Aquatic Plant Management In Lakes and Reservoirs
- CHAPTER 5 -

AQUATIC PLANT MANAGEMENT TECHNIQUES


There is more than one way to skin a cattail. - Roberta Goodwin

Introduction

The individual who perceives an aquatic plant problem should first determine what state agency or agencies are responsible for aquatic plant management. The agencies should then be contacted to determine what assistance is available and what the individual can legally do on his or her own. Problems on public lakes, which affect the public's access and use of the lake, will normally be the responsibility of a public agency. Decisions concerning perceived whole-lake problems on private lakes should be addressed through consensus of a home owners association after obtaining recommendations from public agencies.

Whole-lake problems should be managed by a public agency or a commercial aquatic plant management firm who has the necessary equipment and expertise. Management of aquatic vegetation in small areas along private beaches or around boat docks may be accomplished by the individual property owner, although even in these situations it usually is best to obtain the services of an experienced aquatic plant manager. It is essential for an individual who decides to conduct his or her own aquatic plant management to determine what can be legally done. If herbicides are used, it is essential to use only herbicides that are registered for use in aquatic sites and to become fully trained in their use. This information should be available from a county Cooperative Extension Service Office, state natural resources agency, or state department of agriculture.

The diversity of lake types and expectations of water resource users demands that commercial and public aquatic plant managers, as well as individual waterfront property owners, carefully choose the most appropriate method or combination of methods to manage aquatic plants for each individual situation. Methods that may be considered for managing aquatic plants include, physical removal, habitat alteration, biological controls, and herbicides. The effectiveness and benefits of methods for controlling the pest plant must be weighed against potential impacts on nontarget plants and animals and impacts on water uses such as swimming, fishing and domestic purposes. This section will discuss the most often used methods of managing aquatic vegetation.

Physical Removal

Hand Removal

Removal of small amounts of vegetation by hand, which interfere with beach areas or boat docks, may be the only vegetation control that is necessary. Of course, hand removal is labor intensive and must be conducted on a routine basis. The frequency and practicality of continued hand removal will depend on availability of labor, regrowth or reintroduction potential of the vegetation, and the level of control desired.

Regrowth of vegetation will depend on the plant species present, lake trophic status, and seasonal growth trends. Plants such as cattails and many grasses, that can reproduce from small root fragments require frequent removal because it is impossible to remove these plants without leaving root fragments in the sediment, where regrowth occurs. Most aquatic plants tend to grow rapidly during spring, while growth slows during fall and may cease during winter. This growth pattern becomes more pronounced as one moves from southern to northern climates.

Introduction or reintroduction of new plants can result from natural seed dispersal; plant fragments generated naturally, by boat traffic or by harvesting operations; wind or current dispersal of floating plants or spread by waterfowl and various human caused activities. Floating booms can minimize reintroduction.

Frequency of hand removal will depend on the combination of factors for each individual situation. For example, daily removal of water hyacinth plants may be necessary from a boat dock area on a eutrophic Florida Lake, while a single spring removal of grasses may be the only effort needed from a beach front on an oligotrophic Wisconsin lake. Hand removal may be used in combination with other methods such as herbicides or benthic barriers to minimize regrowth.

Hand removal for control of aquatic vegetation also has the advantage that it can be very selective for removing undesired vegetation and maintaining desired plants. Objectionable dying or muck causing dead vegetation can be eliminated, but it will take a lot of hard work from the whole family and neighbors.

Mechanical Removal

Specialized machines are available in a wide variety of sizes and with various accessories for removing aquatic vegetation in a variety of situations. Small machines are available that are practical for limited areas, as well as large machines in combination with transports and shore conveyors for large whole-lake operations. These machines are commonly called mechanical harvesters or weed harvesters and the process is called mechanical harvesting or removal.

Mechanical removal is an important method of aquatic plant management in certain circumstances because of several advantages it has over other methods. Immediate control can be achieved in small areas. Water can be used immediately, as compared to water-use restrictions that may be associated with herbicide use. Objectionable dead and dying vegetation that may be associated with other methods is minimized.

Use of mechanical removal for aquatic weed control is limited in many regions because of several disadvantages. It is usually higher in cost, slower, and less efficient than other methods and there are high maintenance and repair costs. Some water bodies are not suitable for mechanical removal because of water depth and presence of obstructions. Plant fragments drift to infest new areas. Temporary Increases in turbidity may result from disturbance of sediments while harvesting aquatic plants. A suitable area for disposal of harvested plants must be available. Wildlife (e.g., small fish, snakes, turtles) and desirable vegetation is also removed with harvested weeds.

Dredging

In extreme cases of overgrown aquatic vegetation, conventional or specially adapted dredging machines may be used to remove vegetation and associated sediments. Dredging is expensive, especially if a nearby disposal sight is not available. Careful consideration to secondary environmental effects must be considered and permits from regulatory agencies are usually necessary before conducting dredging operations. Following dredging, other methods should be used to maintain vegetation growth and prevent recurrence of the extreme situation. Dredging is usually short lived if not done deeper than the photic zone.

Habitat Alteration

Water Level Manipulation

Water level manipulation refers to the raising of water levels to control aquatic vegetation by drowning or lowering to control aquatic vegetation by exposing them to freezing, drying or heat. Use of water level manipulation for aquatic plant management is limited to lake and reservoirs with adequate water control structures.

