Multi-toxin biotech crops not silver bullets, scientists warn

March 29, 2013 by Daniel Stolte
A major agricultural pest, the moth Helicoverpa zea and its caterpillar go by many common names, depending on the crop they feed on: Shown here is a "corn earworm." Credit: José Roberto Peruca

( —The popular new strategy of planting genetically engineered crops that make two or more toxins to fend off insect pests rests on assumptions that don't always apply, UA researchers have discovered. Their study helps explain why one major pest is evolving resistance much faster than predicted and offers ideas for more sustainable pest control.

A strategy widely used to prevent pests from quickly adapting to crop-protecting toxins may fail in some cases unless better preventive actions are taken, suggests new research by University of Arizona entomologists published in the Proceedings of the National Academy of Sciences.

Corn and cotton have been genetically modified to produce pest-killing proteins from the , or Bt for short. Compared with typical insecticide sprays, the Bt toxins produced by genetically engineered crops are much safer for people and the environment, explained Yves Carrière, a professor of entomology in the UA College of Agriculture and Life Sciences who led the study.

Although have helped to reduce insecticide sprays, boost and increase farmer profits, their benefits will be short-lived if pests adapt rapidly, said Bruce Tabashnik, a co-author of the study and head of the UA department of entomology. "Our goal is to understand how insects evolve resistance so we can develop and implement more sustainable, environmentally friendly pest management," he said.

Bt crops were first grown widely in 1996, and several pests have already become resistant to plants that produce a single Bt toxin. To thwart further evolution of pest resistance to Bt crops, farmers have recently shifted to the "pyramid" strategy: each plant produces two or more toxins that kill the same pest. As reported in the study, the pyramid strategy has been adopted extensively, with two-toxin completely replacing one-toxin Bt cotton since 2011 in the U.S.

Most scientists agree that two-toxin plants will be more durable than one-toxin plants. The extent of the advantage of the pyramid strategy, however, rests on assumptions that are not always met, the study reports. Using lab experiments, computer simulations and analysis of published experimental data, the new results help explain why one major pest has started to become resistant faster than anticipated.

"The pyramid strategy has been touted mostly on the basis of simulation models," said Carrière. "We tested the underlying assumptions of the models in lab experiments with a major pest of corn and cotton. The results provide empirical data that can help to improve the models and make the crops more durable."

One critical assumption of the pyramid strategy is that the crops provide redundant killing, Carrière explained. "Redundant killing can be achieved by plants producing two toxins that act in different ways to kill the same pest," he said, "so, if an individual pest has resistance to one toxin, the other toxin will kill it."

The same pest is called cotton bollworm when plaguing cotton plants. Credit: Thierry Brévault

In the real world, things are a bit more complicated, Carrière's team found out. Thierry Brévault, a visiting scientist from France, led the lab experiments at the UA. His home institution, the Center for Agricultural Research for Development, or CIRAD, is keenly interested in factors that could affect pest resistance to Bt crops in Africa.

"We obviously can't release resistant insects into the field, so we breed them in the lab and bring in the crop plants to do feeding experiments," Carrière said. For their experiments, the group collected cotton bollworm – also known as corn earworm or Helicoverpa zea –, a species of moth that is a major agricultural pest, and selected it for resistance against one of the Bt toxins, Cry1Ac.

As expected, the resistant caterpillars survived after munching on cotton plants producing only that toxin. The surprise came when Carrière's team put them on pyramided Bt cotton containing Cry2Ab in addition to Cry1Ac.

If the assumption of redundant killing is correct, caterpillars resistant to the first toxin should survive on one-toxin plants, but not on two-toxin plants, because the second toxin should kill them, Carrière explained.

"But on the two-toxin plants, the caterpillars selected for resistance to one toxin survived significantly better than caterpillars from a susceptible strain."

These findings show that the crucial assumption of redundant killing does not apply in this case and may also explain the reports indicating some field populations of cotton bollworm rapidly evolved resistance to both toxins.

Moreover, the team's analysis of published data from eight species of pests reveals that some degree of cross-resistance between Cry1 and Cry2 toxins occurred in 19 of 21 experiments. Contradicting the concept of redundant killing, cross-resistance means that selection with one toxin increases resistance to the other toxin.

