florpyrauxifen-benzyl in water is by photolysis. The half-life in shallow water is < 12 hours and 1-2 days in deeper water. Breakdown is slightly enhanced via hydrolysis in waters > pH8. Very high turbidity or algal content may also subtly reduce florpyrauxifen-benzyl uptake by target aquatic weeds following in-water application due to the herbicide’s strong binding properties and merit the use of higher rates within the label range. Florpyrauxifen-benzyl is an auxin mimic herbicide and is subject to the DACS organo-auxin rule – 5E-2.033 F.A.C. While having low volatility, it should not be applied aerially in winds less than 2 mph or greater than 10 mph (see spray drift management on product label). Due to its short half-life and rapid plant uptake, florpyrauxifen-benzyl is recommended to be expeditiously applied in tight swaths for spot treatment of target submersed plants. Consequently, minor dissipation is expected from treatment plots during in-water applications. It is recommended that the herbicide manufacturer (SePRO) be contacted for site-specific use recommendations.
Florpyrauxifen-benzyl is registered to manage a broad range of invasive plants that are prevalent in Florida waters. Floating and emergent plants include water hyacinth (Eichhornia crassipes), floating heart (Nymphoides cristata, and water primrose (Ludwigia spp., Eurasian watermilfoil (Myriophyllum spicatum, hydrilla (Hydrilla verticillata), and hygrophila (Hygrophila polysperma) are on the list of submersed species. While collaboratively assessed in mesocosm and pond studies by the U.S. Army Corps of Engineers, the University of Florida, and other state/university partners, as of mid-2018, there is little operational experience for management in large systems because of the recent registration. Early field evaluation/demonstration will proceed with florpyrauxifen-benzyl applied alone and in combination with other herbicides including diquat, fluridone, imazamox, penoxsulam, and potassium endothall.
Flumioxazin was registered for aquatic use in 2011. It is a contact herbicide with the same mechanism of action as carfentrazone and the onset of rapid injury is similar to other contact herbicides. However, flumioxazin has a broader spectrum of activity compared to carfentrazone. Flumioxazin is a protoporphyrinogen oxidase (PPO or protox) enzyme inhibitor, listed in the WSSA Resistance Management Group #14. It moves within treated leaves but does not translocate to other areas of the plant. Once inside the plant cell, flumioxazin inhibits the PPO enzyme. Through a series of cascading reactions, cell membranes are destroyed by lipid peroxidation reaction. The result is cell leakage, inhibited photosynthesis, bleaching of the chloroplasts, and cell death. Plant necrosis and death is rapid, taking a few days to a week or two. In general, at least four hours of contact time is required for good control of susceptible plants. The primary breakdown pathway of flumioxazin in water is by hydrolysis and is highly dependent on water pH. Under high pH values (> 9), flumioxazin half-life in water is 15-20 minutes. Under more neutral pH values (7-8), half-life in water is around 24 hours.
Flumioxazin is applied in FWC programs to control a wide variety of aquatic weeds and algal species. Submersed plants include hydrilla (Hydrilla verticillata) and cabomba (Cabomba caroliniana). Floating water lettuce (Pistia stratiotes), duckweeds (e.g. Lemna and Spirodella spp.,), giant salvinia (Salvinia molesta), watermeal (Wolffia spp.) and surface mats of some algae are susceptible to flumioxazin. Label application rate recommendations range from 200-400 ppb for submersed plants and 6-12 oz per acre applied as a foliar spray for floating plants. Flumioxazin use patterns are still being developed, but rates as low as 2-4 oz per acre foliar spray or 50 ppb submersed application are reported to be effective for water lettuce control. Field evaluations have shown that surface and submersed applications of flumioxazin also provide good control of spatterdock (Nuphar spp.), waterlily (Nymphaea spp.) and American lotus (Nelumbo lutea). Flumioxazin can be tank-mixed with other contact or systemic herbicides to enhance control of emergent weeds like large flower primrose willow (Ludwigia grandiflora) when used in combination with glyphosate, imazamox or auxin mimic herbicides.
