by Matthew Niederhuber
figures by Kaitlyn Choi
Summary: Bacillus thuringiensis (Bt) is a common bacteria that has played a very uncommon role in agriculture and the development of genetically modified foods. The natural insecticidal abilities of these bacteria have made them an important pest control tool for nearly a century. While their use as a natural biopesticide is widely accepted and approved for organic applications, the engineering of Bt genes into major crops has been more controversial.
Protecting our food from pests has been an ongoing battle ever since humans began cultivating food. The U.S. Food and Agriculture Organization estimates that some 20-40% of global crop totals are lost annually due to disease and pests, and the Environmental Protection Agency estimates that around 5 billion pounds of pesticides are used globally each year, costing more than 35 billion dollars [1-2].
However, with the advent of genetic engineering, new highly targeted strategies for pest control have become available in the form of transgenic plants that are designed to have insecticidal traits. Specifically, these bug-fighting plants were developed by moving some of the genes from the bacterium Bacillus thuringiensis (Bt) into corn and cotton. So called Bt crops are highly effective at combating pests such as European corn borer, rootworm, corn earworm, tobacco budworm, and bollworm [3-4].
Since the 1990s, corn and cotton with Bt genes have become the predominant varieties planted in North America . Yet despite their long-term usage and widespread presence in the U.S. food supply, there continues to be heated debate and rampant consumer misinformation about the safety of Bt and other genetically modified (GM) crops.
So what exactly do we know about the safety of Bt crops?
A bit of history
Bacillus thuringiensis (Bt) is a very common bacterium found in a variety of distinct environments, from soil, to dessert, to tundra. It was first isolated in 1901 by Japanese biologist Ishiwata Shigetane as he studied the causes of a disease afflicting silkworms. Then in 1911, the German scientist Ernst Berliner re-isolated Bt from flour moth caterpillars that had been collected from Thuringia, Germany (hence the species name). Soon Berliner determined that the Bt bacterium was specifically toxic to certain insect larva and not others. However, it wasn’t until 1928 that anyone attempted to harness Bt as a tool for pest control .
Figure 1. Bacillus thuringiensis has been used to control pests for almost a century, with its first agricultural application dating back to 1928 and first commercialization a decade later.
In this first instance, the bacteria were used to fend off European corn borer (Ostrinia nubilalis), which historically has been a common and very damaging corn pest. This initiated the development of the first commercial Bt based biopesticide, Sporine, which was introduced in 1938 in France . Since then, Bt-based biopesticides have been a significant pest control strategy, and are actually a common pest control strategy in organic agriculture. By the 1990s, tens of thousands of Bt strains had been isolated, with toxicity to a broad range of insect species .
Still, it was a game changer when the first GM corn engineered with genes from Bt became available in 1995. Since then, crops with Bt genes have come to dominate the majority of varieties planted in the U.S., representing 81% of total corn and 84% of total cotton acreage .
The mechanism of Bt toxicity
After Bt was first discovered, the mechanism of its toxicity still remained a mystery for many years. But in the 1950s, scientists discovered that the crystalline proteins that formed in Bt spores, previously observed by Berliner, were responsible for Bt toxicity . These crystal proteins, called Cry proteins, exhibit such a high degree of target specificity because of their mode of action within insect larvae.
Figure 2. The production of Bt toxins is coupled to the organism’s sporulation, and the multi-stage toxic mechanism by which Bt kills insects directly benefits the proliferation of the bacteria.
When the Cry protein reaches the gut, it is partially degraded, releasing a smaller and potentially toxic part of the protein . But this toxin will only be active if it finds the right matching protein receptor sticking off the cells lining the gut of a larval insect. This is the most important aspect of the Cry toxin mechanism. Much in the same way that a certain key will only open a certain lock, the Cry toxin can only exert its toxic effect on a particular cell receptor. Consequently, the toxin tends to only impact insects within a particular taxonomic order.
Once the toxin is bound, the process is fairly straightforward. The toxin recruits other Cry toxins to the same cell and together they form a hole in cell’s membrane that ultimately causes the cell to burst . The cumulative effect of this happening to many cells is the irreversible destruction to the midgut membrane, compromising the barrier between the body cavity and gut. Without this barrier, Bt spores and other native gut bacteria can infiltrate and grow within the nutrient-rich body of the insect [4-5].
