Editorial “Careless farming could cost us benefits of GM crops“
Farmers are walking a tightrope between reaping the benefits of genetically modified crops and getting it wrong and losing all their advantages. This is the message from an analysis of the first 400 million hectares of “Bt crops”, which have been tweaked to be toxic to the pests that feed on them.
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Along with weedkiller-resistant crops, Bt crops are the jewels in the crown of agricultural GM, allowing farmers to save money and avoid polluting farmland by reducing the amount of pesticide they spray. But that crown could now be slipping as insects become resistant.
A review published this week reveals that resistance has evolved in five of 13 key pest species, three against Bt maize and two against Bt cotton. In 2006, just one species was resistant.
The news comes in the wake of the furore over the appearance of long-discontinued GM wheat in a field in Oregon, the against international GM giant Monsanto, and the passing of a law requiring GM food to be labelled in Connecticut, the first US state to do so.
The latest finding will be grist to the mill for opponents of GM, but researchers are taking comfort in the fact that Bt crops, modified to include a bacterial chemical toxic to crop-eating insects but benign to others, remain effective against eight of 13 major pests despite the planting of more than 400 million hectares around the world since 1996.
“Overall, the picture is much rosier than anyone predicted,” says lead author at the University of Arizona in Tucson. “We’re not talking about species being resistant to all Bt toxins worldwide, just resistant to one of the toxins in areas such as western India for one pest or the US Midwest for another,” he says.
Kevin Folta at the University of Florida in Gainesville agrees: “After more than a decade of use in huge acreage, the resistance is still relatively minor and localised.”
In the US, for example, the crops have given farmers complete resistance to three major pests: the pink bollworm, the tobacco budworm and the European corn borer. In Arizona alone, Tabashnik says, this has eliminated the need for pesticides against the pink bollworm and in Bt cotton by 70 per cent.
Tabashnik and his colleagues used data from 77 published studies to analyse 24 cases, each representing resistance by one pest to one toxin in one country. Only five of the 24 cases revealed hard-core resistance, defined as more than 50 per cent of the pests collected identified as resistant, coupled with the crops’ stunted ability to kill pests. Fourteen of the 24 case studies gave a good bill of health, with less than 1 per cent of insects resistant.
Tabashnik’s review also reveals that resistance develops mainly where farmers have failed to implement measures designed to keep pest resistance from evolving. These measures include planting “refuges” of non-Bt crops near their Bt counterparts. The idea is that pests on the refuge crops breed with any Bt-resistant bugs on the plants nearby, helping to ensure that any resistance genes passed on are not expressed in their descendants. Other measures include rotating crops and, increasingly, using crops which make more than one toxin so bugs still die even if they have become resistant to one (see “Second-generation modification“).
Tabashnik found that where the rules for refuges and other measures were sufficiently stringent and adhered to by farmers, such as in Australia and the south-western US, resistance was largely non-existent. In places where farmers did not plant refuges, such as South Africa, Puerto Rico and India, resistance usually evolved quickly.
The message, he says, is that farmers risk losing the benefits of Bt crops unless they comply with the instructions. The benefits are worth fighting for: a by consultancy PG Economics of Dorchester, UK, which regularly analyses government and industry data on GM crop performance, estimates that between 1996 and 2011 the total global consumption of pesticides decreased by 9 per cent as a result of Bt crops. Bt cotton and Bt maize farmers saved $31 billion and $26 billion on pesticides, respectively.
of the University of Minnesota at St Paul is an adviser to the US Environmental Protection Agency on refuge management. He says industry surveys have shown that compliance with refuge rules in the US has been diminishing. A found, for example, that although around 90 per cent of US farmers complied from 2003 to 2005, the figure had fallen to 78 per cent by 2008. “Having a refuge can be costly,” says Hurley.
A quick fix is available, he says. For the past three years, seed companies such as Monsanto and Dow AgroSciences have been selling Bt crops as “refuges in a bag”. They mix Bt and non-Bt seeds together in exactly the correct proportions to give the required size of refuge, making it much easier to adhere to the rules.
In poorer countries, however, the problems are exacerbated. “In places like India and Africa, it’s hand-to-mouth survival for many farmers, and they simply don’t have the buffers against failure that rich farmers do,” says Stephen Morse of the University of Surrey, UK. “Their incentive is to maximise production, so devoting part of their land to a refuge that’s less productive… Well, would you do it?”
Tabashnik’s review may hint at a way forward, by following China’s example. There, Bt crops continue performing well even though farmers do not follow the rules on refuges. The explanation may be that the varieties grown are all thoroughly tested independently beforehand to make sure that they contain enough Bt to deliver the required toxic punch – another measure proven to keep resistance at bay.
If farmers can find ways to follow such measures, Tabashnik says existing Bt crops could work for decades yet. “It’s an incredibly valuable resource and we want to mine it for all it’s worth, not use it wastefully.”
Journal reference:
Second-generation modification
Even though munching Bt crops still kills the majority of 13 major pests (see main story), the agro-giants know they are in an evolutionary arms race. To keep ahead of the game, these firms have developed “second generation” crops that contain not one but two, or even three, toxins.
Dow AgroSciences has developed the , a variety of cotton. It contains two conventional Bt toxins plus a new one, Vip3A, a “vegetative insecticidal protein” that, like Bt, was originally bacterial. Each toxin works differently, and together they can thwart a range of cotton pests. “With intelligent stacking of these traits, you can considerably extend durability,” says Garry Hamlin of Dow, which hopes to launch the new varieties next year.
Several Monsanto crops with two toxins are already available, and company spokesman Mark Buckingham says that these plants are being used in regions where bugs had become resistant to their single-toxin predecessors.
Looking further ahead, companies are working on crops that protect themselves by making tiny fragments of RNA that sabotage genes unique to pests. Such RNA interference works by blocking genetic material from genes vital to the pest’s metabolism, for example. Monsanto is developing these methods to control the corn root worm in maize, says Buckingham.
A milder way to control pests is under development at Rothamsted Research in Harpenden, UK, which is testing a variety of wheat engineered to repel aphids. It contains a gene found in mint that produces a volatile chemical whenever the plant is being eaten, repelling the attacker.
This article will appear in print under the headline “Save killer crops from super-pests”