Fluridone is a systemic herbicide, discovered in the mid-1970s and used for weed control in cotton. Fluridone was later shown to be effective for the control of submersed plants and was registered by the U.S. EPA and DACS for aquatic use in 1986. Fluridone is a phytoene desaturase (PDS) enzyme inhibitor listed in the WSSA Resistance Management Group #12. Fluridone is absorbed through the leaves and then translocated to shoot and root tissues. It inhibits the synthesis of light-shielding carotenoid pigments and allows ultraviolet light to destroy essential chlorophyll pigments. Without chlorophyll, the plant is unable to photosynthesize. Shoot tips bleach pink or white, and the plant slowly starves and dies. The primary breakdown pathway for fluridone is through photolysis, although microbes also degrade the molecule. The half-life in Florida depends on light intensity but is usually 20 days or longer. Fluridone half-lives of 7-10 days have been reported in Florida. This has been attributed to enhanced microbial degradation in waters that have received repeated large-scale fluridone applications for hydrilla (Hydrilla verticillata) control. Fluridone does not accumulate in fish or zooplankton and does not affect algae. It has a low toxicity to fish at typical treatment rates in Florida waters between 8 and 15 ppb. Fluridone is approved for use in waters at rates up to 150 ppb.
Fluridone is a broad-spectrum herbicide that controls a variety of submersed and floating plants such as duckweed and Salvinia species. It is formulated as both a liquid, and as slow or fast-release pellets. Application rates for plant control are in the 8-15 ppb range. The exposure period for submersed plant control is measured in weeks or months and hours or days for floating plants. Fluridone must be kept at prescribed concentrations for at least 45-80 days for optimum long-term control of hydrilla. Even longer exposure is required for mature plants with high carbohydrate reserves. Once applied, fluridone can break down or dissipate from the application zone via wave action or water currents. Periodic monitoring and reapplications are essential to ensure the proper fluridone concentration is maintained in the hydrilla control zone long enough to kill the hydrilla standing crop.
Resistance to this herbicide was confirmed at several research institutions in 2000 after repeated large-scale fluridone applications for hydrilla control in Florida from the late 1980s through the 1990s. This was the first occurrence of resistance for a bleaching-type herbicide and the first for a plant species based solely on somatic mutations. Hydrilla produces only asexually in Florida leaving no avenue for gene recombination. Fluridone attacks only one gene location in hydrilla and several genotypes have been reported in Florida, each with different fluridone susceptibilities. Repeated fluridone use effectively removed the susceptible hydrilla genotypes, leaving the more tolerant plants to expand.
Once the mainstay of hydrilla control in Florida public waters, Fluridone is now used on a limited scale in FWC management programs. Strategies using fluridone have evolved due to the unexpected resistance development. Pre-application bioassays are essential to determine each hydrilla population’s current level of fluridone tolerance. Applying too little fluridone is wasteful in that it will not control the target plant. Applying too much fluridone is not only wasteful, but it may also injure non-target vegetation in the system.
Glyphosate is a broad-spectrum herbicide used to control annual and perennial broadleaf weeds and grasses, trees, and certain floating plants. It does not control plants that are completely submersed or have most of their leaves under water as it lacks activity in water. Glyphosate is a systemic herbicide that is applied to the foliage and moves throughout the plant to cause damage. Its mechanism of action is an enolpyruvyl shikimate-3-phosphate synthase (EPSP) enzyme inhibitor and is listed in the WSSA Resistance Management Group #9. Glyphosate works by inhibiting the synthesis of specialized plant amino acids. Without the ability to manufacture these essential components, plant death occurs slowly over a period of 2-3 weeks. Animals do not produce these enzymes, explaining the very low toxicity of this herbicide to animals. Visible effects on most annual weeds occur within 4-7 days (more on most perennial weeds), and 30 days or more on most woody plants and trees. Glyphosate breaks down in water microbially with a variable half-life reported from 12-60 days. It binds readily with soil or suspended organic particles, effectively inactivating the chemical activity. Also, hard water (containing calcium or magnesium cations) used to make up a spray solution may bind some of the glyphosate in the mix tank prior to application.
