by Jennifer Hsaio
figures by Krissy Lyon
Summary: Pesticides are ubiquitous. Because they are used in agriculture and food production, pesticides are present at low levels in many of our diets. Less obvious is the fact that many people use pesticides around their homes, and even on their skin (i.e. in the form of insect repellents). According to the NIH, the health effects of pesticides are still not well understood . Potential effects include cancer and damage to the nervous, endocrine, and reproductive systems. Genetically modified organisms (GMOs) are often engineered to be more resistant to pesticides or produce pesticides themselves. How are GMOs changing the landscape of pesticide usage in our crop fields, and ultimately, the pesticide dosage in our dinners?
Pesticides are substances used to repel, kill, or control animals (insecticides) or plants (herbicides) that are considered to be pests. There are different types of pesticides, which include synthetic pesticides and biopesticides (Figure 1). Pesticides are used extensively in agriculture and they are also used at a lower scale in our homes and on ourselves. According to the National Institutes of Health (NIH), the health effects of pesticides are not well understood, but their use has been associated with conditions such as cancer, diabetes, and neurological effects. GMOs have been changing the way that pesticides are used in agriculture. Herbicide-tolerant genetically modified (GM) crops have led to an increase in herbicide usage while insecticide-producing GM crops have led to a decrease in insecticides. To understand whether GMOs make us better or worse off in our interaction with pesticides, let’s explore the relationship between pesticides and GMOs in some detail.
Figure 1. Pesticides are grouped under several classes.
The Upside of Pesticides
According to the US Environmental Protection Agency (EPA), pesticides are often the only effective way to control disease organisms . As a result, their use has become deeply entrenched in our lives. We as consumers often reap the benefits of pesticide use with lower costs and a wider selection of food and clothing. As a way of conserving food supply and lower food costs, they also help to combat hunger and related problems in various parts of the world (see this article). Pesticides can protect our homes and buildings from structural damage by creatures such as termites. They can protect our health, too – disease outbreaks are prevented by controlling insect and rodent populations. Pesticides can even disinfect our drinking water and medical instruments .
The Downside of Synthetic Pesticides
Despite their agricultural, economic, and safety , pesticides can also have negative impacts on our health. Many conventional pesticides are synthetic materials that kill or inactivate the pest directly. These chemical pesticides include compounds such as organophosphates, carbamates, pyrethroids, and sulfonylureas. Short-term exposure to a large amount of certain pesticides can result in poisoning. Exposure to large amounts of pesticides is usually more likely for people such as farmers who may frequently touch and/or breathe in pesticides. The effects of long-term exposure to small amounts of these pesticides are unclear, but studies have linked them to a variety of chronic health conditions such as diabetes, cancer, and neurological defects (for more detailed information, the EPA has an extensive table of health effects of different pesticides). Specifically, carbamates and organophosphates are known to affect the nervous system by disrupting a neurotransmitter called acetylcholine . Studies have shown preliminary evidence that chronic, low-dose exposure to pesticides increases the risk of cognitive impairments and diseases such as Alzheimer’s and Parkinson’s later in life . A study of 50 pesticides and more than 30,000 licensed pesticide applicators linked exposure of seven pesticides that contain chlorinated compounds (including two herbicides, two organophosphate insecticides, and two organochlorines) to increased risk of diabetes . Exposure to pesticides has also been associated with increased infertility in women and developmental problems in children .
Natural Pesticides and GMOs
Biopesticides are derived from natural materials such as plants, animals, bacteria, and minerals. There are three main categories of biopesticides: 1) microbial pesticides, which are microorganisms (e.g. bacteria, fungi, viruses, or protozoa) that have relatively specific pest targets; 2) biochemical pesticides, which are naturally occurring substances that control pests using nontoxic mechanisms (e.g. mimics of insect sex hormones that interfere with their mating); and 3) plant-incorporated-pesticides (PIPs), which are pesticides that the plants themselves produce after genetic material has been added to them. An example of a PIP is Bacillus thuringeinsis (Bt) crops.
