Management Methods: Chemical Methods

Impacts of CHEMICAL METHODS

Herbicides kill or suppress plants by interfering with essential plant processes such as photosynthesis. All of the interactions between an herbicide and a plant from application to the final effect are referred to as the mode of action. Understanding the mode of action of an herbicide is essential in selecting the proper herbicide, diagnosing herbicide injury symptoms, preventing herbicide resistance problems, and avoiding nontarget environmental impacts.

Diagram of leaf cross-section showing mode of action of a foliar-active, systemic herbicide (e.g., atrazine) that inhibits photosynthesis. In this example, the herbicide is applied to and absorbed through the leaf, is translocated to the chloroplasts (site of action), and subsequently inhibits photosynthesis (mechanism of action).

The mode of action involves

• contact and absorption
  contact, penetration, and movement of the herbicide into the plant through the cuticle or epidermal root tissue
• translocation
  movement of the herbicide to the site of action
• site of action
  specific location within the plant where the herbicide exerts toxicity at the cellular level
• mechanism of action
  specific biochemical or biophysical process that is affected by the herbicide

The terms mode of action and mechanism of action are often used interchangeably. However, mechanism of action refers to the plant’s specific biological process that is interrupted by the herbicide, whereas mode of action is a general term referring to all of the plant-herbicide interactions.

Contact and Absorption

Herbicides must contact the plant surface to be effective. Herbicides with limited mobility that are effective at the site where they contact the plant are known as contact herbicides. Herbicides that must be absorbed and translocated to the site of action to be effective are called systemic herbicides. Contact herbicides typically affect only the portion of the plant with which they come into physical contact. Contact herbicides are fast acting, and injury symptoms can appear within hours of application. Conversely, injury symptoms from systemic herbicides can take from several days to weeks to appear, but the entire plant may eventually be killed. Soil-applied herbicides are applied to the top few inches of the soil and eventually absorbed through root tissue, whereas foliar-applied herbicides are applied to leaves or stems. Most contact herbicides are foliar-applied, whereas systemic herbicides can be either soil- or foliar-applied.

Choosing the appropriate herbicide depends upon target species biology, herbicide selectivity, application method, and site conditions. It is important to understand these factors to ensure that an effective herbicide is selected. For example, contact herbicides are most effective against annual invasive plants and in situations in which plant regrowth is not a concern. Conversely, systemic herbicides are more effective on perennial invasive plants and can limit regeneration of treated plants. Soil-applied herbicides are most effective on seedlings or germinating plants prior to their emergence above the soil. Established plants may require a foliar-applied herbicide for effective control. Mature plant tissues absorb herbicides less easily than young plant tissues due to thickening of the outer tissues in older plants.

Translocation

Systemic herbicides move, or translocate, from the point of application to the site of action through either the phloem (tissue that transports sugars from the leaves to the roots), xylem (tissue that transports water from the roots to the leaves), or through both. Some herbicides move more easily and farther within plants than others.

Site of Action

To be effective, an herbicide must reach the site of action. An herbicide binds to a specific location within the plant, typically a single protein, and as a result disrupts a physiological process essential for normal plant growth and development.

Mechanism of Action

Herbicides can affect various sites of action within plants, and they are often categorized into different mechanisms of action based on how they work and the injury symptoms they produce.

(adapted from Radosevich et al. 1997, Zimdahl 1999, Monaco et al. 2002)

Repeated use of an herbicide with the same mechanism of action can result in resistance of the plant population to that herbicide because selection pressure for the resistant portions of the population increases with each application. Using herbicides with different mechanisms of action, or combining them with other control methods, can reduce the risk of developing herbicide-resistant populations.

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Because herbicides are inherently toxic to plants, they are effective tools to manage undesirable plant species, but they can also have unintended, adverse effects on desirable plant species. Thus, it is important to understand the fundamentals of how herbicides affect plants as well as to focus herbicide use to meet particular invasive plant management objectives.

Selectivity

Plants vary in their susceptibility to different herbicides. For example, the selective herbicide 2,4-D injures or kills broadleaved plants but has little effect on grasses. Selectivity is the result of complex interactions between the plant, the herbicide, and the environment.

