7.5 Pests, pesticides, and genetically modified organisms in agriculture

Pests are organisms that occur where they are not wanted or that cause damage to crops or humans, or other animals. Thus, the term “pest” is highly subjective. Researchers estimate that crop losses to pests and diseases for major crops average 17-30%.^[1]

A pesticide is a term for any substance intended for preventing, destroying, repelling, or mitigating any pest. Though often misunderstood to refer only to insecticides, pesticides also apply to herbicides, fungicides, nematicides, and other substances used to control pests. By their very nature, most pesticides create some risk of harm—pesticides can cause harm to humans, animals, and/or the environment because they are designed to kill or otherwise adversely affect living things. At the same time, pesticides are useful to society because they can kill potential disease-causing organisms and control insects, weeds, worms, and fungi.

“Pesticide” is a general term; more specific terms relate to the specific kind of target pest: herbicides, insecticides, nematicides (nematodes), ovicides (eggs of pests), etc. Pesticides vary in their level of specificity, and many of them kill beneficial species as well as pest species. The term “broad-spectrum” refers to a pesticide that kills a wide range of species.

Pesticide use is common in agriculture

Conventional agriculture almost always plants monocultures – fields or entire landscapes comprising a single crop. Large populations of a single crop can support large populations of pests that specialize on that crop. Once established, pest populations can be difficult to control or eradicate. As a result, pest loads in conventional agriculture can be high, and pesticide use can be similarly high.

Chemical pest management has helped reduce losses in agriculture and increase food and fiber production. Chemical pesticides can be effective, fast acting, and adaptable to a variety of crops and situations. When first applied, pesticides often result in impressive production gains. However, despite these initial gains, excessive use of pesticides can be ecologically unsound, leading to the destruction of natural pest enemies, increased pesticide resistance, and outbreaks of secondary pests. Impacts to human health, including health of agricultural workers applying the chemicals, are also of concern. Most synthetic pesticides persist in the environment, undergo bioaccumulation and biomagnification, and are classified as “forever chemicals.”

Insecticides and genetically modified (GM) plants

Synthetic chemical insecticides began with DDT, developed in the 1940s to control malaria and other insect-borne diseases. Its use was then supported by the US government and by industry in agriculture and households. The resulting loss of beneficial insects and birds and harm to human health led Rachel Carson to write Silent Spring, a book contributed significantly to the environmental movement of the 1960s. DDT was banned in 1972, but it, like succeeding synthetic pesticides, is very slow to break down in the environment, and it bioaccumulates. One of the breakdown products, of DDT, DDE, is also harmful, continuing the damage caused by the initial use. The international treaty called the Stockholm Convention on Persistent Organic Pollutants bans use of DDT in agriculture, but because of its effectiveness against mosquitoes, the treaty makes an exception for DDT use for public health, primarily against malaria (mostly indoor applications), so long as that use falls within World Health Organization guidelines.

Insecticides that have followed DDT have included chemicals with shorter-acting lifetimes such as organophosphates and carbamates, which are active for less than a season. However, PFAS are becoming more common in insecticides, as inert ingredients (for example, to reduce drift or facilitate uptake on stems and leaves). In the US, manufacturers do not need to disclose toxicity information for inert ingredients, making it harder to detect these forever chemicals in agricultural products. Fluorination is also being used with active ingredients (thus moving them into the PFAS category), to extend active lifespan, creating more forever chemicals and chemicals that can have unwanted side effects and direct effects in the environment for longer.^[2]

Figure 1. Spray application of pesticides from a tractor. AdobeStock – Vesna.

Fertilizers and pesticides (for plants, insects, nematodes, etc.) may be applied as sprays. Application may be by backpack sprayer in smaller areas, by tractor (above), or by light aircraft.

