Pesticides Kill Pests and Wildlife Both on and off the Farm

The U.S. Environmental Protection Agency estimates that 2 billion kg of pesticides are applied in the United States each year, and 10 billion are applied around the world. Use of synthetic pesticides increases our dependence on them in a process called the "pesticide treadmill." Insecticides and fungicides do not destroy only pests; they also kill their natural enemies.

Three combines harvest a large wheat field, Washington State. With the mechanization, chemical fertilizers and pesticides, and irrigation of industrial agriculture, more food is produced than ever before in earth's history. This success, however, carries tremendous costs to the sustainability of agriculture and to biodiversity. (W. Wayne Lockwood/Corbis)

Three combines harvest a large wheat field, Washington State. With the mechanization, chemical fertilizers and pesticides, and irrigation of industrial agriculture, more food is produced than ever before in earth's history. This success, however, carries tremendous costs to the sustainability of agriculture and to biodiversity. (W. Wayne Lockwood/Corbis)

The natural enemies of insect pests include other insects that are parasites and predators, as well as pathogenic fungi. Pest species evolve resistance to pesticides much faster than their enemies, and thus pest populations quickly recover. Loss of natural enemies also leads to outbreaks of "secondary pests"—species that are not a problem until pesticides eliminate their natural enemies. As a result of pest resistance and secondary pest outbreaks, increasing amounts of pesticides must be applied, or more toxic chemicals must be developed. This is an arms race that we are losing. Despite the constant increase in pesticide use (Figure 1), loss of crops to insect pests was greater in 1989 (13 percent loss) than in 1945 (7 percent loss).

Pesticides have impacts far beyond their target organisms. Scientists at Cornell University estimate that 67 million birds are killed each year in the United States from pesticides. Many individuals of some bird species have died after eating sprayed insects. Pesticides from agriculture flow into aquatic systems via runoff of surface water, soil erosion, and drainage into groundwater. Pesticide residues in streams, lakes, bays, and coral reefs kill aquatic plants and zooplankton (microscopic animals) that fish require for food. More directly, very low concentrations of pesticides in water have been shown to increase the mortality of young fish and amphibians.

Pesticides and other toxins have an impor tant effect on wildlife through "bioaccumulation." Certain kinds of pesticides are persist-ent—that is, they do not break down as they pass through the food chain. They can be taken up by small aquatic organisms and insects and are then passed on to the fish that eat them. Those fish are eaten by larger fish, which are eaten by predators such as eagles, pelicans, seals, and bears. The toxins become increasingly concentrated in the higher levels in this food chain, so top predators accumulate dangerous concentrations. The decline in the bald eagle population in the 1900s was linked to bioaccumulation of persistent pesticides, especially DDT (dichlorodiphenyltrichloroe-thane). Such toxins affect the eagles' nervous systems and cause their eggshells to become fatally thin. The use of DDT was banned in the United States in the 1970s, and eagles have since recovered. Today many persistent pesticides have been replaced by alternative chemicals that are more short-lived—but more acutely toxic.

Wildlife has also been harmed by endocrine disrupters—toxins that interfere with hormones that regulate animal development. These toxins include some persistent pesticides and industrial pollutants. They appear to be linked to developmental abnormalities that have been increasingly found in wild animals, especially impaired growth of immune and sexual organs. Such deformities are appearing in many threatened species, including alligators, panthers, polar bears, and dolphins. Endocrine-disrupting insecticides have been linked in frog populations to extra limbs emerging from the stomach and neck.

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