What makes Bt such a great candidate for pesticide and GM applications is that while these Cry toxins are highly effective against insects, they have been shown to be safe for consumption by mammals. Tests by the EPA have demonstrated that Cry proteins, like any other benign dietary protein, are very unstable in the acidic stomach environment. Furthermore, an oral toxicity test, which involves giving mice exceptionally high doses of purified toxic Bt proteins, showed no significant health impacts. In their 2001 reassessment of several Bt Cry proteins, the EPA concluded from these findings that “there is reasonable certainty that no harm will result from aggregate exposure to the U.S. population, including infants and children, to the Cry1AB and Cry1F proteins and the genetic material necessary for their production.” Similar conclusions were drawn about the Cry1Ac protein of Bt cotton . Other mouse studies on have shown that even high doses of truncated Cry proteins, such that only the toxic region is conserved, have no deleterious effects . A paper in Annual Review of Entomology from 2002 also makes the strong point that, in addition to no demonstrated toxicity of Bt toxins, their use provides important health benefits to livestock and humans by preventing certain insect-caused crop diseases that produce toxic and carcinogenic compounds .
The environmental safety of Bt
Two major questions about the environmental impacts of Bt crops must be addressed. First, to what extent does the use of Bt crops reduce the application of more harmful pesticides? Second, do Cry proteins have significant off-target effects on other organisms?
Bt crops have enormous potential to reduce the use of both synthetic and organic pesticides (see this article). By relying on their Bt corn or cotton, farmers can decrease pest control-related costs and increase their yield. The USDA reports that “generally, Bt adoption is associated with lower insecticide use,” based on a collection of surveys from1998 to 2007 .
In 2001, Bt adopters were using approximately 36% less insecticide than non-adopters. The major caveat of this data, though, is that over the following decade the use of pesticides, on both Bt and non-Bt crops, has dramatically decreased overall. According to the USDA, between 1995 and 2010, the amount of pesticide used per acre of corn decreased by 99%, while insecticide use on cotton crops decreased by about 95% .What is interesting about these numbers, though, is that some studies have found evidence that the use of Bt corn and cotton is associated with a broad suppression of the overall population of damaging pests like corn borer, bollworm, and aphids [9-10].
One notable example of potential concerns for off-target effects occurred in the late 1990s, when it was widely published that high levels of pollen from Bt crops were toxic to the larvae of Monarch butterflies, commented on by David S. Pimentel and Peter H. Raven in 2000 . While this initially raised concern, it has since been shown that the conditions under which this toxicity was observed do not exist in real-world applications of Bt. Specifically, butterfly larvae are not likely to be exposed to levels of Bt pollen that would be toxic, and are less likely to directly ingest toxic Cry proteins, as they do not feed on corn or cotton . The comparison must be made between plants that have been engineered to produce Bt toxins and the application of Bt-based pesticides. The more targeted and localized action of GM Bt crops appears by all accounts to have less of an ecological impact than non-Bt methods.
Bacillus thuringiensis has a long agricultural history dating back nearly one hundred years. Even within the relatively recent age of genetic engineering, Bt has been one of the longest-running applications and successes of GM foods in the United States. The targeted mechanism of the Bt Cry toxin makes it an excellent pesticide since it has been shown to be safe for human consumption, reduces the use of insecticide application, improves crop yield, and reduces the amount of management crops require . The engineering of Bt insecticidal traits into crops like corn, cotton, and potatoes demonstrates the potential benefits and possibilities that advances in biotechnology are now providing. Perhaps most importantly, the story of Bt spotlights the thorough regulatory oversight that governs the development and application of these GM foods, ensuring their safety and sensible use.
Matthew Niederhuber is a Research Assistant in the Harvard Medical School Department of Systems Biology.
This article is part of the August 2015 Special Edition, Genetically Modified Organisms and Our Food.
