Bt Cotton has created major pest problems in China

Bt cotton in China has resulted in unintended negative consequences.

Updated: May 14, 2010, 20:45 PM IST

Bangalore: Even as India debates the feasibility of introducing Bt cotton farming, growing of Bt cotton in China since 1997 has resulted in unintended negative consequences, researchers at the Chinese Academy of Sciences have reported.

The genetically engineered cotton carries a toxin gene from the Bacillus thuringiensis (Bt) bacterium that protects the plant from being attacked by cotton bollworm (Helicoverpa armigera), a major pest, thereby decreasing the need for externally applied insecticides.

But a 10-year field study of crops in northern China, where 95 per cent farmers grow Bt cotton, has revealed that the use of Bt cotton has led to an explosive growth in the population of Mirid bugs (Heteroptera: Miridae), previously only a minor pest in the area.

Because the Mirid bugs eat a wide variety of plants, Yanhui Lu and his colleagues at the academy say the pests are emerging as a threat to other crops, including grapes, apples, peaches, and pears, for the first time in China.

"Certainly this problem has to be addressed before it becomes a serious issue in India," says Chavali Kameswara Rao, executive secretary of the Foundation for Biotechnology Awareness and Education in Bangalore and a proponent of Bt crops.

"The Mirid bugs have gained ground with less and less of pesticide application but my advice is that even Helicoverpa armigera on Bt cotton requires two or three pesticide sprays," Rao told IANS. "A proper integrated pest management strategy should take care of the Mirid bugs."

Data compiled by Lu and colleagues from their field work show the "unintended consequences" of cultivating Bt cotton. In a paper published online Friday in "Science Express", the researchers report that populations of Mirid bugs have increased dramatically because of fewer insecticides use after the introduction of Bt cotton.
In China, Mirid bugs have historically been considered occasional or minor pests in most crops, occurring at relatively low population levels and only sporadically requiring pest management intervention, the report said.

"Our work shows that a drop in insecticide use in Bt cotton fields leads to a reversal of the ecological role of cotton -- from being a sink for Mirid bugs in conventional systems to an actual source for these pests," the report said.

Throughout the study the Chinese scientists monitored Mirid bug abundance and insecticide use in cotton at 38 locations during 1997-2008. They found that Mirid bug population levels "gradually increased over time and were significantly related to Bt cotton planting proportion."

The report said that prior to Bt cotton adoption, broad-spectrum insecticide use knocked down early Mirid bug populations. "Current absence of insecticide sprays in Bt cotton permits unrestrained Mirid bug population build-up and subsequent spread or spill-over to a multitude of other flowering crops," they said.

"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," they reported.

The scientists warn that their findings demonstrate how alterations to pest management strategies targeting a particular pest (bollworm in the case of cotton) can lead to the spread of other non-target pests.

They stress the importance of analysing such possibilities before implementing country-wide strategies.

So far, long-term ecological effects of transgenic Bt crops on non-target pests have received limited attention, especially in diverse smallholder-based cropping systems of the developing world. With Bt crops presently adopted in over 20 countries "the ecological risks of their commercial cultivation have received considerable scientific scrutiny," the report said.

"To our knowledge, this study is the first confirmed report of a landscape-level emergence of non-target pests with the adoption of Bt crops due to reductions in insecticide applications," the scientists said.