GM crop use makes minor pests major problem

Growing cotton that has been genetically modified to poison its main pest can lead to a boom in the numbers of other insects, a ten-year study in northern China has found.

In 1997, the Chinese government approved the commercial cultivation of cotton plants genetically modified to produce a toxin from the bacteria Bacillus thuringiensis (Bt) that is deadly to the bollworm Helicoverpa armigera. Outbreaks of larvae of the cotton bollworm moth in the early 1990s had hit crop yields and profits, and the pesticides used to control the bollworm damaged the environment and caused thousands of deaths from poisoning each year.

More than 4 million hectares of Bt cotton are now grown in China. Since the crop was approved, a team led by Kongming Wu, an entomologist at the Chinese Academy of Agricultural Sciences in Beijing, has monitored pest populations at 38 locations in northern China, covering 3 million hectares of cotton and 26 million hectares of various other crops.

Numbers of mirid bugs (insects of the Miridae family), previously only minor pests in northern China, have increased 12-fold since 1997, they found. "Mirids are now a main pest in the region," says Wu. "Their rise in abundance is associated with the scale of Bt cotton cultivation."

Wu and his colleagues suspect that mirid populations increased because less broad-spectrum pesticide was used following the introduction of Bt cotton. "Mirids are not susceptible to the Bt toxin, so they started to thrive when farmers used less pesticide," says Wu. The study is published in this weeks issue of Science.

"Mirids can reduce cotton yields just as much as bollworms, up to 50% when not controlled," Wu adds. The insects are also emerging as a threat to crops such as green beans, cereals, vegetables and various fruits.

Rise of the mirids

The rise of mirids has driven Chinese farmers back to pesticides — they are currently using about two-thirds as much as they did before Bt cotton was introduced. As mirids develop resistance to the pesticides, Wu expects that farmers will soon spray as much as they ever did.

Two years ago, a study led by David Just, an economist at Cornell University at Ithaca, New York, concluded that the economic benefits of Bt cotton in China have eroded2. The team attributed this to increased pesticide use to deal with secondary pests.

The conclusion was controversial, with critics of the study focusing on the relatively small sample size and use of economic modelling. Wus findings back up the earlier study, says David Andow, an entomologist at the University of Minnesota in St Paul.

"The finding reminds us yet again that genetic modified crops are not a magic bullet for pest control," says Andow. "They have to be part of an integrated pest-management system to retain long-term benefits."

From the ashes

Whenever a primary pest is targeted, other species are likely to rise in its place. For example, the boll weevil was once the main worldwide threat to cotton. As farmers sprayed pesticides against the weevils, bollworms developed resistance and rose to become the primary pest. Similarly, stink bugs have replaced bollworms as the primary pest in southeastern United States since Bt cotton was introduced.

Along with genetically modified crops, says Andow, farmers need effective systems for responding to changes in pest abundance. This needs to be based on research into the timing, dosage and frequency of pesticide use needed to tackle new pests. "When farmers decide how to control pests, they tend to overuse pesticides," he says.

Wu and his colleagues are seeking the most effective way to use pesticide, and trying to reduce mirid damage to cotton by growing crops the pests prefer nearby. Meanwhile, Chinese researchers are trying to develop cotton plants that kill both bollworms and mirids.

Wu stresses, however, that pest control must keep sight of the whole ecosystem. "The impact of genetically modified crops must be assessed on the landscape level, taking into account the ecological input of different organisms," he says. "This is the only way to ensure the sustainability of their application."