5.6 Pest Control Methods

A pesticide is a chemical that kills or controls pests. The term includes insecticides (for insects), herbicides (for weeds), fungicides (for fungi) and rodenticides (for rodents). Pesticides have greatly reduced crop losses and helped control disease-spreading insects.

But many are broad-spectrum, meaning they kill a wide range of organisms, not just the target pest. They can kill pollinators like bees, natural predators of pests, fish and other aquatic life when they run off into water, and birds. Some, like DDT, persist in the environment and build up in food chains. Rachel Carson's 1962 book Silent Spring described how DDT was harming birds and helped spark the modern environmental movement; the U.S. banned most uses of DDT in 1972.

Pests evolve resistance through natural selection. In any large pest population, a few individuals happen to have genes that let them survive a pesticide. Spraying kills most of the others, so the resistant ones survive and reproduce. Their offspring inherit resistance, and after several generations, most of the population is resistant. The AP course framework calls this artificial selection, because people's spraying is what applies the pressure. Either term earns credit if you explain the steps: variation, survival of resistant individuals, reproduction and inheritance.

Farmers then have to use higher doses, spray more often or switch to new chemicals, which raises costs and harms more non-target organisms. This cycle is sometimes called the pesticide treadmill. Insects and weeds with short generations evolve resistance quickly.

Genetically engineered (GE) or genetically modified (GM) crops have had genes added, often from another species, to give them a useful trait.

• Bt crops contain a gene from the bacterium Bacillus thuringiensis that makes a protein toxic to certain insect pests. This can reduce insecticide spraying.
• Herbicide-tolerant crops survive a particular herbicide, so farmers can spray fields to kill weeds without harming the crop. This has led to heavy use of that herbicide and the spread of herbicide-resistant weeds.
• Other GE traits include drought tolerance and added nutrients.

GE crops can raise yields, reduce some pesticide use and cut crop losses. But they're usually planted as large monocultures of nearly identical plants, which lowers genetic diversity and makes crops more vulnerable to a new pest or disease. Pests can evolve resistance to the built-in traits too. To slow this, farmers growing Bt crops are often required to plant refuges, areas of non-Bt crops where non-resistant pests survive and mate with any resistant ones, diluting the resistance genes. Concerns about gene flow to wild relatives and the control of seed supplies by a few companies are also debated.