Pesticides and herbicides (environmental impact)

• DEFINITION: Chemicals designed to kill or inhibit the growth of unwanted organisms

Although the use of pesticides, including herbicides, has been beneficial to humankind, enabling increased crop yields and helping to prevent disease, many of the chemicals that have been used to kill pests have had detrimental effects on the environment as well as direct negative effects on human health.

The major types of pesticides are insecticides (to kill insects), nematocides (to kill nematodes), fungicides (to kill fungi), herbicides (to kill weeds), and rodenticides (to kill rodents). Although the use of pesticides has mushroomed since the introduction of monoculture (the agricultural practice of growing only one crop on a large amount of land), the application of chemicals to control pests is by no means new. The use of sulfur as an insecticide dates to before 500 BCE. Salts from heavy metals such as arsenic, lead, and mercury were used as insecticides from the fifteenth century until the early part of the twentieth century, and residues of these toxic compounds are still being accumulated in plants that are grown in soil where these materials were used. In the seventeenth and eighteenth centuries, natural plant extracts such as nicotine sulfate from tobacco leaves and rotenone from tropical legumes were used as insecticides. Other natural products, such as pyrethrum from the chrysanthemum flower, garlic oil, lemon oil, and red pepper, have long been used to control insects.

In 1939, the discovery of the utility of dichloro-diphenyl-trichloroethane (DDT) as a strong insecticide opened the door for the development of a wide array of synthetic organic compounds to be used as pesticides. Chlorinated hydrocarbons such as DDT were the first group of synthetic pesticides. Other commonly used chlorinated hydrocarbons include aldrin, endrin, lindane, chlordane, and mirex. Because of the low biodegradability and long in the environment of these compounds, their use was eventually banned or severely restricted in the United States. Organophosphates such as malathion, parathion, and methamidophos replaced the chlorinated hydrocarbons. These compounds biodegrade in a fairly short time, but are generally much more toxic to humans and other animals than the compounds they replaced. In addition, they are water soluble and, therefore, more likely to contaminate water supplies. Carbamates such as carbaryl, maneb, and aldicarb have also been used in place of chlorinated hydrocarbons. These compounds biodegrade rapidly and are less toxic to humans than organophosphates, but they are also less effective in killing insects.

Herbicides, which are used specifically to kill or retard the growth of unwanted plant life, are classified according to the ways in which they work rather than their chemical composition. As their name suggests, contact herbicides such as atrazine and kill when they come into contact with a plant’s leaf surface; these herbicides generally work by disrupting the photosynthetic mechanism. Systemic herbicides such as diuron and fenuron circulate throughout the plant after being absorbed. They generally mimic the plant hormones and cause abnormal growth to the extent that the plant can no longer supply sufficient nutrients to support growth. Soil sterilants such as trifluralin, diphenamid, and daiapon kill microorganisms necessary for plant growth and also act as systemic herbicides.

Pesticide Use

In the United States, pesticide use grew by an estimated 183 million kilograms annually from 1996 to 2011. However, by the 2020s, the use of pesticides, including insecticides and fungicides, had decreased, while the use of herbicides had increased. This is largely attributed to the prevalence of genetically engineered seeds, which were introduced in 1996 by agrochemical giant Monsanto and which were capable of surviving being sprayed with much higher doses of herbicides than other seeds. Indeed, the use of glyphosate, the main herbicide in use in the US (typically found under the brand name Roundup), rose from being used to spray an estimated 84 million pounds of agricultural crops in 2000, to over 270 million pounds of crops in 2019, according to the US Geological Survey. By the early 2020s, glyphosate was sprayed on close to half of all corn and soybean crops planted in the US.

Still, approximately 55,000 different pesticide formulations were available in the early 2020s. These pesticides were used primarily on four crops: soybeans, wheat, cotton, and corn. In 2007, the annual expenditure on pesticides each year in the United States had reached approximately $8 billion, and in 2018, this number hit $15 billion. Worldwide, more than 2.7 million tons of pesticides were applied in agricultural contexts in 2020, and total expenditure on pesticides was more than $41.1 billion. A 2022 market research report on the use of biopesticides, which are pesticides that are made from natural materials such as plants, animals, and bacteria, valued the global biopesticide market at $7 million in 2020 and estimated the market value would reach $33.6 million by 2031.