Drawdown, which refers to the lowering of water level is more commonly used than raising water levels. Drawdown has been used in lake management for many years to oxidize and consolidate flocculent sediments, to alter fish populations, and for aquatic weed control. In addition to the need for an adequate water control structure, use of drawdown for aquatic plant management may also be restricted by considerations such as water-use patterns and water rights (e.g., disruption of recreational or agricultural use) or a predictable source of water for refilling.

Drawdown is usually conducted during winter months so that plants are exposed to both drying and freezing. Summer drawdown can also be effective but usually results in greater impact to agricultural and recreational water use, stresses fish populations and has a greater potential to enhance the spread of emergent plants such as cattails, rushes and willows.

Drawdown alters the composition of aquatic vegetation, but doesn't always produce desirable changes. The responses of various aquatic plant species to drawdown vary widely (Table 2) and sometimes unpredictably. Brazilian elodea (Egeria densa) is sensitive to drawdown and is often controlled for up to three years by this method. In contrast, drawdowns only partially control hydrilla, a near relative of Brazilian elodea, when it is growing in sandy lake bottoms and has little effect when hydrilla is growing in organic sediments. The hydrilla tubers that are produced deep within the sediment are protected from desiccation and can survive several consecutive drawdowns. In general submersed aquatic plants have variable responses to drawdown, while emergent plants tolerate or are stimulated by drawdown.

Table 2. Response of some aquatic plants to drawdown. Sensitive plants are those species that have been shown to decrease after drawdown activities. Sensitive to Tolerant plants are those species that have been shown to decrease, remain the same, or increase after draw down activities. Tolerant plants are those species that have been shown to remain the same or increase after drawdown activities.

The advantages of drawdown as a method of aquatic plant management includes low cost (unless recreational or power generation is lost) and secondary benefits of sediment oxidation and consolidation and fisheries enhancement. Potential undesirable effects of drawdown include reductions of desirable species, increases of undesirable tolerant species like hydrilla, expansion of undesirable species to deeper areas, the creation of floating islands, and the loss of storage water and recreational benefits if insufficient water is available to refill the basin.

Effects on Light Penetration

All plants require a certain amount of light to grow. Submersed aquatic plants can sometimes be controlled or suppressed by reducing light penetration into the water. Light penetration can be reduced by the use of special dyes, special fabric bottom covers or fertilization.

Even though dyes are not pesticides, only those that are approved for use in water should be used. These specially produced dyes block light that plants need for photosynthesis and are not toxic to aquatic organisms, humans or animals that might drink the treated water. Dyes are only effective in ponds that have little or no flow through them and they are generally effective only in water greater than 3 feet in depth.

Various materials, including black plastic and specially manufactured bottom covers, have been used to prevent rooted aquatic plants from growing. Gases that are produced on pond bottoms accumulate under nonpermeable bottom covers, such as plastic, and cause them to float to the surface. However, specially made bottom covers can be effective for preventing submersed aquatic plant growth. In addition to preventing light from reaching the pond bottom these materials also physically prevent rooted aquatic plants from becoming established. These special materials are expensive and must be maintained to prevent sediment accumulation on top of the cover. Therefore, their use is generally restricted to ornamental ponds, swimming areas or around boat docks (care must be taken to prevent the bottom cover from becoming tangled in boat propellers).

Nutrient Limitation

Plant growth can be limited if at least one nutrient, which is critical for growth, is in short supply. Nitrogen, phosphorus or carbon are usually the nutrients limiting plant growth in lakes. Therefore, if at least one of these nutrients can be limited sufficiently so that plants do not grow to an objectionable level, this nutrient limitation can be used as a method of aquatic plant management. Generally, however, unless a lake is truly oligotrophic there are enough nutrients in the sediment to sustain abundant rooted aquatic plants.

In some areas nutrients are naturally in short enough supply that aquatic plants do not grow to problem levels. Where human caused inputs have accelerated plant growth, nutrients can be limited by identifying and abating the nutrient source(s). If the lake has received external phosphorus inputs for a long period of time it may also be necessary to affect internal nutrient availability by precipitation with agents such as alum. While nutrient limitation is theoretically possible there are no good examples in the literature where nutrient limitation has managed nuisance populations of aquatic plants.

A problem that should be considered when attempting to manage nuisance populations of aquatic plants with nutrient control, is that it may actually aggravate an existing aquatic plant problem. There are well-documented cases where nutrient limitation has controlled planktonic algae populations. This control increased light penetration to the sediment allowing aquatic plants to expand their coverage in the lake or reservoir.

Biological Control

Insects

Biological control is the purposeful introduction of organisms, such as insects and pathogens to keep the growth of problem plants in check. Biocontrol agents have to be released into the problem plant's range to help suppress its growth. Small numbers of biocontrol agents are released so that they can increase to a point where they control the problem plant and are in balance with the target plant, so a self-perpetuating population is established. In some cases, like the milfoil weevil, a native insect shows a preference for the exotic nuisance plant over its previous plant habitat and helps control the exotic species.

The most attractive aspect of biological control is that it can be permanent and self-perpetuating. Once established, additional releases are usually unnecessary so additional expenses are avoided. However, exceptions occur when it becomes necessary to move field-collected bioagents to new locations. While the initial expense is high, over the long run biocontrol agents are among the least expensive control options. Benefit to cost ratios of this approach have been estimated at 50 - 100:1 or even higher.

