Fighting with DNA

September 2012

Adult stage of the Western Corn Rootworm which damages plants' ability to absorb water.
Picture from Wikimedia Commons

What better way to create resistance to a pesticide than by growing vast monocultures of plants able to suffuse themselves perpetually with the same, single chemical?

This logic didn't escape the biotech industry when it created commodity crops genetically transformed to generate 'Bt' insecticidal proteins. However, by using simplistic modelling and peddling a few glib assumptions regarding the science of evolution, it was possible to persuade regulators that Bt-resistant insects could be kept at bay.

Bt-toxins bind to species-specific proteins on cells in the gut of insects. Once attached, they damage the cells' membrane and the cell dies, soon followed by the insect. The weak point in this is that if the insect can alter the structure of the specific Bt-binding protein at the basis of the technology, the toxin will no longer work.

Biotech industry scientists and regulators have always worked on the assumption that, because natural genes exist in interacting pairs (one on each of a pair of chromosomes), both binding-protein genes would have to mutate to allow the insect to survive Bt poisoning. The resistant individual would then have to mate with another identical double-mutant individual to produce resistant offspring: unless this happened on a large scale, the resistance wouldn't ever spread enough to become a problem.

To ensure that resistant insects were unlikely to encounter each other, the strategy adopted has been to plant areas of non-Bt crops near the Bt ones to act as a 'refuge' in which, with no pressure to evolve mutant genes, normal mates would always be in plentiful supply.

The questions of how big an area of 'refuge' is needed and how close it should be seem to have been solved with arbitrary answers. However, statistical modelling and the study of how resistance develops in laboratory situations have both produced evidence to support the strategy.

Whether the 'refuge' model works in practice is uncertain. Not all farmers plant them. When they do, the conventional crops have to be sprayed with conventional insecticides which will, of course, decimate the required breeding population.

Field-evolved resistance to Bt toxin certainly happens: it's been documented for several pests. But, whether it ever becomes great enough to affect the farmers' bottom line isn't clear.

Another uncertainty comes from the field data. This is because Monsanto itself has been charged with monitoring the pest-resistance around its GM crops. The Company cites its own data to insist that Bt resistance is not a problem in America.

Last year the U.S. Environmental Protection Agency (EPA) reported at least four, and possibly seven, States experiencing “severe efficacy issues for Monsanto's Cry3Bb1 trait” which is the GM trait in most U.S. Bt maize crops. This Bt insecticide is designed to kill western corn rootworm (WCR), a grub which damages the plants' ability to absorb water and makes them vulnerable to wind. EPA data indicate that the amount of Bt toxin now needed to kill WCR has increased a hundred-fold in some areas. At the same time, the Agency has pointed out how “inadequate” Monsanto's monitoring procedures are. For example, fields were sampled 1-2 miles away from where problems were reported, existing problem areas were used to establish 'baseline' susceptibility data, and follow-up sampling was not carried out in problem areas which had been sprayed to kill the pests.

A recent study looked, for the first time, at whether the genetic basis for the refuge model is living up to the theory. In laboratory-selected resistant strains of the cotton bollworm pest, the genes for Bt protein-binding were found to have mutated much as expected: both of the pair of genes had to have changed to make the pest withstand the Bt-toxin.

However, in the field, the genetic changes were very different. Alongside the anticipated mutation, the majority of individuals had evolved other tandem genetic changes unconnected with the binding protein, and which were dominant, i.e. they didn't require a second mutant gene. Such individuals will rapidly spread their Bt-resistance genes through the pest population no matter what kind of mate they have. In such a scenario, the refuge strategy is pointless.

The biotech answer to the (denied) emergence of Bt resistance in America is to create crops with two GM toxins against the same pest. At the same time, the refuge strategy is deemed no longer necessary and is being scaled down. Why this two-trait model should work when there are resistance genes to one of the Bt-toxins already in the population seems wishful thinking. Also, why it should be any more difficult for two genetic adaptations to evolve than one under the same extreme pressure isn't clear. After all, the CBW in the GM cotton fields had evolved more than one resistance gene. If the resistant cotton bollworm genome adaptability is anything to go by, the corn pest may well have a selection of mutations at its disposal, some of them dominant.

A group of scientists, including most of the non-corporate researchers currently trying to evaluate the extent and consequences of the CRW resistance, has approached the EPA to urge a re-think of the whole Bt-GM crop practice. They are suggesting a return to older, established methods of pest control, such as crop rotation and an increased supply of varied (especially non-Bt) crops strains to farmers.

Monsanto has claimed that what's being seen in the corn fields may just be due to abnormally high pest populations. This may be true, but with all that Bt in the system year on year supposedly killing off CRW grubs before they get old enough to reproduce and supply next year's pests, where did these 'abnormally high pest populations' come from?

One U.S. pest-management specialist University Professor summed up the situation: “Instead of making things easier, we've just made corn rootworm management harder and a heck of a lot more expensive”.

OUR COMMENT

The moral is: let's jettison all the clever high-tech genes which insects will always outsmart, and use the wealth of natural genetic diversity already at our disposal to create our food.

The UK government is pressing our scientists to create fancy man-made genes to protect crops. Remind it that, in a battle fought with DNA, all an insect needs to do is change a gene or two of its own. And, the pests will always win

SOURCES

• Dan Charles, Insect Experts Issue 'Urgent' Warning On Using GM Seeds, 9.03.12
• Mark Steil, Corn farmers struggle to cope with rootworm resistance, Minnesota Public Radio, 3.08.12
• Haonan Zhang et al., 2012, Diverse genetic basis of field-evolved resistance to Bt cotton in cotton bollworm from China, PNAS 109:26, 26.06.12
• Monsanto EU GM Maize Plants Undermined by Damning US Report, GM Freeze Press Release, 7.12.11
• Tom Philpott, Monsanto (Still) Denies Superinsect Problem, Despite Evidence, Mother Jones, 8.12.11
• United States Environmental Protection Agency Office of Chemical Safety and Pollution Prevention Memorandum, Decision 441144, 447967, 22.11.11
• Pablo Correa, GM crop efficacy may be jeopardised by pest mutations, Science and Development Network, Agriculture & Environment News: GM crops, 26.07.12