(adapted from Radosevich et al. 1997)

Repeated use of an herbicide such as picloram can select for picloram-tolerant plant species such as hoary cress (Cardaria draba) and other members of the mustard family (Brassicaceae). Photo credit: MSNWAEP, www.forestryimages.org

Species Shift

Because of herbicide selectivity, continued use of a particular herbicide may result in a shift within a plant community from susceptible to more herbicide-tolerant species. For example, repeated use of herbicides, such as clopyralid, that select for broadleaved species can result in an increase in grasses (Tyser et al. 1998).

Removal of invasive plants from highly degraded sites can result in one undesirable species being replaced by an equally undesirable species. For example, using picloram to control spotted knapweed (Centaurea maculosa) with high canopy cover (> 60%) resulted in a plant community dominated by cheatgrass (Bromus tectorum) and Japanese brome (B. japonicus) (Kedzie-Webb et al. 2002). In these cases, revegetation with desirable and competitive plant species is often necessary (DiTomaso 2000).

Seedbank Persistence

If viable seeds remain in the soil after treatment, undesirable plants can reestablish. The relative importance of the seedbank to seedling recruitment and subsequent increase in an invasive plant population varies with the species as well as the plant community and site conditions. Depending upon the plant species, seeds can remain viable in the soil for many years. Thus, management must account for the potential of plant populations to persist even after multiple herbicide treatments. Some herbicides such as picloram can be persistent in the soil for several years after application and can control new plants germinating from seedbanks (Tu et al. 2001).

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Federal laws and policies regulate many aspects of herbicides including labeling, registration, and application, but these regulations are not a substitute for a thorough knowledge of the risks associated with herbicide use. The benefits of herbicides must be weighed against the potential for exposure and impacts to human health, nontarget organisms, and the environment. Risks are always present with any herbicide use, but improper use or misapplication can increase these risks.

Herbicide Registration

The federal government, in cooperation with individual states, regulates herbicides to ensure that they do not pose unreasonable risks to human health or the environment. The EPA requires extensive test data from herbicide producers to show that products can be used without harming human health and the environment. EPA scientists and analysts carefully review these data to determine whether to register (license) an herbicide product and whether specific restrictions are necessary.

The process of registering an herbicide is a scientific, legal, and administrative procedure through which the EPA examines ingredients of the herbicide; sites or target species on which it is to be used; amount, frequency, and timing of its use; and storage and disposal practices. In evaluating an herbicide registration application, the EPA assesses a wide variety of potential human health and environmental effects associated with use of the product. The producer of the herbicide must provide data that address the following:

1. What ingredients are in the herbicide?
2. What is the environmental fate of the herbicide?
3. What does the herbicide do in organisms and the environment?

Herbicide product tests follow EPA guidelines and evaluate whether an herbicide has the potential to cause adverse effects on humans, wildlife, fish, and plants, including federally listed species and nontarget organisms, or to contaminate surface water or groundwater through leaching, runoff, and spray drift. Testing is conducted on only a few faunal species of specific age and under limited environmental conditions. Care should be taken when extrapolating these data to other circumstances. Furthermore, there is essentially no testing on herbicide mixtures, and most testing is done with the technical grade of the active ingredient rather than with actual formulated products.

Read more about herbicide risk assessments:

• EPA Assessing Health Risks from Pesticides
• EPA Protecting Wildlife from Pesticide Risks

Read more about herbicide registration:

• EPA Pesticide Registration Program
• Laws Affecting EPA’s Pesticide Program
• EPA Pesticides: Regulating Pesticides

Exposure

The EPA evaluates both exposure and toxicity to determine the risk associated with use of an herbicide. People, nontarget flora and fauna, water, and soil can be exposed to herbicides during herbicide application or from subsequent offsite movement. Herbicide exposure can be minimized or avoided by following the herbicide label and understanding what happens to herbicides after application.

For animals (including humans), herbicides have three modes of entry into the body: through the skin, by swallowing, and by breathing. Exposure can occur both during and following herbicide application. Individuals who mix, load, and apply herbicides are at the greatest risk of exposure. Exposure may also occur when the labeled restricted-entry interval (REI) is not observed and people reenter an area too soon following treatment.

Once herbicides have been applied, the potential for exposure is further influenced by the many biotic and abiotic processes that affect the fate of herbicides in the environment. Some processes may move or transfer the herbicide away from the target plant to nontarget organisms while other processes degrade or break down herbicides after application.