Glyphosate is used alone or in combination with diquat, flumioxazin, imazapyr, or 2,4-D to control a wide variety of plants in FWC management programs. Invasive grasses include torpedograss (Panicum repens), paragrass (Urochloa mutica), Tropical America watergrass (Luziola subintegra), and West Indian marsh grass (Hymenachne amplexicaulis). Other plants include Cuban club-rush (Cyperus blepharoleptos), cattail (Typha spp.), primrose willow (Ludwigia spp.) and floating islands of mixed herbaceous and woody species. Glyphosate is most effective when applied to actively growing plants that have not recently been mowed, grazed or otherwise disturbed. However, an effective management strategy for torpedograss control is to burn off the standing crop to the soil surface and wait for regrowth to expend the plant’s carbohydrate reserves; applying glyphosate alone or in combination with imazapyr where appropriate to the actively growing leaves and stems. Selective management of plants using glyphosate is achieved only by careful application because, in general, glyphosate damages most plants it contacts. Heavy rainfall or irrigation within 2 hours of application may wash away the herbicide, requiring a repeat treatment; rainfall or irrigation within 6 hours of application may also reduce effectiveness. There are an increasing number of glyphosate-resistant weeds reported in agricultural settings. While none have been reported in Florida for aquatic use, it is important to rotate or combine herbicides with different MOAs to the extent possible when applying glyphosate.
Sodium carbonate peroxyhydrate is a granular or liquid substance registered by the U.S. EPA and DACS for use in Florida waters in 2002. Peroxides are oxidizing agents and are unclassified under the WSSA Resistance Management Grouping. Sodium carbonate peroxyhydrate is transformed into hydrogen peroxide and sodium carbonate in the presence of water. Hydrogen peroxide is the active component and works by oxidizing critical cellular components of the target organism and kills it. For example: in lipid peroxidation, oxygen radicals react with unsaturated fatty acids in cell membrane phospholipids, sequentially damaging them and killing the cell in a chain reaction. Oxygen radicals also react with other fatty acids, nucleic acids, and proteins in a similar manner. Sodium carbonate peroxyhydrate rapidly dissociates via hydrolysis into hydrogen peroxide and sodium carbonate. Hydrogen peroxide is further degraded to water and oxygen while sodium carbonate is neutralized to sodium bicarbonate. The half-life for this process is approximately eight hours. When applied according to label directions, no harm is expected to birds or freshwater fish and invertebrates.
Products containing sodium carbonate peroxyhydrate are used in aquatic systems for control of planktonic algae, especially blue-green algae (also known as cyanobacteria). They are also used for controlling problematic algae in domestic water supply sources. Planktonic algae control is currently not a priority of the FWC; therefore, FWC has not permitted or funded the use of hydrogen peroxide algaecides. The granular formulation has been evaluated to control dense mats of the filamentous cyano bacteria, Lyngbya (now Microseira wollei) that is becoming increasingly common in Florida’s freshwater spring runs. Repeated applications of hydrogen peroxide algaecides with amine endothall or copper have provided Microseira control in hydropower reservoirs in neighboring states, but this method is not operational in FWC-funded aquatic plant management programs.
Imazamox was registered for aquatic use in 2008. Imazamox is a systemic herbicide that is applied to plant foliage to control floating or emergent plants, or to the water for submersed plant control. The WSSA identifies imazamox mechanism of action as an acetolactate synthase (ALS) enzyme inhibitor in Resistance Management Group #2. It works by stopping the production of three amino acids (isoleucine, leucine, and valine) which, in turn inhibit the production of ALS enzymes and other proteins that are built from these amino acids. Although the exact mechanism is not understood, when ALS is inhibited, plants die. Animals do not produce these particular enzymes, so imazamox has low toxicity to animals. Imazamox is classified as practically non-toxic to fish and birds. Enzyme inhibiting herbicides act very slowly. Imazamox is broken down in the water by photolysis and microbial degradation. Its half-life in water is 7-14 days.
