Widespread use of insect-resistant cotton in China is creating pest problems for fruit farmers, say researchers.
Dr Kongming Wu of the Chinese Academy of Agricultural Sciences in Beijing and colleagues report their findings today in the journal Science.
In 1997, China approved the planting of cotton engineered to exude the bacterial toxin Bacillus thuringiensis (Bt).
Bt kills a major pest of cotton called Helicoverpa armigera, and the pest-resistant cotton was adopted rapidly and now makes up most of the cotton grown in China.
Because Bt cotton was controlling H. amigera, cotton farmers were able to cut the number of pest sprays they were using
But in doing this, says Wu, other pests, not affected by the Bt toxin, were able to increase in number.
In particular, there was an increase in mirid bugs, which are sucking pests that affect cotton, cereals, vegetables and fruit crops.
"Our analyses show that Bt cotton has become a source of mirid bugs and that their population increases are related to drops in insecticide use in this crop," says Wu.
In a field trial conducted over 10 years, Wu and colleagues analysed the impact of 3 million hectares of Bt cotton (making up 95% of the cotton in northern China), on 26 million hectares of other crops that were susceptible to mirid bug attack.
Wu says as the planting of Bt cotton increased, so did the number of mirids in chinese date, grapes, apple, peach and pear crops.
"A reduction in insecticide use for H. Armigera control in Bt cotton correlates with mirid bug outbreaks in cotton and various fruit crops in the broader agro-landscape," write Wu and colleagues in Science.
Wu says prior to the advent of Bt cotton in China, mirids were not a problem for other crops but now they need to be sprayed with pyrethroids and organophosphates.
He says the problem calls for a new pest management system in Bt cotton.
Wu says options include a new transgenic cotton that is also toxic to mirids along with biological controls and "trap" crops, such as mung beans, which are attractive to mirids.
Mirid expert Dr Melina Miles, of the Queensland agriculture department in Toowoomba, wonders whether the Bt cotton in China is the major factor behind increasing mirid numbers in such a large area of fruit crops.
She says in Australia mirid numbers are largely controlled by other factors such as environmental conditions.
Still, says Miles, Australian farmers also saw an unprecedented burst in mirid numbers when Bt cotton was first planted in the early 1990s.
"It was exactly the same situation as they are describing in China," says Miles, who also works with the Cotton Research and Development Corporation.
But, she says controls with pesticides, biological control and lucerne trap crops mean aphids are kept in check.
Miles says the Chinese problem will go away once cotton farmers start properly controlling mirids.
Fending of resistance
Pest expert Professor Rick Roush of the University of Melbourne says the failure to keep mirids in check in China's reflects a larger difficulty of managing an industry made up of millions of farmers.
Roush says Bt cotton in China has reduced the use of toxic insecticides by about 80% and reduced the number of farmers poisoned by such chemicals.
But, he says he and other scientists have previously advised China they will need to control for mirids and also to protect against the development of resistance to Bt toxin.
In Australia, farmers must set aside a certain percentage of land to non-Bt crops to fend against resistance developing in the pests that are controlled by it.
But in China there is no such resistance management strategy, says Roush.
"It is a worry," he says. "It's very difficult to do when you're dealing with millions of growers."
Roush says resistance to Bt cotton has developed in some areas of India and it's a matter of time before resistance to the current variety of Bt cotton becomes a problem in Australia. He says by then the industry will be relying on the next technological breakthrough in pest management.
"The strategy really is to delay resistance long enough until other, and hopefully even better, alternatives come out of the marketplace."