Processes that Affect Environmental Fate of Herbicides
Transfer absorption uptake of herbicides into plants or microorganisms adsorption binding of herbicides to soil particles volatilization conversion of solid or liquid herbicides into a gas spray drift airborne movement of spray droplets away from the application area surface runoff movement of herbicides from the land into surface water or groundwater leaching movement of herbicides into water through the soil	Degredation microbial breakdown breakdown of herbicides by microorganisms (e.g., fungi, bacteria) chemical breakdown breakdown of herbicides by chemical reactions in the soil biochemical breakdown breakdown of herbicides by enzymatic reactions such as metabolism within living plants photodegradation breakdown of herbicides by sunlight

(adapted from British Columbia Ministry of Agriculture and Lands 2007)

Persistent herbicides can remain active in the environment for long periods of time, potentially causing soil and water contamination and adverse effects to nontarget organisms. In some cases, compounds that result from herbicide degradation may continue to be significantly toxic in the environment.

Toxicity

The EPA uses toxicity tests as standard reference experiments to evaluate potential harm of herbicides to animals. Various rating systems describe the relative toxicities of herbicides. The EPA has category guidelines for acute and subchronic toxicity, which are used on herbicide labels.

Toxicity tests on mammals are segregated into three categories based on the length of exposure to the pesticide:

• acute tests
  Studies evaluate the effects of large dosages in a single exposure (dose) or short time period. Observations are conducted over a span of days to weeks.
• subchronic tests
  Studies involve exposing the test subject to compounds repeatedly over a longer period of time (e.g., 30 to 90 days).
• chronic tests
  Impact studies expose a test subject to a pesticide for a majority of its life span to determine the effects of long term, low level exposure. The potential for mutagenicity, carcinogenicity, hormone disruption, and developmental and reproductive effects is evaluated.

The amount of a substance to which a subject is exposed is as important as its toxicity. For example, small doses of aspirin can be beneficial to people, but at very high doses this common medicine can be deadly. A dose-response assessment involves considering the dose levels at which adverse effects are observed in non-human test subjects, and using these dose levels to calculate an equal dose in humans.

The median lethal dose (LD50) is the most commonly used index of herbicide toxicity. The LD50 is the dose that is lethal to 50 percent of the treated population (expressed as milligrams (mg) of compound ingested per kilogram (kg) of body weight). An herbicide’s toxicity to aquatic organisms is quantified with the LC50, which is the concentration of the herbicide in water required to kill half of the study animals. The LC50 is typically measured in micrograms of compound per liter of water.

The following table compares LD50 values for various products from caffeine to herbicides. Oral rat LD50 values are expressed as mg of compound per kg of body weight, and are listed from most to least toxic. These values were used to estimate LD50 in humans by converting mg/kg for rats to lb/180 lbs for humans. The column on the right shows estimated LD50 values for humans expressed in various metrics to demonstrate how much of a product would need to be consumed to be lethal.

Although some substances are more toxic pound for pound, oral exposure may be more difficult to achieve considering normal concentration and exposure to these products. For example, caffeine is nearly 10 times more toxic than dicamba herbicide when comparing milligrams of a pure substance ingested by a rat. A human would need to consume about 100 cups of coffee (containing caffeine) compared to 1 cup (0.5 pint) of formulated dicamba herbicide to reach a lethal dose. However, normal oral exposure is more likely to occur in much smaller quantities than if one were to drink a cup of the herbicide.

* Acute lethal dose in mg of compound/kg of body weight for 50% of the test animals (rats); the lower the number the more toxic the substance. Adapted from WSSA 1994.

**Estimated acute lethal dose in various measures of compound for a 180 lb person; estimated from male rat data using a conversion factor for mg/kg to lbs/180lbs. These numbers are not precise but provide a relative idea of lethal doses.

> Values that are preceded by a “greater than” sign (>) mean the LD50 is higher than the quoted figures, which are the highest amounts tested.

Human Health

Measures should always be taken to minimize exposure to herbicides. Photo credit: USFWS

Following herbicide label instructions and established safety procedures minimizes herbicide exposure. Herbicide applicators generally face the greatest risk, particularly during mixing and loading. The general public can be affected by direct contact through spray drift, accidental spills, indirect contact through consumption of contaminated food or water.