A foreign insect species must be extensively tested and proven to be host-specific (can not reproduce in the absence of the host) before it can be released in the United States. These tests are designed to demonstrate that the bioagent will not feed appreciably or reproduce on any plant other than the target weed. This ensures that it will not harm crop plants or other desirable species.

The first aquatic weed target for biocontrol was alligator-weed (Alternanthera philoxeroides). Three host-specific South American insects were found and eventually released. These include the alligator-weed flea beetle (Agasicles hygrophila), which was released in 1964; the alligator-weed thrips (Amynothrips andersoni), which was released in 1967; and the alligator-weed stem borer (Vogtia malloi), a moth, which was released in 1971. These insects are very effective and usually suppress the growth of alligator-weed below problem levels. However, their effectiveness is diminished toward the northern limits of the plant's range in North Carolina. These insects are naturalized throughout the southeastern United States, but populations sometimes are diminished following harsh winters. When this happens control can be enhanced on a localized level by importation of insects from more southerly regions.

Three species of insects have been released for control of water hyacinth (Eichornia crassipes). The first was the mottled water hyacinth weevil (Neochetina eichhorniae), which was released in Florida in 1972. The second was the chevroned water hyacinth weevil (Neochetina bruchi), which is quite similar to the first. It was released in Florida in 1974. The third insect was a moth, the water hyacinth borer (Sameodes albiguttalis), which was released in 1977. These three insects are naturalized throughout the Southeast. A good indication of the presence of water hyacinth weevils is the occurrence of distinctive adult feeding scars on the leaves. Mature larvae can often be found in the petiole bases or in the stem. The weevils (especially the chevroned) have been the most effective of the water hyacinth insects. It has been difficult to quantify the impact of these insects on water hyacinth populations, but suppression has not been sufficient to diminish the need for aggressive maintenance management of water hyacinths with herbicides.

Several insect biological controls are in various stages of research, quarantine, and early release for control of water lettuce (Pistia stratiotes), hydrilla (Hydrilla verticillata), and Eurasian watermilfoil (Myriophyllum spicatum). The interested reader is urged to contact an information source such as the University of Florida/Institute of Food and Agricultural Sciences, Aquatic Plant Information Retrieval Service for current information on biological control progress (APIRS University of Florida/Institute of Food and Agricultural Sciences, Center for Aquatic Plants, 7922 N. W. 71st., Gainesville, Florida 32653-3071; http://aquat1.ifas.ufl.edu/).

Pathogens

The introduction approach would seem ideal for the use of pathogens. However, restrictions regarding the importation of plant pathogens from abroad tend to prohibit this approach and limit the scope to native pathogens. Pathogens also tend to be environmentally sensitive and populations do not remain high enough for sustained suppression of weed populations. Therefore, the use of pathogens for biological control of aquatic weeds has more promise as an augmentation approach. Suspensions of fungal spores can be formulated and applied to weed populations. One fungal pathogen (Cercospora rodmanni), has been formulated as a mycoherbicide for water hyacinth. However, it has not been very effective and research in this area is continuing. Research is also currently being conducted to develop methods for biological control of hydrilla and Eurasian watermilfoil with pathogens. Insects, especially stem borers and piercing-sucking types often provide points of entry for native plant pathogens. While neither the insect nor the pathogen has a substantial impact on the nuisance plant population, in combination they can help control nuisance situations.

Snails, manatees, etc.

Two snails (Marisa cornuarietis and Pomacea australis) have been studied as potential biocontrol agents for aquatic weeds. Large numbers will control several species of submersed aquatic plants under confined conditions. However, snails are not currently under consideration as biocontrol agents for aquatic weeds because of environmental risk associated with the purposeful propagation of prolific, generalized herbivores. They are intermediate hosts for certain fish and human parasites and they are not effective under natural, unconfined conditions.

Manatees or sea cows (Trichechus manatus) have been experimentally used, mainly in canals, for aquatic weed control in Florida. Manatees effectively removed submersed and floating plant species. During winter, however, heaters were required to keep manatees warm. In a study of King's Bay (Crystal River, Florida), conducted by the U.S. Fish and Wildlife Service, biologists found that 10 times as many manatees as normally wintered there could not consume the existing hydrilla biomass, much less keep up with the growth of plants.

Other biological controls for aquatic weeds that have been suggested and/or tested include ducks, geese, crayfish, nematodes, viruses, and water buffalo. Any of these may be useful under highly specialized conditions, but none have proven practical. Some of these agents may also cause more harm to aquatic systems than any aquatic plant nuisance. For example, the rusty crayfish (Orconectes rusticus) has denuded some northern lakes of plants vital for fish habitat and prey on fish eggs.

Triploid Grass Carp

Grass carp (Ctenopharyngodon idella) are the most commonly used and effective biological control currently available. The success of grass carp is also the primary reason this biocontrol agent is so controversial. If stocked at a high enough densities grass carp can remove virtually all aquatic vegetation for a decade or longer. Because of the fear that grass carp would escape and reproduce in United States waters, sterile triploid grass carp are now required by most states that allow grass carp for aquatic plant control.

Triploid grass carp are specially produced in hatcheries and possess three sets of chromosomes instead of the normal two. This abnormal condition causes sterility, so these are the only non-indigenous fish that can be legally used for aquatic weed control in most states. A permit is usually required for possession and use of triploid grass carp. Because they cannot reproduce, the number of fish will not increase beyond the initial stocking. However, they cannot be effectively removed from large bodies of water and they are often hard to contain.

