Pesticide use rising as Chinese farmers fight insects thriving on transgenic crop.
Jane Qiu
Mirid
bugs have filled the gap created by killing other pests of cotton.Science/AAAS 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 week's issue of
Science1. "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. 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. Wu's 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." 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." http://www.nature.com/news/2010/100513/full/news.2010.242.html
Rise of the mirids
From the ashes
References