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Insects are rapidly developing resistance to the toxins in genetically engineered Bt corn [Image: Snbetor via Flikr]

GM crops are outwitted by nature as insect resistance grows

Crops that have been genetically engineered to produce their own insecticide (Bt crops) were meant to reduce the amount of insecticide that farmers spray on their crops and in the surrounding environment.

But in the US, insects are resisting, chemical warfare in the fields is escalating and GM is failing.

Mike Gray, a University of Illinois professor of agricultural entomology says that in 2013, he anticipates a sharp increase in the use of soil insecticides with corn (maize) crops genetically engineered against corn rootworm.

These plants, known as Bt hybrids, have been engineered to produce their own insecticide by inserting a gene from the bacterium, Bacillus thuringiensis, hence Bt.

But according to Gray, on average nearly half the producers asked indicated they intend to use a soil applied insecticide with their corn rootworm Bt hybrid this spring.

“I think it is worth mentioning that one of the key benefits touted concerning the use of Bt hybrids for corn rootworm management was the reduction of soil insecticide use,” he said. “It is a bit surprising that 10 years after the first Bt hybrids entered the marketplace for corn rootworms in 2003 that a heightened interest in the use of soil insecticides has surfaced in such a significant fashion.”

Stacking up problems

Bt crops have been grown in the US since 1996. After a very brief honeymoon period when the crops were effective against insects, signs of insect resistance emerged and the problem has escalated, leading to increased chemical spraying.

The response to this resistance from the biotech companies has been to develop other alternative toxin producing genes and to insert them into the plant as well.

These ‘stacked traits’ are meant to negate the resistance to one toxin, so if the first toxin doesn’t kill the pest, the second or third one will.

‘Stacked traits’ are also used to try to confer herbicide tolerance to the plants as well.

This ‘gene stacking’ is known as ‘the pyramid strategy’ and has been adopted extensively in the US, with two-toxin Bt cotton completely replacing one-toxin Bt cotton since 2011.

But nature doesn’t give up easily and resistance has continued to increase despite ‘stacking’.

Now a study from the University of Arizona (UA), published in the Proceedings of the National Academy of Sciences (PNAS), has shown that earlier understanding of how insect resistance develops is flawed.

Scientists thought that plants with more than one toxin would stave off resistance and would be more effective than plants with a single mode of defence. However, using lab experiments, computer simulations and analysis of published experimental data, UA researchers have discovered that this is not the case.

Nature resists

The team, led by Yves Carrière, a professor of entomology, bred caterpillars of the main crop pests; the cotton bollworm – also known as corn earworm or Helicoverpa zea – a species of moth that is a major agricultural pest in fields of maize, cotton and other crops, and selected caterpillars that were resistant to one of the Bt toxins.

Predictably, the resistant caterpillars survived after munching on cotton plants producing only that toxin. The surprise came when Carrière’s team put the caterpillars on Bt cotton that had two toxins.

On previous assumptions, the caterpillars that are resistant to the crops with one toxin would be killed when consuming plants that contain two, because the second toxin should kill the caterpillars that resist the first.

But as Carrière explained, this wasn’t the case: “on the two-toxin plants, the caterpillars selected for resistance to one toxin survived significantly better than caterpillars from a susceptible strain.”

The assumed success of using two different toxins to defeat resistance is flawed. Whatever mechanism drives resistance to one toxin drives resistance to multiple toxins.

The team’s analysis of published data from eight species of pests revealed that some degree of cross-resistance between multiple toxins occurred in 19 of 21 experiments.

Nature kicks arse

In an attempt to hold back resistance biotech companies and the Environmental Protection Agency (EPA) advises farmers to plant “refuges” of non-GM plants around GM crops to provide an area for non-resistant insects to breed and ‘dilute’ the resistant insect stock.

However the UA team found that the inheritance for toxin resistance is dominant. In lay terms, even if resistant insects breed with non-resistant insects, the resulting offspring will almost certainly be resistant too.

Carrière pointed out: “Our simulations tell us that with 10 percent of acreage set aside for refuges, resistance evolves quite fast, but if you put 30 or 40 percent aside, you can substantially delay it.”

So far few agricultural professionals in the US have picked up on just how ludicrous this advice is. In essence; if you want to keep growing GM crops you have to stop growing GM crops on 40% of the acreage you want to grow GM crops on.

It’s like Catch 22.

• Thanks to Citizens Concerned About GM for permission to reprint this article