Triploid grass carp prefer to consume submersed plants, so they are effective controls of this type of vegetation. Grass carp also browse tips of young, tender emergent plants which often provide control of emergent species, which may be nontarget species. Although young grass carp feed on filamentous algae such as Cladophora and Spirogyra, they are not effective for control of most filamentous algal species unless all other aquatic plants are gone and they are stocked at high rates (>50 per acre). Grass carp do not control phytoplankton.

The ability of grass carp to consume aquatic plants depends on the size of both plants and fish. Factors such as age, gender, and population density of the fish can determine the consumption rate of the stocked fish. The species, abundance, and location of the aquatic vegetation also influence the feeding behavior of the grass carp.

Because predators like birds, snakes, other fish, and some mammals are normally present, grass carp that are 1 pound (10-12 inches) or larger should be stocked to maximize survival. Some mortality will occur even when these larger fish are stocked, therefore it is not possible to know how many fish are present when stocked into large systems.

Stocking rates of 20 - 25 grass carp per acre of lake effectively controls all aquatic plants in southern latitudes but rates as high as 150 grass carp per acre are required before control is achieved in northern lakes. At any latitude, if enough grass carp are stocked where the consumption rate of the grass carp exceeds the growth rate of the aquatic plants, grass carp are an effective method of controlling aquatic vegetation (except for a few nonsusceptible species, such as spatterdock Nuphar luteum). Because of their nonselective feeding behavior and lack of predictability, grass carp should only be used in lakes where complete control of aquatic plants is an acceptable part of a management plan.

Many management agencies are currently attempting to use low stocking densities of grass carp (2-5 per acre) with herbicides to control nuisance aquatic plants while maintaining certain levels of aquatic vegetation. This technique is unpredictable and should only be used with the understanding that total control of aquatic plants, especially submersed plants, is a possibility, if not a probability.

Tilapia

Tilapia are tropical species that can suppress growth of softer aquatic vegetation such as filamentous algae and bladderwort (Utricularia spp.) when stocked at high density (300 per acre). Two species of Tilapia have been considered for aquatic weed control. The Blue Tilapia (Tilapia aurea) feeds entirely on algae (planktonic and filamentous) but does not readily consume larger, coarser vegetation. The redbelly Tilapia (T. zilli) feeds on larger submersed vegetation rather than algae. However both species reproduce rapidly and consume both vegetation and small animals that are important food sources for desirable fish populations. Therefore, use of Tilapia can have unwanted environmental consequences.

Tilapia will not overwinter in water below 65¡ F. This is a benefit from an environmental standpoint, but annual restocking is necessary in temperate climates unless a warm water supply (such as a thermal spring or power plant cooling effluent) is available as a refuge during winter. In tropical climates, where they do overwinter, they are prolific and can be detrimental to sportfish populations.

Before stocking any type of biological control of aquatic weeds, check with appropriate state agencies to determine state regulations!

Herbicides

The Herbicide Label

Before an aquatic herbicide is labeled by the United States Environmental Protection Agency (USEPA), research that requires about 10 years to complete must be conducted. In addition, aquatic herbicides that were registered prior to guidelines that were established by 1978 amendments to the FIFRA (Federal Insecticide, Fungicide, Rodenticide Act) must be reregistered by conducting tests to correct data gaps if they exist. Data required for pesticide registration includes, but are not limited to the following:

Based upon registration data, residue tolerances are set by dividing the amount of residue that causes no observable effect to chronically exposed test animals by 100 or 1000 and estimating how much residue can be allowed in a commodity so that an average sized person would ingest or come in contact with less than that amount.

Based upon tolerances, residue data, and environmental fate, water-use restrictions or precautions for swimming, fishing, irrigation (additional irrigation precautions are often based upon crop sensitivity to the herbicide to prevent crop damage), watering livestock, and domestic uses may be placed on the label. This process insures that the public will not come in contact with a herbicide at concentrations which may cause harmful effects.

All herbicide containers must have attached to them a label that provides instructions for storage and disposal, uses of the product, and precautions for the user and the environment. The label is the law. It is unlawful to alter, detach, or destroy the label. It is unlawful to use a herbicide in a manner that is inconsistent with or not specified on the label. Note that weeds not specified on the label may be treated and application methods not mentioned on the label may be used as long as they are not prohibited on the label. It is unlawful to transfer a herbicide to an improperly labeled container.

Misuse of a herbicide is not only a violation of federal and state law, but also herbicides used in water contrary to label directions may make water unfit for fishing, irrigation, swimming, or domestic use.

The herbicide label contains a great deal of information about the product and should be read thoroughly and carefully before each use. Before applying a herbicide, read the label to determine the following:

    - Is the product labeled for the site, i.e., ditch banks only, canal banks, ponds, lakes, rivers, etc.?
    - Can the weed be controlled with the product?
    - Can the herbicide be used safely under particular application conditions?
    - How much herbicide is needed?
    - What restrictions apply to watering livestock, fishing, swimming, consuming as potable water and irrigation?
    - What is the toxicity to fish and nontarget vegetation?
    - When should the herbicide be applied (time of year, stage of plant growth, etc.)?
    - Is the herbicide classified restricted use?
    - What is the signal word? (DANGER, WARNING, CAUTION)
    - What safety equipment should be worn?

