Management Methods: Chemical Methods

Introduction

Chemical herbicides are one of the primary methods used to manage invasive plants. Herbicides can efficiently and effectively suppress or kill unwanted plants and should be used judiciously, safely, and in a way that minimizes adverse effects on nontarget resources.

Safe and effective use of chemical methods to manage invasive plants requires a working knowledge of how to select and apply herbicides properly, a solid understanding of how herbicides kill or suppress plants, and knowledge of the risks associated with their use, such as effects on water quality or nontarget organisms. In-depth knowledge of herbicides is also advantageous when chemical methods are discussed with stakeholders and the public.

An herbicide is a type of pesticide that is designed to suppress or kill plants by targeting biochemical processes that typically are unique to plants. Other types of pesticides include fungicides, insecticides, nematicides, and rodenticides. Herbicides are used to kill or suppress unwanted plants by decreasing their growth, seed production, and competitiveness. Although some herbicide products are made from living organisms (e.g., bioherbicides) or derived from natural materials (e.g., organic herbicides), this module focuses on synthetic chemical herbicides.

The synthesis of 2,4-D and the discovery of its growth-regulating and herbicidal properties during World War II marked the beginning of modern chemical control of undesirable plants. The rapid expansion of research into chemical herbicides and their subsequent use quickly revolutionized agriculture.

With the dramatic increase in the use of synthetic chemical herbicides and insecticides in agriculture, disease control, and other areas, scientists and the public raised concerns about environmental contamination, harm to human health, and pest resistance. These concerns led to the development of Integrated Pest Management (IPM), which promotes using synthetic pesticides judiciously and in combination with other methods in a way that increases effectiveness and minimizes harm.

The US Fish and Wildlife Service (USFWS) National Wildlife Refuge System implements IPM in a variety of habitats, ranging from actively managed areas such as water impoundments to wildlands composed of native vegetation. Many land managers and scientists agree that the management of invasive plants in wildland systems presents new challenges. The ecological complexity of wildland systems demands not only integrating herbicides with other control methods to manage invasive plants, but also, according to D’Antonio et al. (2004), demands a broader ecosystem focus than solely killing unwanted plants. Scientists suggest that managing terrestrial invasive plants in wildland systems may be improved by considering lessons learned from weed management in agriculture (Smith et al. 2006) and by incorporating ecological principles and processes (Sheley and Krueger-Mangold 2003).

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Becoming familiar with basic terminology associated with herbicide product labels, formulations, and classification systems provides a foundation for understanding how herbicides work, their potential impacts, and how to select and apply herbicides properly and safely.

Product Labels

Herbicide product labels provide directions for proper use; specifically, where, when, and how to apply them. The label also contains safety information such as personal protective equipment that must be worn by applicators and restricted-entry intervals (REIs), which is the amount of time that must lapse between applying herbicide and entering the treated area.

Herbicide labels include three names: the product or trade name, common name, and chemical name.

The partial product label above is used as an example only and is not an endorsement by the USFWS. Roundup is a registered trademark of Monsanto Company.

Individuals who mix, load, and apply herbicides must use products in conformance with the product label. The label is a legal document, and use of any herbicide in a manner inconsistent with its labeling is a violation of federal law. Using the product in accordance with label directions is also important for ensuring safety and effectiveness.

Prior to any herbicide applications, the applicator must read the label and ensure label compliance. Label instructions can be confusing, so it may be necessary for the applicator to read the label throughout the treatment process. The label contains mandatory statements that relate to actions that are necessary to ensure the proper use of herbicides and prevent adverse effects to the environment. These statements direct the applicator to take or avoid specific actions such as “do not apply directly to water.” The label also contains advisory statements that provide information on ways to maximize safety and efficacy while using the herbicide such as “latex gloves provide the best protection.” Cautionary statements or “signal words” such as “caution,” “danger,” or “poison” that indicate the relative toxicity of that herbicide to humans are also included.

