GMOs Demystified- Part 3: Environmental Impact

Rural System, above all else, is an approach to land management that emphasizes reasonableness in making management decisions. “How much profit can be made?” is balanced with “How long will profit be made?” “What’s good for humans?” is balanced with “What is good for the ecosystem?” All management decisions are made according to the most recent scientific knowledge on the topic.

The debate over genetically modified food has been extensive and multi-faceted. Like many sensationalized issues, it can be difficult to sort out truth from bias. The series of posts before you is a guide to GMOs based on available science rather than uninformed opinion. Our third question: how do GMOs currently affect the environment?

Bt crops reduce the need for sprayed pesticides. This helps prevent killing non-target insects such as useful pollinators.

Bt crops reduce the need for sprayed pesticides. This helps prevent killing non-target insects such as useful pollinators.

As noted in “Part 2: Health Effects,” GM is not inherently harmful and so is best evaluated on a case-by-case basis depending on how humans are applying the technology. The impact of GM crops on the environment is best measured against the environmental impact of conventional crops. Is a particular GM crop better or worse than the alternative? Let’s take a look at the main questions and issues.

  1. Decreases in Pesticide Application: Many GM crops are modified to incorporate some form of pest resistance, usually the delta endotoxins of a bacterium, Bacillus thuringiensis (Bt). This Bt pest resistance has resulted in decreased use of sprayed insecticides, which have much more generalized effects on target and non-target species alike. GM pest resistance targets only the insects feeding on the tissues of the plant, thus allowing for the pest’s natural predators to proliferate in the surrounding environment.

    “If the planting of GM pest-resistant crop varieties eliminates the need for broad-spectrum insecticidal control of primary pests, naturally occurring control agents are more likely to suppress secondary pest populations, maintaining a diversity and abundance of prey for birds, rodents, and amphibians.” –Nature Publishing Group

    If the benefits from Bt crops are to be felt, farmers need to take precautions to prevent the development of resistance to Bt toxins. Fortunately, the biotech industry is trending toward stacked Bt traits in crops, providing multiple forms of resistance in the same crop. There are about 600 strains of Bt bacteria, each producing different toxins. Diverse forms of resistance combined with the use of non-Bt refuges may be enough to preserve the efficiency of Bt crops and also of Bt sprays, which are preferred for use in Organic certified farming.

    “Commercial Bt insecticides are classified as Generally Regarded as Safe (GRAS) by the EPA, and are approved for most organic certification programs.” –Colorado State University Extension

    Bt resistance is a recessive trait in pests, so the refuge patches of intentionally vulnerable, non-Bt plants may be enough to encourage the dominant phenotype to persist and so prevent a predominance of Bt resistant pests.

  2. Increases in Herbicide Use & Herbicide Resistant Weeds: Unfortunately, decreases in insecticide use don’t offset increases in herbicide application in the U.S. The development of herbicide resistant crops, particularly soybeans, has lead to dramatic increases in the use of the general herbicide glyphosate, known more commonly as Roundup. This shouldn’t come as a surprise; Monsanto engineered crops to be resistant to its own brand of herbicide, Roundup, and the more farmers use the more the company stands to gain. It is estimated that pesticide use in the U.S. has increased 404 million pounds, or about 7% between 1996 and 2011. (The general term “pesticide” encompasses insecticides, herbicides, and fungicides.) Decreases in insecticide application didn’t go nearly far enough in offsetting herbicide use since the introduction of herbicide resistant crops. And then there is the issue of herbicide tolerant weeds.

    “Several studies have demonstrated that tolerance to a particular herbicide is often more likely to develop by evolution from within the weed gene pool rather than by gene flow from herbicide-tolerant crops (40,41).” –Nature Publishing Group

    Some have been concerned that herbicide resistant crops would hybridize with genetically similar weeds or wild plants. The likelihood of this depends on four factors: maximum distance of pollen travel, synchrony of flowering between GM plants and recipient plants, sexual compatibility between the two, and the ecology of the recipient species. Fortunately transference of GM herbicide resistance to surrounding weeds has been negligible, but widespread, large-dose, repeated applications of one herbicide has been more than enough to accelerate the evolution of resistance. Glyphosate resistant weeds have literally sprouted up everywhere. I found one case of the hybridization of herbicide resistant rice with wild rice.