Bacillus thuringiensis (Bt) is a naturally occurring bacterium in the soil that produces proteins specifically active against certain insects. Some crops such as corn, cotton, and soybeans have been genetically engineered to express the Bt genes that act as insecticides (see this article). Bt corn is designed to control corn pests such as the European corn borer, corn earworm, and southwestern corn borer, and Bt cotton effectively controls cotton pests such as the tobacco budworm, cotton bollworm, and pink bollworm . The use of Bt crops has led to a reduction in conventional synthetic insecticide use  (Figure 2). The EPA has analyzed Bt crops and found that they do not pose any significant risks to human health . Specifically, the EPA has done studies showing that the Bt protein in GM plants behaves as would be expected for a dietary protein, is not structurally related to a known food allergen or protein toxin, and does not show toxicity when administered orally at high doses .
Figure 2. Timeline of the introduction of Bt corn into cornfields and the concurrent reduction of insecticide usage in these fields. The two quantities are strongly anti-correlated, suggesting that this Bt crop has made synthetic insecticides unnecessary.
Roundup tolerance and the development of herbicide resistance
GM herbicide-tolerant crops enable farmers to use certain herbicides that will kill weeds without harming their crop. The prime example of GM herbicide-resistant crops is the suite of “Roundup-resistant” GMOs, which are designed to tolerate the herbicide glyphosate, an ingredient in the weed killer Roundup (see this article). Glyphosate is the most widely used herbicide in the world by volume . It is employed extensively in agriculture and can be found in garden products in many countries.
The use of these herbicide-tolerant crops has allowed farmers to switch from traditional herbicides to glyphosate (Figure 3). The good news is that glyphosphate is thought to be less toxic and less persistent than traditional herbicides, which means that it carries fewer health risks for humans .
However, the World Health Organization recently announced that glyphosate is a probable carcinogen, so we still need to be cautious  (for more information, the EPA also has a list of other pesticides and their carcinogen status). Although studies have shown conflicting conclusions about the link between glyphosate and cancer in humans, glyphosate has been linked to cancer in rats and mice and experiments in human cells have shown that exposure to glyphosate can cause DNA damage .
Plants may develop resistance to herbicides over time . Weeds that have developed resistance to herbicides such as glyphosate may require higher amounts of glyphosate and perhaps other herbicides to keep them in check, and this means that herbicide-tolerant crops will be exposed to higher levels of herbicides as well.
Figure 3. Timeline of glyphosate-based herbicide use on corn, cotton, and soybean in response to the growing popularity of their GMO versions. Since the introduction of Roundup-tolerant crops, herbicides have experienced a significant increase in application. (Adapted from )
Ways to reduce pesticide exposure
The lack of conclusive evidence ruling out negative effects of chronic exposure to low doses of pesticides may mean that we should still work to minimize exposure to pesticides when possible. It is especially important to limit the pesticide exposure of more susceptible groups of people such as pregnant women and growing children. Pesticide use should be regulated in a way that will limit development of herbicide and insecticide resistance in their target organisms. This can help prevent an increase in the amount and toxicity of pesticides used. Making sure that farmers are aware of the best ways to limit unwanted pesticide resistance will also be essential.
Fortunately, pesticide use is studied, monitored, and regulated by organizations such as the EPA and the World Health Organization. The EPA regulates pesticides in food by evaluating every new pesticide for safety and every new use before it is registered . The EPA evaluates hundreds of scientific studies on pesticides to ensure their safety to humans. After a pesticide is registered, the EPA reevaluates its safety every 15 years . Before the EPA allows a pesticide to be used on crops, it sets a maximum legal residue limit (called a “tolerance”) for each treated food, and if that residue limit is exceeded, government action will be taken .
Furthermore, it is essential to strike a balance in pesticide usage: we want to minimize the consequences induced by the toxicity of synthetic pesticides, while maximizing their beneficial effects for crops. GMOs have played a mixed role in this development, helping reduce pesticide use in some cases (e.g. with Bt crops) while increase pesticide use in other cases (e.g. with herbicide-resistant weeds). Thus, their use has not resolved our pesticide conundrum. Encouragingly, research is ongoing to find synthetic pesticides that have high specificity for their target pests. Alternative, non-chemical forms of pest control that are less toxic to humans and other organisms are also being studied . Chances are good that these efforts will become part of the permanent solution.
Jennifer J. Hsiao is a Ph.D. candidate in the Biological and Biomedical Sciences Program at Harvard University
This article is part of the August 2015 Special Edition, Genetically Modified Organisms and Our Food.
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