A particularly problematical class of insecticides – nicotine-like, synthetic chemicals called neonicotinoids or neonics – have been widely used in agriculture in the US since the 1990s. Unlike other insecticides, which are applied as sprays (Fig 1), neonics are often applied as coatings on seeds, to protect the seeds from predators of seeds and seedlings. The seed coating is absorbed by the plant as it sprouts and grows, so that the toxins are incorporated into the growing plant, making it toxic to insects. During planting of coated seeds, neonic dust can be spread throughout the field and nearby areas, where it can be taken up by nontarget plants. Neonics are also applied by spraying). Once absorbed (seed coatings) or applied (sprays), the toxin is expressed in all plant parts as well as pollen and nectar, and affects the central nervous systems of insects of most kinds as well as other kinds of organisms including soil organisms.^[3] Neonicotinoids are implicated in severe declines of bees, including honey bees.

The European Union has banned use of three neonicotinoids since 2018, but permits use of others  that show less harm to bees. However, even neonicotinoids that cause lower mortality may have harmful side effects that affect mortality indirectly.^[4] The EU, in 2020, withdrew permission for thiacloprid, one of the neonicotinoids believed to be less harmful to bees, after researchers found it strongly suppressed immune responses in bees.  The US has no federal bans, but some states regulate some uses, as do some Canadian provinces.

Genetic modification has been used to enable plants to produce a toxin that is produced in nature by the bacterium Bacillus thuringiensis. Common Bt crops include cotton, canola or rapeseed, maize or corn, papaya, potato, soybean, and summer squash. Bt crops are preferable to broadcast spraying of insecticides because the toxins are contained within the plant. Following widespread use of Bt crops in the US, levels of insecticide use dropped, and this was seen as evidence of the success of the GMO approach in protecting crop yields. However, the toxicity of insecticides being employed has increased since the advent of Bt crops, offsetting the decrease in quantity applied. Toxicity has become more specific, with impacts to birds and mammals (including humans) declining. However, impacts to insects and plants have increased.^[5]

As the amount of Bt toxins has increased in the US, resistance in the targeted insect pests has also increased. The EPA requires that farmers using Bt crops plant 20% of their crop area in non-Bt varieties, so that selective pressure for resistance is reduced, and resistance develops more slowly. However, monitoring and enforcement of this requirement has been limited. Farmers resist the requirement to plant so-called refuges, concerned about yield decreases in the refuge. However, evidence suggests that , at least for corn, yields for non-Bt corn are similar to yields for the genetically engineered strains [note that most of the corn grown in the US is  field corn destined for livestock feed, not sweet corn destined for the dinner table].^[6]

Herbicides and GM plants

Figure 2. A monarch butterfly on a milkweed host plant. Jim Hudgins/US Fish and Wildlife Service. Public domain.

Prior to development of genetically modified crops, and accompanying increases in herbicide use in agriculture, milkweeds were common roadside plants in the US Midwest, and monarch butterflies were common. Increasing herbicide use led to the loss of approximately 70% of monarch-hosting capacity of milkweeds. Monarch butterflies have been proposed for “threatened” status on the US endangered species list, in part due to loss of milkweed.

In the 21st century, most herbicide applications in agriculture are of broad-spectrum herbicides that kill most plant species, regardless of whether or not they are pests. Herbicides can leave agricultural fields as drift during aerial application, and through run-off and leaching via water movement. As a result of this movement, they kill plants in areas beyond fields. Herbicides can also have impacts on wildlife and humans (Fig 2). Herbicide impacts include developmental disorders, endocrine disruption including reproductive problems, and damage to DNA.

Readers may notice the apparent contradiction of applying broad-spectrum herbicides to crops. How does the crop escape harm? Most of the major crop varieties now planted in conventional agriculture have been genetically modified to be resistant to one or more of the most commonly used herbicides.  With these crops in the fields, farmers can apply herbicides freely, knowing that only non-crop plants will be killed.

In contrast to insecticides use, which decreased dramatically after the advent of Bt crops, herbicide use has increased dramatically since the advent of genetically modified, herbicide-resistant crops. Use of glyphosate, the most commonly applied herbicide, increased by 1500% between 1996 and 2016.^[7] However, during this time, no-till and reduced-till agriculture also increased dramatically, requiring greater use of herbicides to compensate for loss of the weed control aspect of tillage. In addition, herbicides are being used for new purposes, such as to kill crops that need to dry before being harvested, for faster drying, rather than waiting for the plants to die naturally.^[8] And research shows that increases in herbicide use in GM crops was lower than in non-GM crops^[9].

