When choosing what kind of cotton to plant in the spring farmer Wang Fentong found herself in a difficult position.

The 40-year-old from Mazhuang Village of Xinjin City, in north China's Hebei Province, eventually decided to continue planting genetically modified (GM) cotton although its anti-insect effect "seems to have been decreasing."

Speaking to China Daily in a phone interview yesterday she said: "After all, its benefits are still higher than those of conventional types of cotton and labor is saved."

Mainstream agricultural experts say a more effective way of managing GM crops is needed if farmers are to keep enjoying the seeds' benefits in the future.

The government approved the sale of genetically modified cotton, tomatoes, pimientos and a species of morning glory in the late 1990s. GM cotton, with a Bacillus thuringiensis (Bt) gene inserted to produce toxins, has proven effective against bollworms, a major cotton pest.

Since its commercialization in China in 1997, Bt cotton sales have expanded rapidly with the total planted area reaching 3.3 million hectares in 2005, accounting for nearly 60 percent of all the cotton growing in the country.

But a study carried out by Cornell University researchers based on data provided by the Center for Chinese Agricultural Policy (CCAP) under the Chinese Academy of Sciences (CAS) last month was less than optimistic.

It found that although Bt cotton reduced pesticide use by more than 70 percent between 2001 and 2004, since then secondary insects mainly mirids which are not targeted by the Bt gene have arisen, and many GM cotton growers have ended up using the same amount of pesticides as farmers who planted conventional cotton.

What's more, the GM cotton farmers, who had been earning 36 percent more than those planting conventional cotton in 2001-04, earned 8 percent less than conventional cotton farmers last year because of higher Bt seed prices, the study said.

"These results should send a very strong signal to researchers and governments that they need to come up with remedial action for Bt cotton farmers," said Per Pinstrup-Andersen, the lead scientist in the study and a professor of food, nutrition and public policy at Cornell.

However, CCAP director Huang Jikun said the Cornell team's conclusions could be based on an incorrect reading of the data. According to Huang 2004 had particularly low summer temperatures and more precipitation so the mirids affected not only cotton but also other conventional crops nearby. CCAP interviews with the same farmers in 2005 and 2006 showed fewer mirids.

"In addition, it's not fair to compare the income of Bt cotton growers with that of nearby conventional cotton farmers because the long-time planting of Bt cotton has dramatically reduced the bollworm population not only on GM cotton farms but also on nearby non-GM farms, decreasing the latter's pesticides costs," said Huang.

But Huang did agree that it was important to study and develop strategies against the secondary insects not dealt with by Bt.

Zhang Yongjun, a senior research fellow at the Institute of Plant Protection of the Chinese Academy of Agricultural Sciences, said the rise of the secondary insect problem was mainly due to the poor management of GM cotton growth in China.

Before planting anti-insect cotton, Chinese farmers widely used broad-spectrum pesticides, which killed both bollworms and mirids. But using the pesticides actually increased costs, caused pollution and harmed farmers' health.

After planting anti-insect cotton, however, farmers used pesticides only in the final stage of the crop's growth, when the Bt cotton's resistance against bollworms is relatively reduced. "But in terms of preventing mirids it's too late," said Zhang.

That situation, coupled with weather factors, eventually led to the outbreak of mirids across cotton-growing provinces in 2004, Zhang explained. If the proper pesticide had been used at the right time the mirids could have been controlled in 2004, he said.

Besides the threat of secondary insects there were too many kinds of GM anti-insect cotton on the market to select an ideal type, said Su Guizhen, a cotton farmer in Dalisu village in Xinsu. After planting Bt cotton for several years, Su and her husband, Wang Changshan, decided to shift to conventional cotton with which they were more familiar, she said. "We were not certain which type of anti-insect cotton has better effects or higher output," said Su.

Hu Ruifa, a senior researcher at CCAP, said that although there was nothing wrong with planting GM cotton, an integrated management system was urgently needed in China, including seed management, pest observation and training in the use of pesticide.

"This is not only needed to maintain the benefits GM cotton can bring," Hu said, "but also for the sustainable development of China's agricultural biotechnology as a whole."