Around 1870, a little fruit-eating insect arrived in San Jose, California, on some nursery stock shipped from Asia. The pest, which became known as the San Jose scale, quickly spread through the United States and Canada, killing orchard trees as it went. Farmers found that the best way to control the scale was to spray their orchards with a mixture of sulfur and lime. Within a few weeks of spraying a tree, the insect vanished completely.

Around the turn of the century, however, farmers began to notice that the sulfur-lime mixture was not working all that well. A handful of scales would survive a spraying and eventually rebound to their former numbers. In Clarkston Valley in Washington State, orchard growers became convinced that manufacturers were adulterating their pesticide. They built their own factory to guarantee a pure poison, which they drenched over their trees, yet the scale kept spreading uncontrollably. An entomologist named A. L. Melander inspected the trees and found scales living happily under a thick crust of dried spray. Melander began to suspect that adulteration was not to blame. In 1912, he compared how effective the sprays were in different parts of Washington. In Yakima and Sunnyside, he found that sulfur-lime could wipe out every last scale on a tree, while in Clarkston between 4 and 13 percent survived. On the other hand, the Clarkston scales were annihilated by a different pesticide made from fuel oil, just as the insects in other parts of Washington were. In other words, the scales of Clarkston had a peculiar resistance to sulfur-lime.

Melander wondered why. He knew that if individuals eat small amounts of certain poisons, such as arsenic, they can build up an immunity. But San Jose scales bred so quickly that no single scale experienced more than a single spray of sulfur-lime, giving them no chance to develop immunity.

A radical idea occurred to Melander. Perhaps mutations made a few scales resistant to sulfur-lime. When farmers sprayed their trees, these resistant scales survived, as did a few nonresistant ones that hadn’t received a fatal dose. The surviving scales would then breed, and the resistant genes would become more common in the following generations. Depending on the proportions of the survivors, the trees might become covered by resistant or nonresistant scales. In the Clarkston Valley region, farmers had been using sulfur-lime longer than anywhere else in the Northwest and were desperately soaking their trees with the stuff. In the process, they were driving the evolution of more resistant scales.

Melander offered his ideas in 1914, but no one paid much attention to him; they were too busy discovering even more powerful pesticides.In 1939 the Swiss chemist Paul Muller found that a compound of chlorine and hydrocarbons called DDT could kill insects more effectively than any previous pesticide had. DDT was cheap and easy to make, it could kill many species of insects, and it was stable enough to be stored for years. It could be used in small doses, and it didn’t seem to pose any health risks to humans. Between 1941 and 1976, 4.5 million tons of DDT were produced DDT was so powerful and cheap that farmers gave up old-fashioned ways of controlling pests, such as draining standing water or breeding resistant strains of crops.

DDT and similar pesticides created the delusion that pests could be not merely controlled but eradicated, so farmers began spraying pesticides on their crops as a matter of course, rather than to control outbreaks. Meanwhile, public health workers saw in DDT the hope of controlling mosquitoes, which spread diseases such as malaria.

DDT certainly saved a great many lives and crops, but even in its early days, some scientists saw signs of its doom. In 1946 Swedish scientists discovered houseflies that could no longer be killed with DDT. Houseflies in other countries became resistant as well in later years, and soon other species could withstand it. Melander’s warning was becoming a reality. By 1992 more than 500 species were resistant to DDT, and the number is still climbing. As DDT began to fail, farmers at first just applied more of it; when more no longer worked, they switched to newer pesticides.