The use of pesticides has had a beneficial impact on the lives of humans by increasing food production and reducing food costs. Even with pesticides, insects and other pests reduce the world’s potential food supply by more than 50 percent. Without pesticides, the losses would be much higher, resulting in increased starvation and higher food costs. Pesticides also increase the profit margin for farmers. It has been estimated that for every dollar spent on pesticides, farmers experience an increase in yield worth three to five dollars. Pesticides appear to work better and faster than alternative methods of controlling pests. These chemicals can rapidly control most pests, are cost-effective, can be easily shipped and applied, and have a long shelf life in comparison with alternative methods. In addition, farmers can quickly switch to different pesticides if the pests they are trying to kill develop genetic to a given pesticide.

Perhaps the most compelling argument for the use of pesticides is the fact that pesticides have saved lives. It has been suggested that since the introduction of DDT, the use of pesticides has prevented approximately seven million premature human deaths from insect-transmitted diseases such as sleeping sickness, bubonic plague, typhus, and malaria. It is likely that even more lives have been saved from starvation because of the increased food production resulting from the use of pesticides. Some argue that this one benefit far outweighs the potential environmental and health risks of pesticides. In addition, new pesticides are continually being developed, and safer and more effective control may be available in the future.

Environmental Concerns

An ideal pesticide would have the following characteristics: it would not kill any organism other than the target pest; it would in no way affect the health of non-target organisms; it would degrade into nontoxic chemicals in a relatively short time; it would prevent the development of resistance in the organism it is designed to kill; and it would be cost-effective. However, no pesticide available in the early twenty-first century meets all of these criteria, and, as a result, a number of environmental problems have developed from the use of pesticides. One of these problems is broad-spectrum poisoning. Most, if not all, chemical pesticides are not selective. In other words, they kill a wide range of organisms rather than just the target pest. The extermination of beneficial insects, such as bees, ladybugs, and wasps, may result in a range of problems, including reduced pollination and explosions in populations of unaffected insects.

When DDT was first used as an insecticide, many people believed that it was the perfect solution for controlling many insect pests. Initially, DDT dramatically reduced the number of problem insects; within a few years, however, a number of insect species had developed genetic resistance to the chemical and could no longer be controlled with it. By the 1990s, approximately two hundred insect species had genetic resistance to DDT. Other chemicals were designed to replace DDT, but many insects also developed resistance to these newer insecticides. As a result, although many synthetic chemicals have been introduced into the environment, the pest problem is still as great as it ever was.

Depending on the types of chemicals they contain, pesticides and herbicides remain in the environment for varying lengths of time. Chlorinated hydrocarbons, for example, can persist in the environment for up to fifteen years. From an economic standpoint, this can be beneficial because the pesticide has to be applied less frequently, but from an environmental standpoint, it can be detrimental. In addition, many pesticides degrade in such a way that their breakdown products, which may also persist in the environment for long periods of time, are often toxic to other organisms.

Pesticides and herbicides may concentrate in animals as they move up the food chain. All organisms are integral components of at least one food pyramid. While a given pesticide may not be toxic to species at the base, it may have detrimental effects on organisms that feed at the apex because the concentration increases at each higher level of the pyramid, a phenomenon known as biomagnification. With DDT, for example, some birds can be sprayed with the chemical without any apparent effect, but if these same birds eat fish that have eaten insects that contain DDT, they lose the ability to metabolize calcium properly. As a result, they lay soft-shelled eggs, which causes the death of most of their offspring. DDT was banned in the United States in 1972.

Pesticides and herbicides can also be hazardous to human health. Many pesticides, particularly insecticides, are toxic to humans, and millions of people have been killed by direct exposure to high concentrations of these chemicals. Many of those who have died have been children who were accidentally exposed to toxic pesticides because of careless packaging and storage. Numerous agricultural laborers, particularly in developing countries where there are no stringent guidelines for the handling of pesticides, have also died as a result of direct exposure to these chemicals. Workers in pesticide factories are also a high-risk group, and many of them have been poisoned through job-related contact with the chemicals. Pesticides and herbicides have also been suspected of causing long-term health problems such as cancer, and some pesticides have been classified as carcinogens by the US Environmental Protection Agency (EPA). Debate about such classifications continued into the twenty-first century. A 2015 report by the World Health Organization (WHO), for example, concluded that glyphosate is likely carcinogenic to humans, though the chemical continues to be widely used throughout the US and the EPA maintains that it is safe.

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