GM Free Cymru

Rootworms develop resistance to multiple Bt crops

Date Added to website 21st March 2014

Comment from GM-Free Cymru: This is an interesting article from "Nature" which shows that certain corn pests can develop resistance to two of the three types of Bt toxin built into GMO corn crops-- with the implication that this "cross resistance" will render multiple types of GMO corn with stacked traits vulnerable to insect damage and yield loss. There have been warnings for many years that this might happen, and now it is confirmed experimentally. This is an inevitable consequence of GMO industrial monocultures. So what will Monsanto and the other GM multinationals do about it? The author of this piece suggests that farmers might think about this rather old-fashioned thing called "crop rotation" as one means of battling pests, since pests in nature are least effective if they have only one year in four or five during which their favoured food crop is present in the vicinity. Old time farmers know all about this, having learnt it over many generations; and organic farmers of course practice rotations today as part of a highly effective farm management system. But there are worrying signs in the article that the the biotech corporations, having created a technical problem with Bt crops, will be tempted to solve it in the only way that is familiar to them -- ie with yet another technofix. So wait for it -- our money is on the development of Bt crops that are twice or three times as toxic as they were before. And they are already so toxic that human beings can no longer eat them................... corn on the cob for supper, anybody?

Nature | News

Pests worm their way into genetically modified maize Broadening of rootworm resistance to toxins highlights the importance of crop rotation.

Brian Owens 17 March 2014

Even with biotech crops, farmers still need to make use of age-old practices such as crop rotation to fight insect pests. That's the lesson to be drawn from the latest discovery of resistance to the pest-fighting toxins added to maize — also known as corn.

According to a team led by Aaron Gassmann, an entomologist at Iowa State University in Ames, in some Iowa fields a type of beetle called the western corn rootworm (Diabrotica virgifera virgifera LeConte) has developed resistance to two of the three types of Bacillus thurinigiensis (Bt) toxin produced by genetically modified maize. Resistance to one type of Bt toxin has cropped up in the worms in recent years, but now there is a twist — the researchers have found that resistance to that type of Bt toxin also confers protection against another, more recently introduced type. Their work appears in this week's Proceedings of the National Academy of Sciences.

"That's two of the three toxins on the market now," says Gassmann. "It's a substantial part of the available technology."

Genetically modified (GM) maize producing the Bt toxin Cry3Bb1, which provided protection against pests such as rootworm, was first approved for use in the United States in 2003. By 2009, farmers had started to see rootworm damage in their GM crops. In 2011, that damage had spread to GM maize containing a second toxin, mCry3A. In lab tests, Gassmann showed that this was a case of cross-resistance — worms that had become resistant to Cry3Bb1 were also resistant to mCry3A, possibly because the toxins share structural similarities and some binding sites in the insect's gut.

Part of the problem is that rootworms are tough, and the Bt maize does not produce enough toxin to fully control them. The Bt toxins used against pests such as the European corn borer (Ostrinia nubilalis) kill more than 99.99% of their targets, whereas more than 2% of rootworms can survive Bt maize. Resistance in the worms can evolve rapidly in fields where the same kind of maize is grown every year — in Iowa it showed up after an average of 3.6 years.

Nicholas Storer, a global science-policy leader for biotechnology at Dow AgroSciences in Washington DC, says that the study illustrates that if GM crops are not used as part of an integrated pest-management policy, resistance can develop quickly in an individual field. Agricultural biotechnology companies such Dow are now 'pyramiding' their seeds so that they produce two different Bt toxins to attack the rootworm. For example, Dow has teamed up with Monsanto of St Louis, Missouri, to sell seeds that combine Cry3Bb1 with Cry34/35Ab1, a toxin that has so far not seen any resistance develop.

Gassmann says that the pyramiding of toxins is an important way to delay the development of resistance, but that the combination is less effective once resistance arises to one of the toxins. So farmers should not rely exclusively on technology to fight pests, and should instead periodically change the crop grown on a field to help disrupt the pest's life cycle. "The rootworm can't survive if the corn's not there," Gassmann says.

Storer agrees that even the best technologies will always need to be combined with the old methods. "Crop rotation was the primary tool to combat rootworm before Bt came along," he says. "We need to keep it up."

Nature doi:10.1038/nature.2014.14887


Gassmann, A. J. et al. Proc. Natl Acad. Sci. USA (2014).