Organisms, particularly bugs, are like humans. They have a powerful drive to survive and perpetuate their species. They, too, are driven to sustainability. If life’s conditions change for insects or viruses or fungi that feed on agricultural crops, the creatures will change to cope with the new menace. They will develop resistance.

Charles Darwin and other scientists of the late 19th century were the first to write about what they called the “selection principle of evolution.” Sometimes this is termed “the survival of the fittest.” Here’s a real-life, modern-day example, from farmer’s fields:

A farmer plants rice in a paddy in Taiwan. (Fred Powledge)

A tiny insect called the brown planthopper was making life miserable for rice growers in Asia. Like everybody else, the planthopper wants to live, and to live, it must eat. The planthopper liked to get its food from phloem in the rice plant — sugary juices that bring the plant’s nourishment from the leaves, down a narrow canal, and out to the growing points.

As is the case with many insect attacks, the plant can survive a moderate infestation. The ‘hoppers’ natural enemies keep things in balance. When there’s a surge in leafhopper population, explains Dr. Peter Kenmore, an expert in all this, there’s an equal surplus in leafhopper eggs, some of which fall off the rice plant into the water (this rice is grown in paddies, so the roots and lower parts are under water), where they become a feast for other bugs.

“Little spiders, water striders, water bugs, the bugs that you see dancing on the water — they specialize in killing the hoppers,” said Kenmore one day in the Philippines, where he worked with the United Nations Food and Agriculture Organization. “They detect vibration with their feet, which are like gigantic amplifiers. So as soon as anything disturbs the water surface, Boom!”

When trouble starts.
The trouble comes when farmers notice the increased leafhopper population and respond to it by spraying pesticides into their paddies. Then, two things happen:

One, the sprayed poisons will kill the beneficial insects along with the leafhoppers.

And two, some of the leafhoppers will escape the lethal poison because they are stronger, or perhaps because their genetic makeup gives them protection. They have resistance.

If the poisons kill off the weaker leafhoppers, guess who's left? The stronger ones. When these survivor bugs mate, they may produce large numbers of offspring with genes that favor survival, and on whom the pesticide has little effect. Kenmore and others refer to these as “superbugs.” More sprayings only increase the selective pressure to produce bugs that are extraordinarily resistant.

The creation of resistant pests is hastened by the fact that many insects produce several generations a year, so their evolution proceeds more quickly than larger creatures, certainly faster than the pesticide manufacturers' ability to develop and market newer, “better” poisons.

Increasing resistance.
This resistance to pesticides among insects we humans consider “harmful” has increased wildly. By one account, in 1944 there were 44 known insect species that had developed resistance. Now there are hundreds. Scientists and farmers have responded, in some cases, by reducing the spraying schedule and by encouraging beneficial insects (See the page on Alternative agriculture.)

The brown leafhopper story is but one of many tales of harmful agricultural pests that were actually made stronger by humans’ attempts to control them. Some chemical companies (and some scientists who work for them or who work in closely-related universities that receive funding from chemical manufacturers) seek answers in ever-more-powerful pesticides. But other scientists have taken a different approach. They seek out the place where the pest originated and search for whatever it is that keeps the pest under control. The answer may be another insect, or it may be a fungus, or some environmental condition.

	USDA biological aide Kyra Williams removes beneficial insects, Encarsia formosa, from plant leaves. They'll then be shipped to scientists who are looking for natural controls from the whitefly, a major agricultural pest. (Scott Bauer, USDA-ARS)

The tiny Trichogramma wasp is an effective example. Over most of the world, a creature known to farmers as the diamondback moth eats its way (when it is in its adult caterpillar stage) through whole fields of cabbage, cauliflower, broccoli, brussels sprouts, radish, collard, mustard, and kale. Heavy use of pesticides against it has only turned the moth into a superbug, capable of driving farmers to the poorhouse.

The Trichogramma species have become the tiny workhorses of biological control agents. Although they are barely more than a fiftieth of an inch long, they do their powerful work by parasitizing the eggs of their victims. Females lay their eggs in the eggs of the hosts, and parasitoids may develop within. Eventually, adult wasps emerge from the host egg, which is destroyed in the process. In their natural, normally occurring state, members of the Trichogramma species are known to be effective controls for many food crops, from corn to avocado to tomato to soybean. Farmers can order them in commercial quantities, still parasitized in host eggs that are stuck to small bits of cardboard.

Resistance, then, is a commonplace part of life on Earth, especially the life of plants that we count on for our food. We should know by now that there is no such thing as a “magic bullet” that will eliminate all the leafhoppers, diamondback moths, corn earworms, and potato blight. We must learn that these creatures, like ourselves, live on a constantly unbalanced biological see-saw — that the struggle for survival is an unending one, for our enemies as well as ourselves.

Click here to explore a plant breeding effort that saved millions of lives.
Click here to return to the page on plant breeding.

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