Imazamox is available in liquid and granular formulations. It must be applied to actively growing plants; preferably early in the season to young plants with lower carbohydrate reserves. It absorbs rapidly into plant tissues and growth of susceptible plants is generally inhibited within a few hours after application. In this regard, it acts somewhat like a contact-type herbicide, requiring a short exposure period. However, plant death is relatively slow. Meristematic areas become chlorotic after 1-2 weeks followed by general chlorosis and death in 2-6 weeks. In Florida, the primary aquatic plant management uses of imazamox include foliar applications to control cattail (Typha spp.), wild taro (Colocasia escuelenta), Uruguayan primrose willow complex (Ludwigia grandiflora / hexapetala), and water hyacinth (Eichhornia crassipes). Imazamox may be applied alone or in combination with other herbicides like glyphostate or carfentrazone. Applying imazamox alone provides a measure of selectivity for comingled non-target plants. Combining with glyphosate or carfentrazone results in more rapid and thorough control, but is usually limited to monocultural stands of target plants or in areas where selectivity is not a concern. Imazamox may provide up to a year of control of water hyacinth via root uptake from in-water applications.
Imazapyr was discovered in the 1970s and first sold in 1984. It was registered by the U.S. EPA and DACS for use in Florida waters in 2003. Imazapyr’s mechanism of action is an acetolactate synthase (ALS) enzyme inhibitor and it is listed in the WSSA Resistance Management Group #2. Imazapyr is a systemic herbicide that is quickly absorbed by leaves and shoots. It moves to areas of new growth where it shuts down plant growth almost immediately. In this regard, imazapyr acts like contact herbicide. It prevents the production of the ALS enzyme, without which, the plant cannot continue growing and eventually starves and dies. Susceptible plants become reddish at the growth tips within 1-2 weeks. Control may take 2-6 weeks. Imazapyr breaks down by light and has a half-life in Florida waters of about 2-3 days. Imazapyr is practically non-toxic (the EPA’s lowest toxicity category) to fish, invertebrates, birds and mammals.
Imazapyr should be applied to plants that are actively growing. If applied to mature plants, a higher concentration of herbicide and a longer contact time will be required. Imazapyr is applied alone or with glyphosate in FWC management programs to control cattail (Typha spp.), tussocks (floating masses of herbaceous and woody species), torpedograss (Panicum repens), Cuban club-rush (Cyperus blepharoleptos), and woody species growing in water, such as melaleuca (Melaleuca quinquenervia). Imazapyr is not recommended for control of any submersed aquatic species. Cautions on the label regarding damage to non-target vegetation and lengthy irrigation restrictions must be carefully followed, limiting imazapyr use to rural settings where irrigation is not a consideration.
Penoxsulam was registered for aquatic use by the U.S. EPA and DACS in 2009. It is a systemic herbicide that is applied to plant foliage to control floating or emergent plants, or to the water column for submersed plant control. Penoxsulam’s mechanism of action is an acetolactate synthase (ALS) enzyme inhibitor. It is listed in the WSSA Resistance Management Group #2. Penoxsulam is a systemic herbicide that is absorbed by foliar tissues and moves to areas of new growth. It inhibits the ALS enzyme that regulates the production of essential amino acids in plants. When ALS is inhibited, plants die. Animals do not produce these enzymes, so penoxsulam has low toxicity to animals. Enzyme inhibiting herbicides act very slowly. Control is highly dependent on contact time. For some species and circumstances, split or multiple applications are necessary to keep the herbicide concentration at a prescribed level for 90-120 days for optimum performance. Penoxsulam is broken down primarily via photolysis and to a lesser extent by microbes. Its half-life in Florida waters is about 2-4 weeks.
While penoxsulam controls a variety of aquatic plants, its primary use in FWC management programs is to control hydrilla (Hydrilla verticillata) and water hyacinth (Eichhornia crassipes). When penoxsulam is applied alone, it may require more than 90 days of exposure to control hydrilla. This usually necessitates one or more subsequent applications after the initial treatment to sustain an appropriate dose for control. Research and operational monitoring have demonstrated that applying penoxsulam in combination with potassium endothall provides several advantages over penoxsulam alone. Resistance to ALS herbicides has been reported in terrestrial plants and a second active ingredient assists in resistance management for both herbicides. Combining with potassium endothall reduces the time necessary for penoxsulam to be exposed to hydrilla to about 7-14 days, reducing the effects of degradation and dissipation and the need for additional applications to sustain appropriate penoxsulam concentrations in the water column. Applying these herbicides in combination requires less of each herbicide to control hydrilla and seems to increase selectivity to conserve non-target native plants. Likewise, applying penoxsulam in combination with flumioxazin or carfentrazone has provided an effective water hyacinth management tool with increased resistance management benefit over penoxsulam alone.