1. Food and Agriculture Organization of the United Nations. Keeping plant pests and diseases at bay: experts focus on global measures. (2015). http://www.fao.org/news/story/en/item/280489/icode/
2. EPA. 2006-2007 Pesticide Market Estimates: Sales. (2013). http://www.epa.gov/opp00001/pestsales/07pestsales/sales2007.htm#2_1
3. USDA. Adoption of Genetically Engineered Crops in the U.S.: Recent Trends in GE Adoption. (2015). http://www.ers.usda.gov/data-products/adoption-of-genetically-engineered-crops-in-the-us/recent-trends-in-ge-adoption.aspx
4. University of California San Diego Aroian Lab. Bacillus Thuringiensis: Bt GM (genetically modified) crops. http://www.bt.ucsd.edu/bt_crop.html
5. Lambert B, Peferoen M. Insecticidal Promise of Bacillus thuringiensis. (Feb 1992). American Institute of Biological Sciences.
6. Roh JY, Choi JY, Li MS, Jin BR, Je YH. Bacillus thuringiensis as a specific, safe, and effective tool for insect pest control. (Apr 2007). Journal of Microbial Biotechnology.
7. EPA. Biopesticides Registration Action Document – Bacillus thuringiensis (Bt) Plant Incorporated Protectants. (2001). http://www.epa.gov/oppbppd1/biopesticides/pips/bt_brad2/1-overview.pdf
8. Lemaux, Peggy. Genetically Engineered Plants and Foods: A Scientist’s Analysis of the Issues (Part I). (2008) Annual Review Plant Biology.
9. USDA. Genetically Engineered Crops in the United States. (2014). http://www.ers.usda.gov/media/1282246/err162.pdf
10. Hutchinson et al. Areawide Suppression of European Corn Borer with Bt Maize Reaps Savings to Non-Bt Maize Growers. (Oct 2010). Science.
11. Pimentel D, Raven P. Bt corn pollen impacts on nontarget Lepidoptera: Assessment of effects in nature. (July 2000). PNAS.
12. Mendelson M, Kough J, Vaituzis Z, Matthews K. Are Bt crops safe? (Sept 2003). Nature Biotechnology.
13. Shelton AM, Zhao JZ, Roush RT. Economic, Ecological, Food Safety, and Social Consequences of the Deployment of Bt Transgenic Plants. (2002). Annual Review of Entomology.
14. Cover image: BT cotton crop. picture taken on 9.8.13 near nagarjuna sagar by Bhaskaranaidu. From Wikmedia commons: https://commons.wikimedia.org/wiki/File:Cotton_crop_2.JPG
27 thoughts on “Insecticidal Plants: The Tech and Safety of GM Bt Crops”
This is actually the most important aspect of the actual Cry toxin system. Much in the same way that the certain key will simply open a certain secure, the Cry contaminant can only exert the toxic effect on a specific cell receptor.
Well I must say this article really helped me to learn more about insecticidal plants and its prevention.
The government need to check more to see if these are affecting our healthy eating habits. Look at a corn or soy field with no weeds, but green crops. Are we taking in these pesticides when we eat corn or soy beans? Yes this new method is saving time and money for farmers.
did you even read the article? you aren’t “eating pesticides,” that’s completely different, and there have been thousands of studies already and the conclusion is GMOs have no more risk than non-GMO products
You do in a sense “eat” the pesticides. But the way they have presented it was rather misleading. They should’ve given an example to back the truth eg. ‘It’s almost like consuming chocolate, good for us, but really bad for dogs.” Idk something along those lines.
This is an interesting article….and there are other “articles” quoted..including news stories. Where are the studies? These are not studies.
Sources 5,6,8,10-13 are all peer-reviewed studies published in scientific journals. Most of the other sources are from governmental agencies.
It is my understanding from a review by Cornell of data that several studies were done on Bt but few 2 year cancer feeding studies and no diabetes induction or effect studies, is that correct? It is my understanding that the Cry toxin type protein effects carbonic anhydrase… is that correct? It is my understanding that the effect of Bt protein on the binding of selenium in corn based diets etc
has not been done, is that correct?
That’s not correct. Several 2-year high-dose studies have been done on rats with no increase in any cancers; and Bt has no effect on selenium binding one way or another.
