TOEFL Reading Practice Test #25

In the late 19th century, a small insect from Asia, known as the San Jose scale, arrived in California and began devastating orchard trees across North America. Farmers initially managed the pest by spraying their trees with a sulfur-lime mixture, and the scales vanished within weeks. However, by the early 1900s, this treatment began to fail in some areas. Farmers in Clarkston Valley, Washington, suspected that manufacturers were diluting the pesticide. To solve the problem, they built their own factory to produce pure sulfur-lime, but the scale infestation persisted.

Entomologist A. L. Melander investigated and discovered that, while sulfur-lime worked perfectly in other regions like Yakima, it failed to kill all scales in Clarkston. A few scales consistently survived the spraying and multiplied, leading Melander to suspect that the problem lay with the insects themselves rather than the pesticide. At first, he rejected the idea that the scales had somehow developed immunity, since such a notion seemed incompatible with the prevailing belief that species were fixed and unchanging. Instead, he proposed in 1912 that rare genetic mutations might render some scales resistant to sulfur-lime. Resistant scales survived and passed their traits to their offspring, gradually increasing the population of resistant pests. Farmers in Clarkston, having used sulfur-lime longer and more aggressively than others, inadvertently accelerated this evolutionary process.

Melander’s theory, presented in 1914, went largely unnoticed because attention shifted to newer pesticides. In 1939, chemist Paul Müller introduced DDT, a highly effective insecticide that killed a variety of pests at low cost and low doses. Farmers eagerly adopted it, abandoning traditional pest control methods such as draining water or breeding pest-resistant crops. DDT also gained widespread use in public health campaigns to control disease-carrying mosquitoes.

Although DDT was initially considered a miracle pesticide, its success was short-lived. By the 1940s, resistance began to emerge as pests like houseflies developed immunity. Over time, more species became resistant, with over 500 identified by 1992. Farmers initially responded by increasing doses of DDT, but when this approach failed, they turned to newer pesticides. Melander’s early warnings about the evolutionary development of pesticide resistance were finally recognized as prophetic.