Have all appropriate labels at the application site, including supplemental labels, special local need labels and emergency use labels. Also have manufacturer's material safety data sheets (MSDS) on hand. Read labels often even if you use the herbicide routinely. You may have missed something or it may have changed. Labels are often changed by industry.

What Are Herbicides?

Generally, herbicide is defined as a plant or weed killer. Weed scientists define herbicides more precisely as chemicals used for killing plants or severely interrupting their normal growth processes. For the aquatic plant manager or waterfront homeowner, herbicides are tools that can be used to manage aquatic vegetation in a safe, efficient, and cost effective manner. A herbicide formulation consists of an organic (carbon-containing) or inorganic active ingredient, an inert carrier, and perhaps adjuvants.

Every herbicide must be registered by the EPA for use in the United States. There are about 200 herbicides (active ingredients) currently registered in the United States. Currently, only eight are labeled for use in aquatic sites. Two of these, xylene and acrolein, are highly toxic and used only in irrigation systems of the seventeen western states under the jurisdiction of the United States Bureau of Reclamation. This leaves six active ingredients (copper, 2,4-D, dichlobenil, diquat, endothall, fluridone, and glyphosate) that are contained in herbicide formulations that are currently labeled for use in aquatic sites in most states.

The reasons there are few aquatic herbicides compared to crop production herbicides is primarily because the uniqueness of the aquatic environment. This sets limits to the number of compounds that will effectively control aquatic plants and also meet the rigid environmental and toxicology criteria necessary for registration. Aquatic herbicides must have the capacity to be taken up by plants quickly in sufficient amounts from water to be toxic to target plants and have sufficiently low toxicity to man and other organisms in the aquatic environment. The market for aquatic herbicides is also small compared to the giant agricultural market.

Contact Herbicides

Contact herbicides act quickly and are generally lethal to all plant cells that they contact. Because of this rapid action, or other physiological reasons, they do not move extensively within the plant and are effective only where they contact plants. For this reason, they are generally more effective on annual (plants that complete their life cycle in a single year). Perennial plants (plants that persist from year to year) can be defoliated by contact herbicides but they quickly resprout from unaffected plant parts. Submersed aquatic plants that are in contact with sufficient concentrations of the herbicide in the water for long enough periods of time are affected but regrowth occurs from unaffected plant parts, especially plant parts that are protected beneath the sediment. Because the entire plant is not killed by contact herbicides, retreatment is necessary, sometimes two or three times per year. Endothall, diquat and copper are contact aquatic herbicides.

Systemic Herbicides

Systemic herbicides are absorbed into the living portion of the plant and move within the plant. Different systemic herbicides are absorbed to varying degrees by different plant parts. Systemic herbicides that are absorbed by plant roots are referred to as soil active herbicides and those that are absorbed by leaves are referred to as foliar active herbicides. Some soil active herbicides are absorbed only by plant roots. Other systemic herbicides, such as glyphosate, are only active when applied to and absorbed by the foliage. 2,4-D, dichlobenil, fluridone, and glyphosate are systemic aquatic herbicides.

When applied correctly, systemic herbicides act slowly in comparison to contact herbicides. They must move to the part of the plant where their site of action is. Systemic herbicides are generally more effective for controlling perennial and woody plants than contact herbicides. Systemic herbicides also generally have more selectivity than contact herbicides.

Broad spectrum herbicides

Broad spectrum (sometimes referred to as nonselective) herbicides are those that are used to control all or most vegetation. This type of herbicide is often used for total vegetation control in areas such as equipment yards and substations where bare ground is preferred. Glyphosate is an example of a broad spectrum aquatic herbicide. Diquat, endothall, and fluridone are used as broad spectrum aquatic herbicides, but can also be used selectively under certain circumstances that will be discussed later.

While glyphosate, diquat and endothall are considered broad spectrum herbicides, they can also be considered selective in that they only kill the plants that they contact. Thus, you can use them to selectively kill an individual plant or plants in a limited area such as a swimming zone.

Selective Herbicides

Selective herbicides are those that are used to control certain plants but not others. A good example of selective aquatic herbicide is 2,4-D, which can be used to control water hyacinth with minimum impact on eel grass. Herbicide selectivity is based upon the relative susceptibility or response of a plant to a herbicide. Many related physical and biological factors can contribute to a plant's susceptibility to a herbicide. Physical factors that contribute to selectivity include herbicide placement, formulation, and rate of application. Biological factors that affect herbicide selectivity include physiological factors, morphological factors, and stage of plant growth.

Application can be selective simply by carefully placing the herbicide on target plants and avoiding nontarget plants. For example, when small amounts of water hyacinth are growing among bulrush, an experienced applicator using a handgun can control water hyacinth with 2,4-D and minimize impact to the bulrush community. Although diquat is a broad spectrum herbicide, it is a contact herbicide and affects only the bulrush stems that are above the water surface, where they are contacted by the herbicide. The extensive underground rhizomes and roots are not effected and the plant quickly regrows after the initial effect of the herbicide. This is an example of selective weed control by herbicide placement.

Selectivity can be affected by the amount of herbicide applied. For example, water hyacinth is selectively controlled among spatterdock using the recommended rate of 2,4-D for water hyacinth, but spatterdock can be controlled by using higher rates and granular formulations.