In addition to the herbicide label, herbicide manufacturers must also provide a Material Safety Data Sheet (MSDS) for each product. Whereas the Environmental Protection Agency (EPA) regulates labels, the Occupational Safety and Health Administration (OSHA) has direct authority over MSDSs. An MSDS provides protection for employees using herbicides or other hazardous chemicals and includes information such as proper handling, storage, disposal, and spill or leak procedures.

When an MSDS is distributed with the herbicide, it becomes a part of the herbicide label (The Federal Insecticide, Fungicide, and Rodenticide Act 2(p)(2)(A)) and as such it must not contain information that conflicts with the EPA-approved label. EPA-approved mandatory statements cannot occur on the MSDS instead of the product label, and the EPA does not review or approve MSDSs.

Read more about labels:

• Environmental Protection Agency Read the Label First
• Crop Data Management Systems Herbicide Labels / Material Safety Data Sheets
• The Nature Conservancy Weed Control Methods Handbook (see Appendix 3)

Product Formulations

Herbicide formulations include active ingredients and other ingredients. Active ingredients kill or damage the target plant, whereas other ingredients such as additives (adjuvants) and carriers are added for application accuracy and safety, chemical stability, and herbicide performance.

Diluents, solvents, or carriers are used to dilute, dissolve, or disperse active ingredients and typically include water, petroleum distillates, or clay particles. Of the herbicide formulation additives, adjuvants can have the greatest impact on herbicidal activity. Specific adjuvants are typically not identified on the label, but may include

• surfactants (surface acting agents)
  to improve herbicide contact, droplet retention, and spread on the leaf surface
• antidrift agents
  to improve application accuracy and minimize nontarget impacts
• color, odor, or emetics
  to improve human safety from herbicide ingestion
• antifreeze or other stabilizers
  to enhance storage and transport capabilities

Adjuvants are included in the formulations of some herbicides, or they may be purchased separately and added at the time of application. Although adjuvants have no herbicidal activity, they are chemically and biologically active and can be toxic to plants and animals as well as pollute surface or groundwater. Applicators should check herbicide and adjuvant labels because some herbicides may have specific adjuvant requirements and restrictions.

Many herbicide formulations are designed to be further diluted in water (such as a water-soluble powder or liquid, wettable powders, and emulsifiable concentrates) so that they can be applied by spraying, wiping/wicking, or injection. Other formulations may be diluted and ready for application right out of the bottle, or dispersed as a dry granule or pellet.

The herbicide label provides information about how the product is formulated. This is typically stated as the amount of active ingredient per gallon (for liquid formulations) or per pound (for dry formulations). Some active ingredients are expressed in terms of their acid equivalent, which refers to the acid portion of the active ingredient and represents the percentage of herbicidal acid in the formulation.

Read more about adjuvants:

• The Nature Conservancy Weed Control Methods Handbook (see Chapter 8)

Herbicide Classification

Given the considerable number of herbicides available, different ways of classifying herbicides have been developed to distinguish among them. Herbicide classification also provides a means of understanding general similarities among herbicides. A working knowledge of herbicide classification is essential in selecting herbicides, diagnosing herbicide injury symptoms, managing herbicide resistance, and predicting herbicide interactions in the environment. The table below provides some ways herbicides are commonly classified.

(adapted from Radosevich et al. 1997, Zimdahl 1999, Monaco et al. 2002, Dewey et al. 2006)

Because herbicides can be highly effective at controlling a number of plant species, chemical methods are probably the most widely used invasive plant management method. Relative to other management methods, herbicides can be inexpensive, easy to use, and fast acting. However, herbicides should be used judiciously and carefully. Misuse of herbicides can result in unintended consequences including herbicide resistance, impacts on nontarget organisms, soil and water contamination, and risks to human health.

Chemical methods for managing invasive plants may include many types of herbicides and a variety of tactics for applying them in terrestrial and aquatic habitats. Before using herbicides, it is important to

• become familiar with herbicides, their properties, and environmental impacts
• acquire training, certificates, or licenses required by regulations and agency policies
• comply with all laws and other authorities pertaining to herbicide use