According to the study's authors, even low levels of cross-resistance can reduce redundant killing and undermine the pyramid strategy. Carrière explained that this is especially problematic with cotton bollworm and some other pests that are not highly susceptible to Bt toxins to begin with.

The team found violations of other assumptions required for optimal success of the pyramid strategy. In particular, inheritance of resistance to plants producing only Cry1Ac was dominant, which is expected to reduce the ability of refuges to delay resistance.

Refuges consist of standard plants that do not make Bt toxins and thus allow survival of susceptible pests. Under ideal conditions, inheritance of resistance is not dominant and the susceptible pests emerging from refuges greatly outnumber the resistant pests. If so, the matings between two resistant pests needed to produce resistant offspring are unlikely. But if inheritance of resistance is dominant, as seen with cotton bollworm, matings between a resistant moth and a susceptible moth can produce resistant offspring, which hastens resistance.

According to Tabashnik, overly optimistic assumptions have led the EPA to greatly reduce requirements for planting refuges to slow evolution of to two-toxin Bt crops.

The new results should come as a wakeup call to consider larger refuges to push resistance further into the future, Carrière pointed out. "Our simulations tell us that with 10 percent of acreage set aside for refuges, evolves quite fast, but if you put 30 or 40 percent aside, you can substantially delay it."

"Our main message is to be more cautious, especially with a pest like the cotton bollworm," Carrière said. "We need more empirical data to refine our simulation models, optimize our strategies and really know how much refuge area is required. Meanwhile, let's not assume that the pyramid strategy is a silver bullet."

Explore further: Bigger refuges needed to delay pest resistance to biotech corn

More information: … /1216719110.abstract

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1.9 / 5 (9) Mar 29, 2013
The only creatures GMO's are killing are animals who eat them. Lab animals fed these untested foods develop multiple tumors. Bon appetit.
2 / 5 (8) Mar 29, 2013
Moreover, the team's analysis of published data from eight species of pests reveals that some degree of cross-resistance between Cry1 and Cry2 toxins occurred in 19 of 21 experiments. Contradicting the concept of redundant killing, cross-resistance means that selection with one toxin increases resistance to the other toxin.

Maybe the problem is the two toxins aren't different enough.

Also I think the refuges theory is BS. Part of the theory behind the redundant killing (which is also applied to antibiotics,) is that you are hoping that "either way, we're going to kill the pest before any resistance appears". Well, if you have plots of crops which don't even make the toxin, then if the pest eats a small amount of the toxin, but doesn't die, and then moves away and eats some of the non-toxin plant, then it's like diluting the toxin.

Also, who do I blame for neuropathy? Is BT corn and such causing my nerve pain with all these poisons in the food supply's genetic molecular code?
2.2 / 5 (10) Mar 29, 2013
We'll have whole generations of people in a few decades who will have been eating BT toxins for decades, or even their entire lives, and the accumulation of the damage this is PROBABLY doing to humans and our pets and livestock will be seen.

It's just like LEADED gasoline.

You absolutely KNOW this is wrong, because of both common sense and INDEPENDENT LAB TESTS, but you do it anyway.

Who do we get to sue when the government finally admits this stuff is hurting human beings? Monsanto? Dow? They won't have enough money to pay for the medical damages they will have caused.
1.9 / 5 (9) Mar 29, 2013
There will be no legal recourse against Monsanto and Dow. Chemical companies are virtually immune to prosecution- General Electric's dumping of PCB's in the Hudson, Dow's Agent Orange, etc. Avoid these foods like your life depended on it. Genuine organic products are the only safe haven. Distrust anything else.
Regarding neuropathy, I recommend Benfotiamine which can be gotten OTC at any decent vitamin store or online.
1.6 / 5 (7) Mar 29, 2013
Interesting. Why the hell haven't my incompetent ER and clinic doctors mentioned this to me?!
1.5 / 5 (8) Mar 29, 2013
Interesting. Why the hell haven't my incompetent ER and clinic doctors mentioned this to me?!

