Aquatic Pest Control
Applicator Training Manual

HERBICIDE TECHNOLOGY


K. A. Langeland and D. D. Thayer




What is a herbicide? 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, 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 for use in the United States by the EPA. There are about 200 herbicides (active ingredients) currently registered in the United States. However, only 10 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, diquat, endothall, fluridone and glyphosate that are contained in herbicide formulations that are currently labeled for use in aquatic sites in most states Several other herbicides, dicamba, diuron, hexazinone, imazapyr, and tebuthiuron are labeled for use on ditch banks and will be included in discussions but they should not be applied directly to water.


The reason there are few aquatic herbicides compared to crop production herbicides is mainly because the uniqueness of the aquatic environment limits the number of compounds that will be effective for controlling 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.


Herbicide Classification


Herbicides are commonly grouped according to chemical similarity or herbicidal properties. Properties by which herbicides are grouped include absorption characteristics, plant processes that they affect, and selectivity (Table 2).


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Table 2. Classification of aquatic herbicides.


ABSORPTION CHARACTERISTICS


Contact Herbicides

Copper, Diquat, Endothall

Systemic Herbicides

2,4-D, Fluridone, Glyphosate


PHYSIOLOGICAL PROCESSES


Tissue Development

2,4-D

Photosynthesis

Copper, Diquat, Fluridone

Respiration

Endothall

Nitrogen Metabolism and Enzyme Activity

Glyphosate


SELECTIVITY


Nonselective (Broad Spectrum)

Copper, Diquat, Endothall, Glyphosate

Selective

2,4-D, Fluridone


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Absorption Characteristics


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), herbaceous plants. Perennial (plants that persist from year to year), woody plants 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 hydrosoil. 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. Still others, such as imazapyr (not labeled for aquatic sites), are absorbed by both roots and shoots. 2,4-D, 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 generally have more selectivity than contact herbicides.


Plant Processes


Cell Division
All plants are composed of one or more cells. Some algae are composed of a single cell (unicellular), whereas many algae and all higher plants are composed of more than one, often billions, of cells. Plants grow by increasing the number of their cells and replacing old cells. This process is called cell division. If a herbicide can stop cell division by affecting one of the many complex processes involved in it, it can cause the plant to stop growing. If cell division is sufficiently affected, the plant will die. This is especially true when cell division involved in root growth is affected. Herbicides that affect cell division are most effective when they are applied pre-emergence (before weed seed germinates and begins to grow) or during early growth.


Tissue Development
During tissue development, plant cells become specialized and organized into units that perform particular functions in the plant. When a herbicide causes abnormal tissue development, abnormalities such as twisting of stems and leaves, may be evident. If sufficient abnormalities occur plant can die. Herbicides that act in this manner are often called plant growth regulators (PGRs). 2,4-D is an example of a herbicide or plant growth regulator that interferes with tissue development. 2,4-D is one of a group of herbicides called organo-auxin-like herbicides which act inside the plant like the naturally occurring plant hormone, auxin, but their action results in abnormal tissue development and abnormalities in other plant functions.


Photosynthesis
Photosynthesis is the process by which plants use carbon dioxide, water, and sunlight to produce relatively simple molecules that are the building blocks of other complex molecules that make up the plant body. Photosynthesis is a very complex process and various herbicides can disrupt it in different ways. Diquat affects photosynthesis directly and causes many secondary effects. Tissue death is very rapid in the sunlight. Copper also affects photosynthesis and its activity as an algicide is more rapid when it is applied on a sunny day when photosynthesis is occurring rapidly. Fluridone affects photosynthesis indirectly by inhibiting production of a pigment (light absorbing compound) that is necessary for photosynthesis to occur. This indirect effect on photosynthesis causes slow plant death.


Respiration
Plants produce compounds like sugars and carbohydrates during photosynthesis. Plants then use these compounds through a series of processes known as respiration. Many herbicides affect respiration, however these are probably secondary reactions.


