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 of scientists has been monitoring 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. Their rise in abundance is associated with the scale of Bt cotton cultivation as they are not susceptible to the Bt toxin and hence thrived when farmers used less pesticide.
Mirids can reduce cotton yields just as much as bollworms, up to 50% when not controlled. The insects are also emerging as a threat to crops such as green beans, cereals, vegetables and various fruits.
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.
[Source]
Chapter: Home and Living :: 2 July 2010