A herbicide must be absorbed directly into cells or move through the plant (translocated) to the site where it is active. Herbicides may be bound on the outside of some plants or bound immediately after they enter the living part of the plant so that they cannot move to their site of activity. For other reasons, not all of which are understood, herbicides are translocated more in some plants than in others and this results in selectivity. Once inside the plant, certain plants have the ability to alter or metabolize a herbicide so that it no longer has herbicidal activity. Some herbicides affect very specific biochemical pathways in plants. Therefore, they may be selective against a particular group or groups of plants because they are the only ones that have that particular pathway.

The physiology of perennial plants changes during the annual growth cycle. During early stages of growth, upward transport of food reserves and other plant compounds are active so that soil active herbicides are most rapidly absorbed and moved upward to the growing points and sites of herbicide activity. Conversely, foliar active herbicides (e.g., glyphosate) are least active during this time and some plants are tolerant. During late- and post-flowering, perennial plants are completing their annual growth cycle. At this time they are translocating materials downward to the roots and are most susceptible to foliar active herbicides (e.g., glyphosate), which move downward to the roots with the plant materials.

Environmental Considerations

Aquatic communities consist of aquatic plants including macrophytes (large plants) and phytoplankton (free floating algae), invertebrate animals (such as insects and clams), fish, birds, and mammals (such as muskrats, otters, and manatees). All of these organisms are interrelated in the community. Organisms in the community require a certain set of physical and chemical conditions to exist such as nutrient requirements, oxygen, light, and space. Aquatic weed control operations can affect one or more of the organisms in the community that can in turn effect other organisms or it can affect water chemistry that in turn affects organisms. The effects of aquatic plant control on the aquatic community can be separated into direct effects of the herbicides or indirect effects.

Aquatic Plants

Aquatic plants are a natural and important component of aquatic communities (see chapter 3). They provide food for other aquatic organisms by fixing the sun's energy through the process of photosynthesis. Aquatic plants, especially phytoplankton, are consumed by small invertebrate animals that are in turn consumed by larger animals such as birds or fish. Large aquatic plants, or macrophytes, provide habitat for animals used as food by game fish and provide protective cover for game fish. They also provide nesting sites and provide food for birds and mammals. In addition, aquatic plants can improve the appearance of a water body. However, water is often naturally rich enough in the plant nutrients nitrogen and phosphorus for aquatic plants to grow so vigorously that they become a nuisance. They can hinder recreational use of water bodies or create hazards such as impeding drainage, which is often vital to low-lying residential communities. This is especially true for hydrilla, water hyacinth, alligator-weed and Eurasian watermilfoil, which are non-native plants.

Although it is sometimes necessary to manage native aquatic plants, the majority of publicly funded aquatic plant management programs are aimed at hydrilla, Eurasian watermilfoil, water hyacinth and alligator-weed. The reason for this is that these plants can cause decreased quality of fish populations, competition with native plants, decreased water quality, and hinder water use. The herbicides used for aquatic weed management can directly impact native aquatic vegetation if not used prudently. However, weed problems can be managed with minimum impact on native plant populations by using appropriate application rates, timing and application techniques of aquatic herbicides. In this way, the aquatic weed problem can sometimes be managed while maintaining a beneficial aquatic plant community for fisheries and waterfowl habitat. However, sometimes it will not be possible to satisfy the demands all water users and certain tradeoffs must be made. For example, it may not be possible to manage aquatic plants in a shallow eutrophic lake for fisheries habitat, waterfowl habitat, and water skiing all at the same time.

Aquatic plant control operations can have an indirect impact on phytoplankton. When large amounts of aquatic vegetation (>30% area covered with aquatic plants, see Chapter 3 for more information) are controlled in a lake with herbicides or grass carp, the plant nutrients nitrogen and phosphorus, which are often limiting to phytoplankton growth, are released into the water. These nutrients can allow additional phytoplankton growth to occur in the lake. This growth causes the water to take on a green coloration and water clarity is decreased.

Effects on Fish and Other Organisms

When used according to the label specifications, currently available aquatic herbicides are not toxic to fish, birds, or other aquatic organisms. They are also short-lived in the environment and do not accumulate in organisms. Environmental conditions are not always predictable, however, and under certain circumstances, fish kills can occur usually as an indirect result of aquatic herbicide applications.

Fish kills are only likely to occur as a direct effect of herbicide application if a herbicide formulation known to be toxic to fish, such as the amine salt of endothall, is applied in an enclosed water body. The concentration of copper that is used for most herbicide applications is below toxic concentrations. However, rates recommended for difficult-to-control filamentous algae can be toxic to fish in enclosed ponds and care should be taken when making this type of application. The greatest concern for copper toxicity is in soft water because the toxicity of copper to fish increases as water hardness decreases (Table 3). This is especially true for most trout species. Most aquatic herbicides have very low toxicity to fish and the concentration that occurs after application of recommended rates is less than concentrations that are toxic to fish (Table 4).

The most common reason for fish kills due to aquatic herbicide application is the indirect effect of lowered dissolved oxygen (DO) in the water. DO in lakes and ponds commonly range between 0.0 and 12 ppm. The lower concentrations occur during early morning hours in productive lakes because aquatic plants consume oxygen during darkness or reduced light. Oxygen is also consumed by other aquatic organisms, especially those associated with decaying organic matter in the water. Fish populations can usually withstand the everyday fluctuations of DO, but many types of fish cannot tolerate prolonged periods of low DO. Natural fishkills can occur in highly productive waters when phytoplankton populations die and cease producing oxygen after prolonged cloudy, still, warm weather.