People with a hypersensitivity to chemicals, or multiple chemical sensitivity, may display extreme adverse effects and should take care to avoid or reduce their exposure to herbicides. These individuals are generally aware of their sensitivities because they have reactions to a variety of natural and synthetic compounds. Posting signs in public use areas during and following herbicide application will help minimize exposure.

Flora

Herbicides can be used to reach invasive plant management objectives and achieve desired vegetation conditions. Photo credit: USFWS

Herbicides can enhance native plant communities by removing undesirable species and increasing native species. For example, treating invasive alligatorweed (Alternanthera philoxeroides) with the selective herbicide triclopyr effectively reduced the biomass and cover of alligatorweed and increased the biomass of native wetland species important to waterfowl on managed wetlands of Eufaula NWR in Alabama (Allen et al. 2007). Similarly on waterfowl production areas of Medicine Lake NWR in Montana, using the nonselective herbicide glyphosate to control Canada thistle (Cirsium arvense) reduced Canada thistle density and biomass while increasing density and biomass of desirable forbs important for nesting waterfowl (Krueger-Mangold et al. 2002).

Herbicides can also have unintended consequences for nontarget plant species, species composition, and plant species richness and diversity. For example, herbicides such as picloram that are selective for broadleaved plants can control broadleaved invasive plants such as spotted knapweed (Centaurea maculosa) and sulphur cinquefoil (Potentilla recta) and promote recolonization of native grasses. However, because of this selectivity for broadleaved species, these herbicides can promote invasion by invasive grass species and negatively impact native broadleaved plants, reducing native species richness and diversity (Tyser et al. 1998, Pokorny et al. 2004, Denny and Sheley 2006).

Fauna

Use of herbicide to control encroaching woody growth increased forage for northern bobwhite. Photo credit: NC Wildlife Resources Commission

Herbicides have been designed to target biochemical processes, such as photosynthesis, that are unique to plants. Thus, they typically are not acutely toxic to animals (Tatum 2004). Some exceptions include paraquat. Data indicate that some herbicides can have a synergistic effect with commonly used insecticides when they runoff into surface waters. In addition, some herbicides can have subtle, but significant, physiological effects on animals, including developmental effects. So, as with all pesticides, the user needs to thoroughly evaluate the range on potential nontarget effects and strive to minimize these effects. In some cases, this may involve best management practices that go beyond the requirements on the pesticide label.

Herbicides have indirect effects on wildlife by altering vegetative cover and structure. Using imazapyr to control encroaching woody growth in longleaf pine stands (Pinus palustris) can increase forage for northern bobwhite (Colinus virginianus) (Welch et al. 2004). Conversely, silvicultural practices that use herbicides to eliminate competitive deciduous shrubs to promote revegetation by conifers can negatively impact songbird reproductive success (Easton and Martin 2002). In aquatic environments, treating invasive aquatic plants such as Eurasian watermilfoil (Myriophyllum spicatum) can result in massive plant die-off and decomposition. The decaying vegetation can deplete dissolved oxygen resulting in fish kills (Langeland 1998).

Water

Herbicides can contaminate groundwater and surface water. Contamination can occur directly due to several factors including spills or leaks, improperly discarded herbicide containers, and rinsing equipment near drainage areas. Contamination can also occur due to surface runoff or leaching of herbicides. Spray drift and volatilization of herbicides can transport the chemical into the atmosphere during and after application, potentially allowing herbicides to reach surface water and groundwater via precipitation.

Read more:

• US Geological Survey - Pesticides in the Nation’s Streams and Ground Water

Soils

The effect of herbicides on soil properties, chemistry, and microbial populations depends upon herbicide concentration and characteristics, and soil type, temperature, and moisture (Haney et al. 2000). Herbicides can influence soil pH (Schreffler and Sharpe 2003) and soil microbial activity (Haney et al. 2000). Although herbicides can have direct toxic effects on soil fauna (Salminen et al. 1996), herbicides typically affect these organisms indirectly via removal of aboveground vegetation and through changes to soil decomposer community structure and reductions in nutrient cycling (Mahn and Kastner 1985, Salminen et al. 1997). Herbicides can also reduce the growth and function of mycorrhizal fungi (Vieira et al. 2007), which increase the ability of plants to absorb and translocate nutrients from the soil.