Nitrogen metabolism and enzyme activity
Nitrogen is an essential plant nutrient and it is involved in many plant processes. Its absorption and incorporation into plant compounds is referred to as nitrogen metabolism. Complex nitrogen-containing compounds called enzymes are essential to all plant processes. Many herbicides affect plant processes by interfering with the enzymes associated with the processes. Fluridone, glyphosate and imazapyr are examples of herbicides that affect plant processes by interfering with the action of specific enzymes.


Selectivity


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.


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 broad leaf weeds with minimum impact on grasses. 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 waterhyacinth are growing among bulrush, an experienced applicator using a handgun can control waterhyacinth with minimum impact to the bulrush community. This is an example of selective weed control by herbicide placement.


Herbicide formulation can also affect the selectivity of foliar-applied herbicides by increasing the herbicide's ability to enter the plant. Adjuvants, which may be added to one formulation by the manufacturer and not to another, can increase a herbicide's ability to pass through the cuticle or waxy coatings on plant leaves or aid in bypassing leaf hairs by reducing surface tension. Certain herbicide formulations may dissolve through waxy coatings on leaves whereas other formulations are partially bound on the outside of the leaf. Selectivity can be affected by the amount of herbicide applied. For example, water hyacinth is selectively controlled among spatterdock at the recommended rate for water hyacinth, but spatterdock can be controlled by using higher rates and granular formulations.


For a herbicide to be effective it must first enter the plant. Morphological characteristics, such as thick cuticles, waxy coatings or hairs can affect a plant's susceptibility to herbicides by physically preventing entry of the herbicide into the plant. Likewise, leaf shape and angle can affect the entrance of herbicide into the plant. Broad, horizontally oriented leaves will intercept and retain a greater amount of herbicide than narrow upright leaves such as those of many grasses and cattails.


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 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.


Growth stage can affect susceptibility in several ways. Young, actively growing, annual plants, which have not developed a cuticle or leaf hairs are more susceptible to foliar-applied herbicides. 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 is active so that soil active herbicides (e.g. tebuthiuron and hexazinone) 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.


Finally, some plant parts may be susceptible to a herbicide whereas other parts of the same plant are not affected. This is why we are able to selectively manage waterhyacinths among bulrush with diquat. 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.


Environmental Factors That Affect Herbicide Application


Weather conditions, water movement, soil chemistry, and water chemistry can greatly affect the success of aquatic herbicide applications. The applicator has no control over some of these factors but other factors can be controlled by the applicator.


Weather Conditions


Rainfall
The most obvious effect that rainfall can have on a herbicide application is to wash foliar applied herbicide off before it can be adequately absorbed into the plant. This is a particular problem with slowly absorbed systemic herbicides such as glyphosate. Contact herbicides such as diquat are absorbed more quickly and therefore are less affected by rainfall. Soil active herbicides applied to ditch banks can also be affected if heavy rainfall washes the herbicide away before it can leach into the root zone. Applying soil active herbicides to moist soil (following rain) is preferable because this promotes quick diffusion of the herbicide into the soil. Lack of rainfall can also affect herbicide efficacy because drought stressed plants are less likely to absorb both foliar and soil applied herbicides. Certain herbicide labels, such as that for glyphosate, have precautionary statements to this effect. The applicator should be aware of potential weather conditions and should schedule applications accordingly.


Wind
Foliar applications are mostly affected by windy conditions because of poor coverage. Wind can also indirectly affect the ability of plant leaves to absorb herbicides. Windy conditions favor herbicide drift (discussed more fully in a later chapter) and applications should not be made when wind is strong enough to cause drift.


Wind can also affect the efficacy of herbicide applications for submersed plant control. As previously discussed, the herbicide must be in contact with submersed plants at sufficient concentrations for sufficient periods of time (the longer the better) to achieve control. Wind can affect the efficacy of submersed weed control applications by causing water movement that carries the herbicide away from the target plant. Windy conditions should be avoided when making herbicide applications.