Sethoxydim is a foliar-applied selective herbicide used to kill and suppress annual and perennial grasses with little to no impact on broadleaf plants. It is applied as a post-emergent herbicide and requires the addition of an oil adjuvant or nonionic surfactant for maximum effectiveness.
Sethoxydim was discovered in the late 1970’s, and first registered on soybean and cotton in the early 1980’s. It was the first of a large group of postemergence herbicides in the chemical family cyclohexanedione ‘DIMs’, that selectively controls grasses in broadleaf crops. Sethoxydim is an amber-colored, oily, odorless liquid. It is a General Use Pesticide (GUP) in EPA toxicity Class III; slightly toxic. Products containing sethoxydim bear the Signal Word WARNING on the label.
Sethoxydim is in the Class 1 Group of herbicides (lipid biosynthesis inhibitors). It is absorbed rapidly through leaf surfaces, transported in the xylem and phloem, and accumulates in meristematic tissues. Sethoxydim inhibits acetyl CoA carboxylase enzyme that prevents fatty acid production, leading to failure of cell membrane integrity, especially in regions of active growth. Lipids are an important component in cell division and plant growth. If plant cells cannot divide, the plant will die. This results in a cessation of shoot and rhizome growth, leading to necrosis and death of shoot meristems and rhizome buds, and ultimately plant death.
Sethoxydim is water soluble, does not bind readily with soils, and therefore has the potential to be mobile. Rapid degradation, however, generally limits extensive movement of sethoxydim in the environment. It is degraded rapidly by microbes and through photolysis, and possibly by hydrolysis. The half-life of sethoxydim in the field ranges from 5 to 25 days. In water, sethoxydim can be degraded by sunlight within an hour. Sethoxydim is not highly volatile.
Symptoms include cessation of growth within 2-3 days after application; growing tissue in the nodes and buds become necrotic, young leaves are first affected and turn yellow (within 1-4 wks) and then brown, depending on growing conditions. Leaf sheaths become brown and mushy at or just above their point of attachment to the node; older leaves show yellowing, light purpling and browning; susceptible plants are typically dead within 2-4 weeks and those not completely dead may show excessive tillering.
Topramezone was registered for aquatic use in Florida waters by the U.S. EPA in late 2013 and Florida DACS in early 2014. It is a systemic herbicide that is applied to the water column for submersed or floating plant control, or directly to foliage of floating and emergent vegetation. Topramezone is the first aquatic-registered herbicide belonging to the chemical class called pyrazolones. In sensitive plant species, topramezone inhibits the enzyme 4-Hydroxy-Phenyl-Pyruvat-dioxygenase (4-HPPD) leading to a disruption of the synthesis and function of chloroplasts. Consequently, chlorophyll is destroyed by oxidation resulting in bleaching symptoms of the growing shoot tissue (white or pink coloration) and subsequent death of the above ground portion of the pant. Topramezone is listed in the WSSA Resistance Management Group #27. Isolated resistance to 4-HPPD compounds has been confirmed in terrestrial species, but there is no evidence of resistance in aquatic plants. Topramezone breaks down via photolysis with a half-life in water ranging from 4-6 weeks. Microbial degradation is a minor breakdown pathway for topramezone that may also adhere somewhat to suspended clay particles.
Topramezone is under evaluation to determine optimum exposure periods and concentrations for hydrilla (Hydrilla verticillata) control. Generally, topramezone is applied at 25-40 ppb and maintained at or near the initial concentration for a minimum of 60 days. This requires monitoring and possible reapplications to sustain a prescribed dose until the plants die. Applications are made to actively growing plants early in the growing season before mature plants can build carbohydrate reserves, mat at the water surface, and slow growth and subsequent herbicide response. Applying early in the growth stage reduces the amount of herbicide and the exposure period necessary to control hydrilla. Topramezone is absorbed into the plant tissue and symptoms generally first appear in 7-10 days. Water hyacinth (Eichhornia crassipes) has been controlled via root uptake of topramezone in waters treated for hydrilla control. Operational use is under evaluation for foliar and in-water applications to control water hyacinth and water lettuce (Pistia stratiotes).
