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• Herbicides: Introduction

Herbicides

• Introduction
• When to List
• Ways to Measure
• Conceptual Diagrams
• Literature Reviews
• References

Herbicides are chemicals used to manipulate or control undesirable vegetation. The most frequent application of herbicides occurs in row-crop farming, where they are applied before or during planting to maximize crop productivity by minimizing other vegetation. They also may be applied to crops in the fall, to improve harvesting. Herbicides are used in forest management to prepare logged areas for replanting; the total applied volume and area covered is greater but the frequency of application is much less than for farming (Shepard et al. 2004). In suburban and urban areas, herbicides are applied to lawns, parks, golf courses and other areas. Herbicides are applied to water bodies to control aquatic weeds that impede irrigation withdrawals or interfere with recreational and industrial uses of water (Folmar et al. 1979).

The potential effects of herbicides are strongly influenced by their toxic mode of action and their method of application. The molecular site of action is challenging to predict because structural associations have not been identified (Duke 1990), but modes of action are well-established. Herbicides can act by inhibiting cell division, photosynthesis, or amino acid production or by mimicking natural auxin hormones, which regulate plant growth, and causing deformities in new growth (Ross and Childs 1996). Methods of application include spraying onto foliage, applying to soils, and applying directly to aquatic systems.

Figure 1 and Table 1 present the ten herbicides most used on agricultural land in the U.S. Glyphosate and atrazine were applied to more than double the crop field acreage than the third leading herbicide, 2,4-D, in 2001.

Figure 1. Agricultural use of herbicides in 2001 in millions of acres.
Reproduced from www.ers.usda.gov/Briefing/AgChemicals/pestmangement.htm.

Herbicides may cause biological impairments of water bodies if they occur in water or sediment at sufficient concentrations. Most commonly, they enter surface water in runoff or leachate, but, because they have relatively low toxicity to fish and invertebrates (Table 2), acute toxicity is likely only when they are deliberately or accidentally applied directly to water bodies. Direct applications may result in direct toxicity to non-target plants and animals or indirect effects due to the death and decomposition of plants. Impairments also are more likely when herbicides are applied together or with other pesticides (Streibig et. al. 1998), resulting in additive or synergistic effects. Tests using the earthworm Eisenia fetida show that atrazine reacts synergistically with chlorpyrifos, an organophosphate insecticide: the mixture is seven times more toxic than the two individual pesticides (Lydy and Linck 2003). Atrazine has been shown to increase the effects of other pesticides in mosquito larvae, fruit flies, houseflies, and midge flies (Belden and Lydy 2000, Lydy and Linck 2003). The surfactants used in herbicide solutions also can be toxic to biota and are not considered when testing active ingredients (Folmar et al. 1979).

Figure 2. A simple conceptual diagram illustrating causal pathways, from sources to impairments, related to herbicides. Click on the diagram to go to the Conceptual Diagrams tab and view a larger version.

Checklist of sources, site evidence and biological effects

Herbicides are addressed in this module as proximate stressors that should be considered in Step 2: List Candidate Causes when potential human sources and activities, site observations, or observed effects support portions of the causal pathways in the herbicides conceptual diagram (Figure 2). The conceptual diagram and other information also may be useful in Step 3: Evaluate Data from the Case. When herbicides are contributing to other stressors (e.g., in stream habitat alteration via reduction in riparian vegetation), rather than causing direct toxicity to organisms, they can be considered as part of the pathway for the proximate cause of impairment.

This checklist will help you determine whether herbicides should be listed as a candidate cause based on the presence of sources and activities, site evidence, or biological effects. For more detailed information on the entries listed here, please see the When to List tab of this module. Like the conceptual diagram (Figure 2), this list also can guide you in collecting evidence that reduces or enhances support for herbicides as a candidate cause.

Other stressors may influence herbicide effects, including:

• Temperature: Elevated temperature frequently tends to increase the toxicity of chemicals.
• Moderate to high pH: pH will determine the ionic state and bioavailability of ionizable herbicides.
• DO: If low DO concentration is a candidate cause of a kill or other impairment, signs of herbicide application and resulting dead plant material are evidence of a causal pathway to low DO.
• Unspecified toxics: Surfactants in herbicide formulations can be more toxic to animals than the active ingredients (Folmar et al. 1979, Diamond and Durkin 1997). For example, Folmar et al. (1979) discovered that elevated temperature and pH (i.e., an increase from 6.5 to 7.5) increased the toxicity of Roundup® to bluegill and rainbow trout; the higher toxicity was attributed to the higher toxicity of the surfactant, rather than the active ingredient, glyphosate. The findings of Diamond and Durkin (1997) reinforce this result, showing the toxicity of glyphosate decreases and the toxicity of the surfactant increases at more alkaline pH levels.

You also may wish to consider other causes with similar evidence:

• Insecticides
• Endocrine disruptors
• Toxic metals

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