Table 3. Toxicity of copper (48 hour TLM) to bluegill at different water hardness and alkalinity. A 48 hour TLM is defined as the median tolerance limit and is an acute test where the critical limit of the test factor is at a level where 50% of the test organism survives in a given time.

Table 4. Theoretical concentrations of aquatic herbicides after application and their experimental 96-hour LC-50 (ppm). A 96-hour LC-50 is an acute toxicity test where the concentration of a chemical in the test environment is at a level where 50% of the test organisms will survive in 96 hours. Theoretical concentrations are based upon low and high label rates applied in 3 feet of water.

    Herbicide Theoretical
    Concentration    		 -- 96-hour LC-50 (ppm) --
    Bluegill Rainbow Trout
    Rodeo (-)1 >1000 >1000
    Aquathol K 1.0-3.0 343 230
    Diquat 0.12-1.5 245 -
    2,4-D, DMA 1.0-4.0 168 100
    Sonar 0.05-0.15 14 11
    Hydrothol 191 1.0-3.0 0.94 0.96
    Copper Sulfate (soft water) 0.5-3.0 0.88 0.14

    1 Application to emergent vegetation only; concentrations in water are insignificant.

When large amounts of aquatic plants are killed by a herbicide application the decaying vegetation and lack of oxygen production may cause DO to become so low that fish cannot survive in the water and a fishkill occurs. If a herbicide that is effective on higher plants and not phytoplankton is used, the potential for a fishkill can be minimized because phytoplankton will continue to produce oxygen. Also, the danger of fishkills is less in cooler water because it can hold more oxygen than warm water. Herbicide applications to large weed populations in warm water during periods of prolonged still and cloudy weather, and where fish movement is restricted should be avoided to minimize the potential for fishkills. Large weed populations should be brought under control by a series of applications to portions of the waterbody and treated during the spring when water temperatures are lower. Once under control, weeds should be maintained at low densities.

Herbicide-related fishkills, either direct or indirect, are not likely to occur as a result of partial area applications in large water bodies because fish have avoidance mechanisms and are mobile. If possible, fish will move to other parts of a lake to avoid adverse conditions. When making partial applications of herbicides, such as the diethylamine salt of endothall, that can be toxic to fish at recommended use rates, applications should be started near shore and proceed toward open water. This allows fish to escape to untreated water. All precautions should be taken to avoid conditions that can lead to potential fishkills when applying aquatic herbicides.

Fate of Aquatic Herbicides in the Environment

The concentration of herbicide in water immediately after proper application of aquatic herbicides for submersed weed control is very low (Table 4). For example, when 2 gallons of diquat are applied to an acre of water 6 feet deep the nominal concentration is 0.12 ppm. Lower herbicide concentrations in water result from foliar applications to floating or emergent plants because the herbicide is directed on to the plants and very little herbicide ever reaches the water.

We refer to herbicides disappearing and dissipating from the environment. Disappearance refers to the removal of the herbicide from a certain part of the environment. Aquatic herbicides can disappear from treated water by dilution, adsorption to bottom sediments, volatilization, absorption by plants and animals or by dissipation. Dissipation refers to the breaking down of a herbicide into simpler chemical compounds. Herbicides can dissipate by photolysis (broken down by light), microbial degradation, or metabolism by plants and animals. Both dissipation and disappearance are important considerations to the fate of herbicides in the environment because even if dissipation is slow, disappearance due to processes such as adsorption to bottom sediments makes a herbicide biologically unavailable.

Aquatic herbicides are nonpersistent in treated water, that is, they disappear rapidly. Disappearance is greatest when spot treatments are made in large bodies of water because the dominant effect is dilution. Aquatic herbicides are water soluble and quickly dilute to non-detectable concentrations. They disappear at different rates and by different methods. Table 5 lists rates of breakdown and major routes of disappearance and dissipation of aquatic herbicides. Because of environmental factors, disappearance is often much faster than listed in Table 5 and these values should be used only for comparison.

Copper
Copper is a naturally occurring element and essential at low concentrations for plant growth. It does not break down in the environment, but it forms insoluble compounds with other elements and is bound to charged particles in the water. It rapidly disappears from water after application as a herbicide. Because it is not broken down, it can accumulate in bottom sediments after repeated high application rates. Accumulation rarely reaches levels that are toxic to organisms or significantly above background concentrations in the sediment.

2,4-D
2,4-D photodegrades on leaf surfaces after foliar applications and is broken down by microbial degradation in water and sediments. Complete decomposition usually takes about 3 weeks in water and can be as short as 1 week. 2,4-D breaks down into naturally occurring compounds. Two pounds of 2,4-D amine will break down into 1 pound carbon dioxide, 1/4 pound water, 1/4 pound ammonia, and 1/2 pound chlorine.

Table 5. Major methods and rates of break down of five aquatic herbicides.

Diquat
When applied to enclosed ponds for submersed weed control, diquat is rarely found longer than 10 days after application and is often below detection 3 days after application. The most important reason for the rapid disappearance of diquat from water is that it is rapidly taken up by aquatic vegetation and bound tightly to particles in the water and bottom sediments. When bound to certain types of clay particles diquat is not biologically available. When it is bound to organic matter it can be slowly degraded by microorganisms. When diquat is applied foliarly it is degraded to some extent on the leaf surfaces by photodegradation, and because it is bound in the plant tissue a proportion is probably degraded by microorganisms as the plant tissue decays.