Temperature
Low temperature affects herbicide efficacy indirectly by affecting plant growth. At less than optimum temperatures plant growth slows down and this may decrease herbicide absorption and activity. Other factors probably override temperature effects. Too little is understood about the relationship between temperature and submersed plant control to make reliable predictions. A good rule of thumb is that if plants are actively growing they will be susceptible to contact herbicides. Excessively high temperatures can affect foliar applications of some herbicides by causing them to volatilize (change from liquid to vapor) before they are absorbed into the plant.


Water Movement


Most herbicides used for submersed aquatic weed control must be absorbed from the water into the target plants. A sufficient amount of herbicide must be available in the water for a long enough period of time for the herbicide to be effective. It is difficult to control submersed aquatic weeds in rapidly flowing water where the herbicide is carried away from the plants with the water flow. Special techniques must be used even in slowly moving water.


Methods used when controlling aquatic weeds in flowing water include:


1. Use of invert emulsions or polymers and trailing hoses to aid sinking the herbicide and adhering it to the plants.
2. Use of special herbicide formulations for flowing water such as slow release pellets.
3. Use of rapidly absorbed herbicides.
4. Use of sequential applications or injection equipment to increase contact times.


Soil Chemistry


Interactions between herbicides and soil are complex and are very important to the use of herbicides in agricultural production. Soil chemistry affects the efficacy of soil active herbicides applied for ditch bank weed control. The most important factor is the ability of different soils to bind herbicides. Soil-applied herbicides usually have label recommendations for use on different types of soils. In general, soils with more organic matter and clay have greater capacities for binding herbicides and require higher application rates than other soil types, such as sandy soils.


Water Chemistry


Water chemistry that affects herbicide efficacy includes pH, turbidity, and hardness. The applicator has little control over these conditions in lake water. However he can decide which herbicide to use under certain circumstances or can adjust the rate of application. More importantly, chemistry of dilution water can affect herbicide performance and the applicator can sometimes control this.


pH
pH indicates weather water is acidic or basic. The pH scale goes from 0 to 14, where 7 indicates neutrality. Values less than 7 indicate acidic water and values greater than 7 indicate a basic condition. Many southeastern lakes have pH values between 6 and 8, with higher values, up to 9.5, occurring in dense submersed vegetation during daylight hours after 10:00 am. pH affects the rate at which some herbicides are absorbed by plants. A good example is 2,4-D amine which has increased activity at pH less than 6. The applicator has no control over the pH of a lake; changing the pH of a small pond to increase herbicide efficacy would not be cost effective and may have other detrimental effects. However, it is beneficial to be aware of pH because lower rates of a herbicide such as 2,4-D may be used when applications are made to low pH (less than 7) ponds.


Turbidity
Particles suspended in the water affect the water's ability to transmit light and this is called turbidity. The particles can be biotic, (plankton), organic, or inorganic (clay, minerals). Organic or clay particles are of most concern to the applicator because they can inactivate herbicides such as diquat by binding with them. When using a herbicide that binds to particles the applicator should be careful not to increase turbidity by disturbing the lake or pond bottom with the boat in shallow water. Particulates in diluent water can also affect herbicide performance. For example, diquat or glyphosate can be bound to particulates in a spray tank and make them ineffective. Always use diluent water that is as clean as possible and be careful not to put the suction end of a filler hose close enough to the lake bottom to draw in sediments.


Hardness
The dissolved metals calcium, magnesium, iron, and strontium cause water hardness. Hardness is usually expressed in terms of ppm calcium carbonate and can range between close to zero and over 500 in southeastern lakes. Hardness will usually be in the range of 10 - 100, but in many regions values of 150 - 300 are common. Above 50 ppm calcium carbonate is usually considered hard. Knowledge of water hardness can be important to the aquatic herbicide applicator because it can have important effects on herbicide performance and on environmental considerations as discussed in a later chapter.