  3. Reduced need for tillage: In addition to loosening soil to prepare for planting, tillage is a weed management strategy where farmers plow deep into the ground, burying the weed seedbank. Though tillage has many benefits, excessive tillage has been found to increase soil erosion, increase carbon loss, and detrimentally affect soil structure. Tillage also accounts for over 25% of agricultural production costs. Herbicide resistant crops have, however, reduced the need for tillage.
  4. Horizontal Gene Transfer & Free DNA: Some have suggested that the DNA of GM crops will transform bacteria in the body and so, for example, equip bacteria in the human body to continue to produce a potential allergen. However, it is highly unlikely that transgenic genes would be taken up by bacteria in the gut or in the environment. The digestive tract breaks free DNA down too fast for it to transform resident bacteria. Residual DNA has not been found in the tissues or eggs of animals fed GM crops.

    “To gain perspective when assessing the potential environmental impact of DNA from GM crops, it is important to consider the amount of DNA in the environment from non-GM origins. Pollen, leaves, and fruit alone result in thousands of tons of DNA per year being released into the environment (62).” –Nature Publishing Group

    Several studies have been done to assess the lifespan of GM DNA in the soil. This depends on the type of soil and the concentration of nucleases, which break down DNA. Results have ranged from 5 days for target GM tobacco sequences to 2 years for sugar beets, but none of the studies found an indication that the DNA was being taken up by native soil bacteria.

  5. Toxins in Soil: That is not to say that the Bt toxins do not persist in the environment. Bt crops release toxins into the soil throughout their lifetime and as they decay. Bacillus thuringiensis is naturally found in the soil, so the question is whether Bt crops add enough to harm soil biota. Saxena & Stotzky studied the soil biota in the rhizosphere of Bt and non-Bt crops and found that there was no difference in the mortality, weight, and quantity of soil organisms (such as nematodes, protazoa, bacteria, and fungi). Bt toxin can affect non-target species such as the monarch butterfly, but exposure levels are not enough to damage monarch populations. Bt toxins are not harmful to humans and must be ingested by insects in order to have effect.

These issues are really only the beginning. I know I still have questions, which I may address in future posts. For example, how toxic is glyphosate as compared with other herbicides? Is it a lesser of many evils? How about the total herbicide that is Roundup? This Scientific American article explains that an “inert” compound in Roundup suffocates human cells, so the toxicity of the total herbicide is much more complicated than just the toxicity of glyphosate.

Yet I am also encouraged by the amount of attention to environmental health the GMO controversy has generated. Perhaps human fear of the unknown, of what we don’t understand, can have its benefits.

“As a consequence of the discussions about the commercialization of GM crops in some parts of the world, questions are being asked that were rarely considered for conventionally bred crops. This is true even though the products of many methods of conventional plant breeding (e.g., induced mutation and hybridization with wild relatives) carry a high level of unpredictability. The questions currently being asked of GM crops frequently demand a degree of forensic precision that would be impossible to apply to the products of more traditional breeding.” –Nature Publishing Group

But the last thing we need in this controversy is more confusion. There is plenty of information on the GMO controversy, but it is much easier to navigate with a guide. Our next post will address both sides of an argument: should companies be allowed to patent genetic sequences?

Read the whole series, or pick a topic that interests you:

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About Laurel Sindewald

Laurel is an alumna of Warren Wilson College with a BS in Conservation Biology and a BA in Philosophy. She is a writer for Rural System, Inc.


  1. Risa Pesapane says:

    Wonderfully informative article Laurel and especially important for people to focus on your statement “GM is not inherently harmful and so is best evaluated on a case-by-case basis depending on how humans are applying the technology.” I wanted to add another bit here about the circulation of DNA in the environment….you could consider it “biological pollution” in some senses but pollution denotes something negative and I’m not suggesting that result is typically negative. In GMO’s Demystified Part I you brought up plasmids and how DNA is “passed on.” I thought I would add that there is the phenomenon of horizontal gene transfer…..the realization that not all DNA is “inherited” but in fact in bacteria and viruses it is simply “exchanged” through contact in some cases, often through plasmids. The reason it is important to use caution, or demonstrate increased mindfulness with genetic modification, is that whatever DNA we put out in the environment in one bacterium for one purpose, could then come in contact with other bacterium and viruses (that perhaps it would or would not have naturally) and those genes can be exchanged. This area of study is fairly new to science…..but be assured that no action goes without effect somewhere in the ecosystem. This topic has been most commonly studied in the transmission of resistance genes between bacteria in the gut…’s the reason your current bacteria (sometimes even healthy bacteria) become resistant to antibiotics that you took to treat a different infection. These resistance genes are now exist throughout our natural environment, soil, water, animals, etc! This example is alarming and it’s important to recognize that it’s a much farther stretch to apply the same logic to tomatoes and corn, BUT it is a great reminder that we are constantly learning about the ripple effects in nature, constantly surprised, and thus must be vigilantly conscious of possible unintended consequences.

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