Endothall
Like 2,4-D, endothall is rapidly and completely broken down into naturally occurring compounds by microorganisms. The by-products of endothall dissipation are carbon dioxide and water. Complete breakdown usually occurs in about 2 weeks in water and 1 week in bottom sediments.

Fluridone
Dissipation of fluridone from water occurs mainly by photodegradation. Metabolism by tolerant organisms and microbial breakdown also occurs, and microbial breakdown is probably the most important method of breakdown in bottom sediments. The rate of breakdown of fluridone is variable and may be related to time of application. Applications made in the fall or winter when the sun's rays are less direct and days are shorter result in longer half-lives. Fluridone usually disappears from pondwater after about 3 months but can remain up to 9 months. It may remain in bottom sediment between 4 months and 1 year.

Glyphosate
Glyphosate is not applied directly to water for weed control, but when it does enter the water it is bound tightly to dissolved and suspended particles and to bottom sediments and becomes inactive. Glyphosate is broken down into carbon dioxide, water, nitrogen, and phosphorus over a period of several months.

Maintenance Control of Aquatic Weeds

Maintenance control (or management) refers to controlling plants at low levels and before they reach a problem level. It has been defined in a Florida Statute as follows:

....a maintenance program is a method for the control of non-indigenous aquatic plants in which control techniques are utilized in a coordinated manner on a continuous basis in order to maintain the plant population at the lowest feasible level as determined by the department [Department of Natural Resources]. FAS 369.22

Maintenance control of aquatic weeds reduces the detrimental environmental effects caused by the weeds and reduces the potential for environmental impacts from aquatic plant control activities. Maintenance control offers the following advantages:

For example, maintenance of water hyacinth to less than 5% coverage under experimental conditions reduced herbicide usage by a factor as great as 2.6; reduced deposition of detritus by a factor of 4.0; and reduced depression of DO that occurred beneath the vegetation mats.

A problem experienced when conducting a maintenance control program is that people do not perceive a weed problem and question the need to spray. Therefore, public education is an important part of a successful maintenance control program. Maintenance management is the most environmentally sound method for managing water hyacinth. Unmanaged, water hyacinth can double every 7 - 10 days. Ten plants can grow to cover one acre in a single growing season, often weighing 200 tons. Therefore, the benefit of controlling those 10 plants early should be obvious.

Maintenance management works for water hyacinth, but is more difficult for submersed weeds such as hydrilla. In South Florida canals, maintenance management of hydrilla has been successfully implemented but further research will be necessary to develop cost effective programs for maintenance management of hydrilla in lakes. Once developed, maintenance management programs for hydrilla in lakes should provide more environmentally sound aquatic weed control. In northern lakes cold weather, ice, and snow perform an annual maintenance management program. Aquatic plant management is often an annual affair but some evidence indicates that when properly planned and applied, management during one growing season may carry over to the following growing season or beyond.

Manipulating Plant Communities

The aesthetics, fish and wildlife habitat values of lakes and reservoirs can sometimes be greatly enhanced by establishing and managing certain desirable aquatic plants. Many lakes have little vegetation, undesirable species, or plants growing in the wrong places. Manipulating habitat (e.g., substrate type, lake slope), selectively removing undesirable plants or plants that occur in undesired locations and planting desired plants in desirable locations are all ways of managing aquatic plants to improve the quality of a lake or reservoir.

Where it is legal, excavation can deepen aquatic environments to exclude plants from areas where they are not desired and the substrate can be used to form shallows for planting desired aquatic plants. When manipulating habitat like this it extremely important to determine the low, average and high water line of the lake. While some wetland plants will tolerate dry and wet seasons, there are many that will die if they are kept too wet or too dry. Individual plant species also require different water depth to be successful. Thus, when creating habitat for aquatic plants it is important to create habitat of the proper depth for the desired plant species.

Some aquatic management techniques that control plants can also promote desirable species and improve habitat. The physical removal of problem aquatic plants like mechanical harvesting of water milfoil can sometimes stimulate wild celery by removing the shading canopy of watermilfoil. The herbicide 2,4-D can sometimes shift plant community composition from watermilfoil and coontail to beneficial pondweeds and wild celery (Nichols 1986). Screens and harvesters can channelize plant beds to produce island habitats, increase edge, and form cruising lanes for boaters and gamefish. Aluminum sulfate (alum) can reduce algae and thus improve water clarity for larger plants to grow. These are only a few of the many methods available to promote desirable aquatic plant growth in lakes and reservoirs. This is also a concept that should be part of any aquatic plant management plan.

Adding plants to lakes may be more important than removing them. Chapter 3 shows, however, that different types of plants (e.g., emersed, submersed) and individual species within each plant type require different conditions to survive. For example, water shield is an excellent food source for waterfowl and a potential plant for revegetation of lakes with no aquatic plants but it only thrives in acidic, softwater lakes (Hoyer et al 1997). Therefore, attempts to plant water shield in alkaline, hardwater lakes would be a waste of money and effort. Before attempting to revegetate it is best to list the types and species of aquatic plants that can grow in a particular lake.

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