Resistance to herbicides since development of GM, herbicide-resistant crops has led to the reintroduction of older, more toxic herbicides that had been replaced by the glyphosate and other, newer herbicides. The fact that one of the reappearing herbicides, 2-4 D, was a component of the defoliant Agent Orange used during the Vietnam War that caused long-lasting environmental harm and harm to human health, added to the controversy surrounding modern herbicide use.

Use of genetically engineered crops and stacking of GM traits in crops

In 2024, the top 6 GM crops by area were soybean, maize, cotton, canola (rapeseed), alfalfa, and sugarbeet, and the top 6 countries in GM crop area were the US, Brazil, Argentina, Canada, and India.^[10] . Many countries have approved one or more GM crops for cultivation (Fig 3).

Figure 3. Countries that have approved use of genetically modified crops. This material is published by ISAAA (www.isaaa.org).

In the US, the leading country in acres of planted GM crops, most of the corn, soybean, and cotton that is planted is genetically modified (Fig 4). Several crops, particularly corn and cotton, carry stacked traits. That is, they carry more than one genetically modified trait. Traits may be stacked for a single purpose – most Bt crops carry genes for the expression of multiple toxins from Bacillus thuringiensis – or for multiple purposes – usually insecticide and herbicide traits. Countries generally approve GM crops by specific agricultural products, so early approvals may be of single-trait GM crops, and stacked-trait crop varieties would require additional approval.

Figure 4. Adoption of genetically modified crops in the US – 1996 – 2024. US Department of Agriculture, Economic Research Service. Public domain.

Pesticides are not the only solutions to pests

A recurring complaint about the increasing dependence of conventional agriculture on synthetic herbicides and other pesticides is that their use has led to a decrease in non-chemical approaches to pest management such as crop rotation and multi-pronged approaches such as integrated pest management for insect and invertebrate pests and crop rotation, cover cropping, intercropping, and integrated weed management for plant pests.

Crop rotation reduces insect and invertebrate pest loads and disease loads simply by depriving pest and disease populations of an ongoing food supply, so that crop-specific pests are regularly starved out and cannot build up in fields. In addition, crop rotation can reduce weed pests, increase resilience, and contribute to soil health and fertility. Crop rotation may require more equipment than a continuous single crop, to address the needs of planting and harvesting different crops. It requires more complicated understanding and monitoring of markets for multiple crops, and it prevents farmers from continually producing the most profitable crop, year after year.

The point of integrated pest management systems – for weeds, insects, or diseases – is that no single approach to pests provides a long-term solution. Whereas herbicides and pesticides are forms of artificial selection that lead to pests that are constantly harder to defeat, integrated pest management uses a variety of approaches, including chemical control; pests are less able to respond to several pressures and less likely to defeat multiple approaches.

Biological control organisms or biocontrols are another non-chemical means of combating primarily insect pests by using a natural predator of the pest.^[11] Natural predators can help to control crop pests, if they are not poisoned by pesticides and if habitat is provided for them. Mites, wasps, beetles, nematodes and other organisms can be purchased and released for one-time application or can occur naturally if strips of natural vegetation are left or planted with specific attractor plants, for habitat. Introduced biocontrols imported from other areas have also been used to control crop pests, but these are more problematical. Early efforts with imported species resulted in considerable harm to native species.

Gypsy moths (now called spongy moths) were brought to North America in the 1600s in shipments of silk moths ordered by entrepreneurs interested in starting a silk industry in the US. They defoliate deciduous trees and rapidly became a problem. To control the moths, a parasitoid fly was introduced repeatedly between 1906 and 1986, and became established. Unfortunately, it was a rather generalist insect and has caused the steep decline of native moths in the same group, including luna, cecropia and polyphemus moths – large and beautiful examples of the taxon (Fig 5). As a result of this and other examples of introduced biocontrol agents gone awry, the US and other countries now impose severe restrictions on them and require extensive testing of biocontrol agents to seek to reduce the chance of harm.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.