At first glance this all looks fine & dandy but I confess to cynicism which less forgiving others may term paranoia. The tentacles of giant corporate agrochem & its filthy lucre extend so far into our scientific & educational establishments that it is nigh impossible sometimes to separate true scientific endeavour from clever disinformation emanating from special interest groups. It is clear that agrochem’s sustained encouragement of monoculture necessitates expensive agrochem solutions to prevent associated crop yield problems that historically were not so prevalent during eras of crop rotation & diversity of plants, creatures & birds. Increasingly, the damage to human health & the environment seems to be acceptable as “collateral damage” by giant corporations in their drive for ever higher profits for the few.
Oh look someone that thinks they’re smart
wow this isreally suprising
people who criticize the doubt are not “less forgiving,” literally thousands of studies have proven no increased risk in GM crops, this is logic and reasoning, the concerns you have is baseless skepticism, stop attacking scientific advancement just because you are unfamiliar with it
GMO are real because my dog die .
misspelled word died*.
It is unfortunate that people want to know the long-term effects of Bt protein contained in GM foods on human and animal health, for a moment forgetting that the alternative to Bt GM crops is loads of chemical insecticides in the food and environmental chain. Are toxic chemical insecticides acceptable? GM crops have considerably reduced the environmental load of insecticides and fungal contamination in caterpillar-damaged Bt corn cobs.
Mohan Komarlingam S
Our latest understanding of the human body renders it so complex that it is impossible to determine what is “safe” from short term, myopic studies. With a significant portion of our overall health dependent on the BACTERIA that live in our gut it should be obvious that anything has the potential for long term problems if it is not compatible with a very diverse set of life forms. One huge issue is intestinal permeability. Maybe BT toxins don’t impact mammals, but if they impact bacteria we’re still at risk. Glyphosate has been shown to be toxic to some bacteria, what’s worse it tends to be worst for the beneficial bacteria……. OOOPS.
We need a lot less OOOPS moments if we’re going to survive this process.
Read “The mechanism of BT toxicity”. There, the article clears up something: https://objection.lol/objection/3678055 this will clear your doubt
I was going through the comments because I’m writing an essay on this topic and did you seriously put an objection.lol thing into this
bro I’m dying I didn’t read the link when I clicked on it
It is also noteworthy that recent GM technology only make use of specific genes that code for the Cry protein and not the whole Bt bacteria, thereby greatly reducing incidences of off-target effects. The effectiveness and benefits of Bt in agriculture cannot be overemphasized given that it cannot be activated in the human gut and is easily degraded in the environment. The challenge is now in educating the general public to fully understand how Bt, and GMOs in general, works.
THE WIDE PRACTICE OF OMITTING LONG TERM HEALTH TESTING IS NOW WELL UNDERWAY. THE LATEST FIASCO BUILDER IS TRYING TO APPROVE GMO TYPE MATERIALS WITH A 90 DAYS FEEDING STUDY. THIS WAS FOSTERED INITIALLY BY DAVE KESSLER ON RBGH ETC AND HAS NOW SPREAD TO OTHER PROJECTS.
HOWEVER SCIENCE MAG WRITERS HAVE BEGUN TO ADDRESS AN IMPORTANT CONCERN, THE SHAPE OF THE PROTEIN AND NOT NECESSARILY A SPECIFIC SEQUENCE MAY DETERMINE BIO ACTIVITY. FOLLOW IT PLEASE. WVM
Pest Control is basically removal of species like insects or pests from a specific area which can be residential, industrial, commercial or housing. You can use pest control service once or twice a year. Usually people go for this service during rainy or snowy season. However price of this service depends on the area that is affected by pests.
wow this is really suprising
pesticide go brrrrrr
insect be dead
misspelled word died*.
A 2018* study by Santos-Vigil et al observed an immune response in mice to Bt toxin, so I don’t know that I’d agree with the notion that it’s harmless to mammals. Given the potential link between chronic inflammation and cancer it seems that anything introduced into the food supply at this scale with the potential to stimulate an immune response in the gut should stay on people’s radar. The fact that the commercialization of Bt bioengineered foods in the mid-90s coincides with an international increase in the incidence of colon cancers in younger adults may not be significant, but I think there’s value in using long-term data to back up theoretical notions of what is and isn’t possible.
*after publication of this article