Other Environmental Themes

Natural Pest Control

In the 1962 classic, Silent Spring, Rachel Carson warned of a world without bird songs—a by-product of heavy herbicide and pesticide use. There are, of course, many other unwanted impacts stemming from the widespread application of toxic substances to control animal pests and weeds. A reliance on synthetic poisons undermines the pest control services provided by nature. These services are jeopardized because chemical spraying can inadvertently wipe out the pests' natural enemies, who tend to be even more vulnerable to the toxic agents than the pests themselves. Another drawback is that pests develop resistance to the chemicals over time, prompting ever-increasing doses to be applied and new compounds to be tried.

Despite these shortcomings, twice as many pesticides are used in the United States today than in 1962, when Carson's book was published. Each year about 2,500,000 tons of chemicals are applied to crops worldwide, making it the dominant form of agricultural pest control employed by humans. Nevertheless, a growing awareness of the problems associated with pesticides has heightened interest in biological control methods, which rely on natural predators to keep pests in check.

Biological techniques often work quite well. In the 1970s, for example, the accidental introduction of the cassava mealybug in Africa led to cassava crop losses of 80 to 90 percent. A tiny parasitic wasp, which feeds on the mealybug, was then introduced, bringing cassava production back to normal. Biological techniques have an intrinsic advantage over chemical strategies: whereas pests often develop resistance to chemicals, natural predators and parasites can evolve in response to changes in the pest.

The U.S. Department of Agriculture is encouraging farmers to explore alternative, nonchemical solutions to common agricultural problems, such as the planting of crimson and clover amidst pecan orchards in the American South. The crimson and clover flowers attract beetles and other bugs that prey on aphids, which normally plague pecan trees. By attracting natural predators, this strategy decreases the need for insecticide spraying. The flowers also contribute nitrogen to the soil, reducing the need for artificial fertilizers.

Although there are many success stories, biological approaches are not infallible. A recent study found that an insect predator imported from Europe to curb the growth of weed thistles in Nebraska pastures spread uncontrollably, threatening two plant species and an insect with extinction. This experience shows that great care must be exercised in the selection of natural predators. It also demonstrates just how difficult it is for humans to reproduce the services of nature.