Doctor's are in the business of managing illness rather than ending it.
2.3 / 5 (6) Mar 29, 2013
Yay! Let's muck with DNA of our food so that they will naturally express poisons!

Then get the our pocket congressmen put a rider in an agricultural bill making us immune to prosecution!

This is like a comic book plot by an evil super-villain.
If you're not growing your own garden and otherwise trying to eat organic and non-gmo, it is definitely time to start.
1.6 / 5 (7) Mar 29, 2013
Where does BT toxin accumulate in the human body?

Is it the liver? Nerves? heart? Stomache? maybe the adipose tissue? Maybe it's at the sub-cellular level, inside the lysosomes.

Who tests all this stuff long-term to know what it's doing to us? Nobody, except the people promoting the toxin...

I wouldn't mind if they engineered the corn to grow bigger ears, or more kernels per ear, or more ears per plant...

Adding poisons to the crops DNA is ridiculous. Adding two or three poisons to the DNA per crop is insane.

This is worse than that "eel growth hormone in the salmon" thing. This is botulism in our food at a genetic level, and they want to add even more of the toxin, and more types of toxin.

Are they literally trying to kill people or reduce average life expectancy or something?

What is being done with all the uncontaminated strains of our crops? Do the seed banks keep the "original" strains of everything from before Monsanto and Dow screwed them up?!
1.3 / 5 (6) Mar 29, 2013
Why not make some sort of miniature robotic insect terminator, which manually hunts down and mechanically destroys the pest insects?

Nature calls them "Spiders".

Maybe we can make a micro-bot or nano-bot to do the task.

Maybe some sort of area of effect EM bug zapper which doesn't harm the humans or other life.
not rated yet Mar 29, 2013
The only creatures GMO's are killing are animals who eat them. Lab animals fed these untested foods develop multiple tumors. Bon appetit.

You mean the Seralini study that has been thoroughly debunked? Check the related articles on the right side.

not rated yet Mar 29, 2013
1 / 5 (4) Mar 30, 2013
You will always lose when you challenge me, dummy-

not rated yet Mar 30, 2013
You will always lose when you challenge me, dummy-


That's fine and dandy, but it's completely irrelevant. That has nothing to do with the actual food itself.
1.8 / 5 (6) Mar 31, 2013
GMO crop fields should be burned! Monsanto as well...Neither should have ever happened!
1 / 5 (3) Mar 31, 2013
"Mystery of honey bees" at the birdseed: bees apparently don't appreciate the GMO corn very much. Many animals aren't so silly and they can recognize GMO corn and they avoid it in food. Why?
Because in reality, the contemporary genetic manipulations aren't well defined chemical reactions, which you may expect from school lab experiments. They usually produce metabolic mess and mixture of proteins, many of them are unknown in the nature. Some of them are causing allergies, but many others have no apparent effect. The animals may recognize some of them by their smell, but people aren't so sensitive about it.
1 / 5 (3) Mar 31, 2013
Not Just the Bees: Pesticide May Harm Birds, Too. Many links to another peer-reviewed studies are provided there..
1 / 5 (4) Mar 31, 2013
I saw some cotton seed compost for sale the other day at the local hardware store.

The seed came from west Texas - no doubt from GM cotton fields where the (cotton) stubble cannot be grazed by cattle because doing so will kill them.
3.7 / 5 (3) Apr 01, 2013
One possibility starry-eyed GMO enthusiasts have missed is that the pests carry dormant genes from the past which give them a resistance to the toxins.
Over hundreds of millennia the pests have been eating plants with bacteria, etc. on them. It is not unlikely that along the way the pests of yore ingested a lot of harmful pathogens, and some survived to reproduce.
In time, the genes can have built up quite an arsenal of coding that kicks in to protect future offspring.

This will be viewed as a scientific heresy, of course. :-)
1 / 5 (3) Apr 01, 2013
IMO it's one of purposes of genetic dark matter, i.e. the "Junk DNA". But in applications of GMO it actually doesn't matter, in which way the pests will gain their resistance. My problem rather is, the mixing of various toxins into single plant is the effective way, how to gain the resistance to all components at the single moment.
not rated yet Apr 01, 2013
The plants are becoming poisonous to the pollinators...

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