Unit 7: Agriculture // Section 5: Combating Pests and Disease
As agriculture became increasingly dependent on technological inputs throughout the 20th century, it also underwent a structural shift, particularly in developed countries. Instead of raising a diverse mix of crops, farmers increasingly planted large holdings of one or a few crop varieties that had been developed for high yields. Monoculture makes it easier to cultivate large acreages more efficiently, especially using mechanized equipment and chemical inputs. However, these artificial ecosystems are vulnerable to outbreaks of pests and pathogens because they do not have natural protection from genetic diversity and they are typically nutrient-rich, thanks to abundant fertilizer use. Moreover, many pest species have adapted to spread rapidly in ecosystems where recent disturbances, such as plowing, have eliminated natural predators (for background, see Unit 4, "Ecosystems").
Agricultural pests include insects, mammals such as mice and rats, unwanted plants (weeds), fungi, and microorganisms such as bacteria and viruses. Humans have controlled pests with naturally-occurring substances such as salt, sulfur, and arsenic for centuries, but synthetic pesticides, first developed during World War II, are generally more effective.
Many of the first pesticides that were widely used for agriculture were organochlorines such as DDT (dichloro diphenyl trichloroethane), aldrin, dieldrin, and heptachlor. These substances are effective against a range of insects and household pests, but in the 1950s and 1960s they were shown to cause human health effects including dizziness, seizures, respiratory illness, and immune system dysfunction. Most organochlorines have been banned in the United States and other developed countries but remain in use in developing countries.
In her 1962 book Silent Spring, biologist and author Rachel Carson drew wide-scale public attention to the environmental effects of pesticides. Carson described how actions such as spraying elm trees with broad-spectrum pesticides to prevent Dutch elm disease severely affected many other parts of local ecosystems (Box 1).
The trees are sprayed in the spring (usually at the rate of 2 to 5 pounds of DDT per 50-foot tree, which may be the equivalent of as much as 23 pounds per acre where elms are numerous) and often again in July, at about half this concentration. Powerful sprayers direct a stream of poison to all parts of the tallest trees, killing directly not only the target organism, the bark beetle, but other insects, including pollinating species and predatory spiders and beetles. The poison forms a tenacious film over the leaves and bark. Rains do not wash it away. In the autumn the leaves fall to the ground, accumulate in sodden layers, and begin the slow process of becoming one with the soil. In this they are aided by the toil of the earthworms, who feed in the leaf litter, for elm leaves are among their favorite foods. In feeding on the leaves the worms also swallow the insecticide, accumulating and concentrating it in their bodies . . . . Undoubtedly some of the earthworms themselves succumb, but others survive to become ‘biological magnifiers’ of the poison. In the spring the robins return to provide another link in the cycle. As few as 11 large earthworms can transfer a lethal dose of DDT to a robin. And 11 worms form a small part of a day’s rations to a bird that eats 10 to 12 earthworms in as many minutes.
Rachel Carson, Silent Spring (New York: Houghton Mifflin, 1962), pp. 107–108 (emphasis in original).
Bioaccumulation of DDT and other organochlorines drastically reduced populations of bald eagles and other large predatory birds that fed at the top of the food chain. The pesticides disrupted birds' reproductive systems and caused them to lay eggs with very thin shells that broke before young birds hatched (Fig. 8).
Figure 8. DDT accumulation in the food chain
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Source: © United States Fish and Wildlife Service.
Organochlorines were replaced in the 1970s with other pesticides that were less toxic and more narrowly targeted to specific pests. However, many of these newer options still killed off pests' natural enemies, and when the insecticides were used repeatedly over time, pests became resistant to them through natural selection (many types of insects can develop through entire generations in days or weeks). Today hundreds of species of insects and weeds are resistant to major pesticides and herbicides.
In response some farmers have turned to methods such as releasing natural insect predators or breeding resistance into crops. For example, U.S. farmers can buy corn seeds that have been engineered to resist rootworms, corn borers, or both pests, depending on which are present locally, as well as corn that has been developed to tolerate herbicides. Others practice integrated pest management (IPM), an approach under which farmers consider each crop and pest problem as a whole and design a targeted program drawing on multiple control technologies, including pesticides, natural predators, and other methods.
In one notable case, Indonesia launched an IPM program in 1986 to control the brown planthopper, a notorious pest that lays its eggs inside rice plant stalks, out of range of pesticides. Outreach agents trained farmers to monitor their fields for planthoppers and their natural predators, and to treat outbreaks using minimal pesticide applications or alternative methods such as biological controls (Fig. 9). Over the following decade rice production increased by 15 percent while pesticide use fell by 60 percent. Yields on IPM lands rose from 6 to almost 7.5 tons of rice per hectare (footnote 7).
Figure 9. Gathering insects for identification during IPM training, Indonesia
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Source: © J.M. Micaud, Food and Agriculture Organization.
Plowing originally developed as a way to control pests (weeds), but created new issues in the process. Bare lands that have been plowed but have not yet developed crop cover are highly susceptible to erosion. The Dust Bowl that occurred in the United States in the 1930s was caused partly by poor agricultural practices. With support from the federal government, farmers plowed land that was too dry for farming across the Great Plains, destroying prairie grasses that held topsoil in place. When repeated droughts and windstorms struck the central and western states, hundreds of millions of tons of topsoil blew away. Today a similar process is taking place in northern China, where over-plowing and overgrazing are expanding the Gobi Desert and generating huge dust storms that scour Beijing and other large cities to the east.
Excessive plowing can also depress crop production by altering soil microbial communities and contributing to the breakdown of organic matter. To conserve soil carbon and reduce erosion, some farmers have turned to alternative practices such as no-till or direct-drill agriculture, in which crops are sown without cultivating the soil in advance. Direct drilling has been widely adopted in Australia, and some 17.5 percent of U.S. croplands were planted using no-till techniques as of the year 2000 (footnote 8).
No-till agriculture enhances soil development and fertility. It is usually practiced in combination with methods that leave crop residues on the field, which helps to preserve moisture, prevent erosion, and increase soil carbon pools. However, no-till requires an alternative strategy for weed control and thus frequently involves substantial use of herbicides and chemical means to control other pests.