Aphid-friendly cotton

July 2012

Cotton. Photo from Wikimedia Commons
China is one of the world's largest producers of cotton. In the six major cotton-growing provinces there, more than 90% of the crops are now 'Bt' insecticidal GM strains designed to kill cotton boll worm (CBW). Scientists in the country have been especially diligent in collecting data to see how the GM cotton is interacting with its environment.

Over the years, several Chinese studies have been published, most of which found very little positive to say about Bt cotton. However, the latest one has finally managed to extract 'good news': just what the media and biotech industry have been waiting for.

The Chinese scientists have evidence that, at landscape level (i.e. looking at the whole agricultural complex over a wide area), the reduction of chemical spraying against CBW following from the use of Bt crops is enough to allow natural predators to creep back into the cotton fields. What they found was that, over the years, three major predators (ladybirds, lacewings and spiders) have been gradually increasing in numbers in Bt cotton fields. When they looked at the associated abundance of aphids, a pest which has been troublesome since spraying first began in the 1970s, the scientists found these are now being controlled by their natural enemies. Non-cotton crops are also benefitting from a spill-over supply of the predators.

The authors remarked that the biocontrol services arising in Bt fields “could lead to the development of sustainable agriculture”.

This all sounds very natural and environmentally-friendly. The Chinese study was duly promoted in Britain, in particular, by the Rothamsted scientists trying to plug their GM wheat, which is also designed to attract insect predators.

How come this research was so positive when the previous ones highlighted concerns?

For the first three years or so after Bt cotton was introduced, spraying for CBW steadily reduced. After that, the boll worm applications continued to decline gradually until more than 90% of the total crop in the ground was GM, after which it attained a low plateau where it stayed for at least the next six years. As the treatments needed to control CBW decreased, the spraying needed for other pests increased proportionately, and ended up at a higher plateau. For the last few years of data collection, there was no further change in the actual total number of insecticidal applications.

The net result? Spraying specifically for CBW from 1997 when Bt cotton was introduced until 2010 decreased by a spectacular two-thirds. Total sprayings for all insect pests during this period was reduced by a less spectacular 14%. Spraying for non-CBW pests increased by 60%. (Lu, 2012)

The reasons for the increase in insecticide spraying have been described in published papers.

One study examined the surge of mirid bugs over 10 years in the six main provinces which had adopted Bt cotton. This, previously minor, pest (actually a complex involving 18 species in China) has become a major one. Also, it found that the GM cotton had come to be a source of mirid bugs from which other susceptible crops (including many important fruit and vegetables) were being infested and damaged. The data used in this paper seem to be a subset from the same survey as the aphid study, and authors were much the same: one of them predicted that farmers will soon spray as much as they ever did. (Lu, 2010)

Another publication was based on a large survey of farmers' experiences. It revealed that each of the five different provinces studied were experiencing very different secondary pests after the adoption of Bt cotton. In the worst-affected area four types of pest were causing problems, and in two areas three types of pest were causing problems. Pink bollworm and lygus bugs (a mirid-type pest), had caused almost universal problems in specific provinces. Aphids were mentioned by farmers in three provinces, but seemed to be a major problem (i.e. experienced by more than half the farmers) in one province only. (Zhao, 2011)

In 2011, another team of scientists published data showing early warning signs of emerging CBW resistance to the Bt toxin in Chinese cotton. The blame for this was laid on the excessively intensive planting of a single type of GM crop practised in the country.

The suggestion that Bt crops might usefully enhance natural predators was put forward (by much the same authors as the aphid paper) ten years ago when they found that mirids managed to be more active on Bt-protected crops than on conventionally sprayed ones (Wu, 2002). This received little attention by the press at the time. But now the UK media has been primed to find pro-GM news to print: it accordingly declared that as a result of the introduction of Bt cotton in China “pesticide use has halved” and that natural insect predators have doubled. The trouble with this is that, average pesticide use may have been reduced a bit on cotton and on other crops in the landscape, but it hasn't been halved and, given the early warning signs of CBW resistance emerging, it's unlikely to stay that way. Predators have certainly increased from a previous very low level, but those needed to control the most troublesome pests, such as mirids and pink bollworm, obviously aren't there. Aphids may be disappearing from Bt cotton fields at a great rate, but other more indigestible insects aren't.

One Ecology professor* was quoted as declaring that the work “ended a long-running debate” involving the argument that, since Bt crops need no pesticide spraying, “other pests would go crazy so you would subsequently have to spray lots more pesticide ... this did not happen for aphids”. No, but it did happen for mirids and a few other pests.

*Guy Poppy, previously GM researcher at Rothamstead; active in CropGen the biotech-funded lobby group; government advisor.


At the level of the farmer, the local nature of pests is a hugely important aspect of the success or failure of Bt cotton. Scientists approach the problem from a different angle: careful selection of the pest and predator model on which their study is based, can extract very different conclusions.

Readers familiar with the pressure put on scientists by the biotech industry and by pro-GM governments might be wondering if the aphid study is something in the nature of a damage-limitation excercise. With such a huge amount of data, spanning 20 years and 6 provinces, something positive had to be findable there to make amends for the negative impression of their previous revelations about the mirid onslaught.

This study suggesting a GM crop is sustainable follows on from several others providing evidence that Bt cotton growing, as being practiced in China now, is not sustainable. However, this lack of sustainability which could be catastrophic for many small farmers, is a plus for the biotech industry. No doubt, Chinese farmers will not doubt find themselves growing increasingly expensive GM cotton stacked with lots of new bacterial insecticidal proteins to combat, not only resistant CBW, but mirids, pink bollworms and anything else with six legs that has moved into the vacuum left by the demise of the CBW of yore. And, just how safe will all these toxins be for the environment and the people in it?

The branding of Bt as aphid-unfriendly will certainly not be the end of the debate.

  • Yanhue Lu et al., 2012, Widespread adoption of Bt cotton and insecticide decrease promotes biocontrol services, Nature Letters
  • Damian Carrington, GM crops good for environment, study finds, Guardian 15.06.12
  • Jennifer H. Zhao et al., 2011, Benefits of Bt cotton counterbalanced by secondary pests? Perceptions of ecological change in China, Environment Monitoring Assessment 173
  • Yanhui Lu et al., 2010, Mirid Bug Outbreaks in Multiple Crops Correlated with Wide-Scale Adoption of Bt Cotton in China, Science 238, May
  • Jane Qiu, GM crop use makes minor pests major problem, Nature 13.05.10
  • Paul Voosen, Biotech Cotton Curbs One Pest Only to Unleash Another, New York Times, 17.05.10
  • Zi-jun Wang et al., 2009, Bt Cotton in China Are Secondary Insect Infestations Offsetting the Benefits in Farmer Fields?, Agricultural Sciences in China 8:1
  • Shengui Wang et al., 2008, Bt-cotton and secondary pests, International Journal of Biotechnology 10:2/3
  • Susan Lang, Seven-year glitch: Cornell warns that Chinese GM cotton farmers are losing money due to 'secondary' pests, Cronicle on line, 25.07.06
  • S. Udikeri et al., Mirid Menace - A Potential Emerging Sucking Pest Problem in Cotton, 2007 study
  • K. Wu et al., 2002, Seasonal abundance of the mirids, Lygus lucorum and Adlephocoris spp. (Hemipter: Miridae) on bt cotton in northern China, Crop Protection 21

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