Triclopyr has been widely used to control herbaceous and woody plants in non-cropland sites, forestry and pastures. It was registered for aquatic use by the U.S EPA and DACS in 2002. Triclopyr is a systemic auxin mimic herbicide and is listed in the WSSA Resistance Management Group #4. Triclopyr is absorbed by foliage and translocates throughout plant tissues. It moves to areas of new growth and causes a disruption in hormone levels, interfering with normal expansion and division of plant cells. It acts like a growth stimulant in some plant tissues and a growth retardant in others. Symptoms include cupped leaves and twisted stems. Vascular tissue becomes crushed, stopping movement of essential nutrients and sugars. Plants essentially grow themselves to death. Photolysis is the primary breakdown pathway in water. Triclopyr has a short half-life depending on season and water depth (e.g. 2.5 days in shallow water during the summer to 14 days in deeper water in winter. Triclopyr does not bind strongly or adsorb to soil particles.
Triclopyr has been used extensively in Florida’s upland invasive plant management program for basal and foliar applications to control Brazilian pepper (Schinus terebinthifolia). It is effective for controlling emergent aquatic plants, some floating plants such as water hyacinth (Eichhornia crassipes), and some submersed aquatic weeds such as Eurasian watermilfoil (Myriophyllum spicatum). However, it is only occasionally applied in aquatic situations in Florida due to extensive irrigation restrictions of 120 days for treated waters. Triclopyr is applied with glyphosate, 2,4-D, or imazapyr to control tussocks (mixed floating masses of woody and herbaceous plants). It is also occasionally used to control primrose willow (Ludwigia octovalvis or peruviana) via foliar or in-water applications with 2,4-D.
2,4-D, or 2,4-Dichlorophenoxyacetic acid, is the oldest organic herbicide registered in the U.S. It is primarily used for weed control in food crops (grains, corn, sorghum, rice, sugarcane), turf, non-crop areas and in certain aquatic environments. 2,4-D was first applied in Florida waters to control water hyacinth (Eichhornia crassipes) in 1959. It is an auxin mimic herbicide listed in the WSSA Resistance Management Group #4. 2,4-D is a systemic herbicide. It is absorbed by roots and leaves and then translocates and accumulates mainly in the growing points of shoots and roots. 2,4-D interferes with the plant’s ability to maintain proper hormone balance. Plants undergo uncontrolled growth in some tissues and halted growth in other tissues. The result is injury to the growing regions of the plant and then a gradual death, usually within 3-5 weeks. Microbial degradation is the primary breakdown pathway for 2,4-D that has a half-life ranging from one to several weeks. The half-life is shorter in warmer months and in waters to which 2,4-D has been previously applied, presumably where microbial activity is greater.
The two main formulations currently in use for aquatic sites in Florida are the liquid dimethylamine salt, and the granular butoxyethyl ester. The granular formulations of 2,4-D sink to the bottom and slowly release herbicide into the water. Granular 2,4-D is applied for the control of water milfoil (Myriophyllum spp.), and for some floating-leaved species. In FWC management programs, the liquid formulations of 2,4-D are mixed with water and sprayed onto the leaves of water hyacinth and other broadleaf aquatic weeds including Cuban club-rush (Cyperus blepharoleptus) and primrose willow (Ludwigia spp.) 2,4-D is applied alone or mixed with other herbicides like diquat, flumioxazin, or glyphosate to improve efficacy or resistance management.
2,4-D is sometimes confused with “Agent Orange” a name given to the military’s plant defoliant mixture that was created and used during the Vietnam War. During the manufacture of Agent Orange, it became contaminated with a cancer-causing dioxin, tetrachlorodibenzo-p-dioxin, known as TCDD. While 2,4-D was one of the components of Agent Orange, it is not Agent Orange, nor does it contain TCDD, nor has it been shown to cause cancer. After numerous lifetime feeding studies in rats and mice, the U.S. EPA has classified 2,4-D as Class D compound – Not Classifiable as to Human Carcinogenicity.