Certain herbicides can react with hardness components in water and cause them to become inactive or precipitate (come out of solution). This can happen either in lake water or in the spray tank. The performance of glyphosate can be reduced by hardness of dilution water. If possible, a source of softened or distilled water might aid efficacy of glyphosate applications.


The herbicidal properties of copper are very sensitive to compounds in water (especially carbonates) that are related to water hardness (these are actually measured as alkalinity). When inorganic copper, such as copper sulfate, is used as an algicide, it is much more effective and lower rates can be used in soft water (<50 ppm calcium carbonate hardness). It is also more toxic to fish in soft water. When using copper in hard water organic forms of copper are much more effective because they stay in solution longer, are more readily absorbed by plants and are less toxic to fish.


Water chemistry is an important factor in the performance of a herbicide application and the applicator has control over some of these factors. This is especially true when considering the source of diluent water for tank mixes. Take the following precautions when obtaining make-up water for tank mixes:


(a) Use the cleanest water available. Avoid sediments.
(b) When tank mixing herbicides that are known to be inactivated by hard water use the softest water available. If possible use softened or distilled water; lake water is next best choice. Avoid using well water.
(c) Minimize the amount of time that herbicides remain mixed in tanks.
(d) Read the label for special precautions or instructions.


Some additives may alleviate the hardwater problem, but these are still under study and results to date have been inconsistent.


Herbicide Formulations


The active ingredient of a herbicide is rarely 100 percent of the formulation. Instead, the herbicide is mixed with water or an oil blend and often includes an adjuvant that facilitates the spreading, sticking, wetting, and other modifying characteristics of the spray solution. These special ingredients usually improve the safe handling, measuring, and application of the active ingredient.


Aqueous Suspension (AS)
Low solubility herbicides are suspended in water on a carrier such as fine clay particles. Adjuvants may be added, which aid in keeping the herbicide and carrier in suspension. Thorough agitation of the herbicide container is necessary before using AS formulations because the herbicide may come out of suspension during storage. Tank agitation is also necessary while applying an AS.


Emulsifiable Concentrate (EC)
Emulsifiable concentrates contain a mixture of petroleum solvents and emulsifiers that allow the formulation to mix with water. Each gallon of EC usually contains 2 - 8 pounds of active ingredient. ECs require little agitation. ECs cause little equipment wear because they are nonabrasive. ECs can be mixed with water or oil based carriers for low volume applications.


Water Soluble Liquids (WSL or L)
Water soluble liquids have the herbicide (e.g. amines) dissolved in a water soluble solvent such as alcohol. Because they form true solutions, they do not require agitation. They are usually not compatible with oil based carriers.


Flowable (F)
A flowable formulation consists of an insoluble solid phase suspended in a liquid. The active ingredients in flowables are insoluble in water and form suspensions when mixed with water. Constant tank agitation is important when using flowables. Flowables share the handling advantages of an EC.


Dry Flowable (DF)
Dry flowables are formulations that are insoluble in water but are formulated in such a way that they can be easily poured and measured. As with flowables, tank agitation is important to keep dry flowables in suspension. They are much easier to handle than wettable powders but usually are more expensive.


Granule (G) and Pellet (P)
Many aquatic herbicides have liquid and dry formulations. The active ingredient is generally mixed into or sprayed on clay particles with the amount of active ingredient ranging from 1% to 15%. Granules are convenient for spot treatments, are ready to use and require no mixing, reduce drift hazards and can be applied easily. Disadvantages of granules are their sometimes high cost and their ineffectiveness as a foliar treatment.


Wettable Powder (WP,W)
WP formulations resemble a fine dust and generally contain greater than 50% active ingredient. When mixed with water, agitation is required to keep the insoluble particles of a wettable powder in suspension. Advantages of a WP are lower cost ease of handling, and ease of measuring. Some disadvantages of WP are the abrasion by suspended particles on spray equipment and the requirement for constant tank agitation.


The Herbicide Label


All herbicide containers must have attached to them a label that provides instructions for storage and disposal, use 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 pesticide 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 labelled 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 manufacturers 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.