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Pesticide Resistance

The chemical arsenal we have developed in an attempt to rid our homes of rodents and our crops of insects is losing its power. We have simply caused pest populations to evolve, unintentionally applying artificial selection in the form of pesticides. Individuals with a higher tolerance for our poisons survive and breed, and soon resistant individuals outnumber the ones we can control.

Credits: 1978 WGBH Educational Foundation

Pesticide Resistance

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Pesticide Resistance:

It has the menacing sound of an Alfred Hitchcock movie: Millions of rats aren't even getting sick from pesticide doses that once killed them. In one county in England, these "super rats" have built up such resistance to certain toxins that they can consume five times as much poison as rats in other counties before dying. From insect larvae that keep munching on pesticide-laden cotton in the U.S. to head lice that won't wash out of children's hair, pests are slowly developing genetic shields that enable them to survive whatever poisons humans give them.

Rachel Carson predicted such resistance in her groundbreaking book Silent Spring, published soon after the chemical insecticide glory days of the 1950s. And the problem is getting worse. Farmers in the U.S. lost about seven percent of their crops to pests in the 1940s. Since the 1980s, some 13 percent of crops are being lost -- and more pesticides are being used.

It's a huge problem, but the pests are only following the rules of evolution: the best-adapted survive. Every time chemicals are sprayed on a lawn to kill weeds or ants for example, a few naturally resistant members of the targeted population survive and create a new generation of pests that are poison-resistant. That generation breeds another more-resistant generation; eventually, the pesticide may be rendered ineffective or even kill other wildlife or the very grass it was designed to protect.

In many ways, human actions are hastening pests' evolution of resistance. Farmers spray higher doses of pesticide if the traditional dose doesn't kill, so genetic mechanisms that enable the pests to survive the stronger doses rapidly become widespread as the offspring of resistant individuals come to dominate the population.

These days, farmers and backyard gardeners alike are trying to outsmart the pests by using a variety of natural methods. With "integrated pest management," scientists encourage the spread of natural enemies of pests, or they lure the pests with a meal that's even more tasty than the vulnerable crop. Pesticides are only used as a last resort if every other method fails. Integrated pest management has had some successes, but pesticides are still the world's most popular way to kill pests. Something to ponder the next time you can't seem to kill a cockroach.

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