In 1840, there were no devastating pests or diseases of crops. And there were few insecticides or fungicides, because there was no real need for them. The only known crop fungicide was flowers of sulphur, used as a dust, to control powdery mildews. The only known crop insecticide was soft soap (i.e., potassium soap) used to control aphids. In those days, it was normal to find a codling moth grub in the core of an apple, a caterpillar in the heart of a lettuce or cabbage, or weevils in stored cereal products. But these were about the limits of the problem. We have to ask the inevitable question "What has changed in a century and a half?" Crop parasites are now a major problem, taking about 25% of pre-harvest crop yields world-wide, in spite of pesticides that cost billions of dollars a year, and cause endless pollution problems. There appear to be three basic reasons.

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