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The Large and Varied World of Pesticides

Hanna's Handbook of Agricultural Chemicals lists about 500 chemical products used in agriculture when it was published. The year was 1952 and the number of insecticides was 182, fertilizers, 96, and weedkillers, 53.¹ The Handbook was designed to be carried in a coat pocket, but such a reference published today would be far less portable.  Today's reference would show some 20,000 pesticide products.²

Over the years, the term “pesticide” has come to include a vast array of substances aimed at “preventing, destroying, repelling or mitigating any insects, rodents, nematodes, fungi, or weeds, or any other forms of life declared to be pests.” The term is also applied to substances that are used as plant regulators, defoliants or drying agents. While pesticide is the general term, “subcategories” under that include herbicides that control weeds and insecticides to kill insects, as well as rodenticides, fungicides, nematocides and acaracides, each targeting a different pest.

Another classification of this large and ever-changing universe of products is based on chemistry rather than target.

• Some of the earliest synthetic pesticides introduced into widespread use were the chlorinated hydrocarbons (organochlorines), such as DDT, aldrin, dieldrin, heptachlor and chlordane. In general, chemicals in this class did not tend to be highly water soluble or show very high toxicity to mammals. Unfortunately, however, the chlorinated hydrocarbons had a tendency to persist in the environment and move through the food chain, building up in living tissues.
  

• Organic phosphorus compounds (organophosphates) include malathion and diazinon, among others. Some of these compounds were initially developed as nerve gases and though they are less persistent than the organochlorines, they can be more toxic and hold the potential to kill and injure birds and wildlife.
  

• Carbamates are generally short-lived and include such pesticides as aldicarb, which has contaminated groundwater in a number of areas, and carbofuran, which has been implicated in bird kills. The carbamates are more persistent than the organophosphates and they tend to be “broad spectrum” pesticides, acting against a number of species rather than selectively against a few.
  

• First developed in 1973, synthetic pyrethroids mimic the active compounds of the pyrethrum plant. These pesticides have low toxicity to mammals but can be very toxic to fish and aquatic life. Since they are highly toxic to insects, they can often be used at lower dosages than other pesticides.

Each pesticide product, no matter how complex, contains at least one component, known as the “active ingredient,” that works on the pest it has been designed to destroy. A mixture of one or more active ingredients is combined with so-called “inert ingredients” to make a particular pesticide formulation.

Though the regulatory terminology describes these “carrier” or mixture ingredients that accompany the active ingredients as “inert,” they do not necessarily meet a chemistry textbook definition of inert or inactive. In fact, some inert ingredients -- like chloroform, carbon tetrachloride, tetrachlorethene, and trichloroethene -- are troubling chemicals that can cause pollution and health problems in their own right.³

Additional bits of pesticide vocabulary to keep in mind include "parent pesticide" and "degradate" or "metabolite." Parent pesticide refers simply to the active ingredient as it is manufactured and sold by its maker.  Generally the pesticidal chemical does not remain stable indefinitely,  but degrades or metabolizes into different chemicals over some period of time.  These chemical offspring, as it were, are generally referred to as degradates or metabolites.

1. Berg, Norman A., Soil and Water Conservation Society, "Why We Use Agrichemicals:  A Historical Perspective and a Look Ahead," in Agricultural Chemicals and Groundwater Protection:  Emerging Management and Policy, Proceedings of a conference held October 22-23, 1987, published by the Freshwater Foundation, 1988.

2.  U.S. EPA, Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs Annual Report for 1996, EPA 735R96001, 1996.

3.  See, for example, Northwest Coalition for Alternatives to Pesticides, What You Don't Know Can Hurt You:  Secret Ingredients in Pesticide Products, 1991 and Worst Kept Secrets:  Toxic Inert Ingredients in Pesticides, 1998 and Toxic Secrets: "Inert" Ingredients in
Pesticides, 1987-1997, 1998.  Note also the comments of the National Research Council in Environmental Epidemiology: Public Health and Hazardous Waste, 1991. The Council cites a 1984 report by Cohen and Bowes that estimates the amount of “inert” pesticide ingredients released to the land between 1971 and 1981 at 200 million pounds. They also note that “In some cases, materials that have been banned as active ingredients continued to be used as inert ingredients.”

4. Barbash, Jack E., et al, "Distribution of Major Herbicides in Ground Water of the United States," U.S. Geological Survey, 1999 available at <http://water.wr.usgs.gov/pnsp/rep/wrir984245/>.

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