fevereiro 14, 2006

GM crops are not the answer to pest control

'Farmer field schools', such as this one in Cambodia, teach farmers the principles of IPMG. V. Ramanjaneyulu

8 February 2006
Source: SciDev.Net

G. V. Ramanjaneyulu argues that insect-resistant crops will eventually require an increased use of pesticides, and that farmers around the developing world will suffer as a result.

Thousands of farmers in the Indian state of Andhra Pradesh have committed suicide since the 1990s, and many of these deaths have been blamed on so-called pest disasters. This refers to the way farmers' heavy use of pesticides has led to increased resistance in pests, which in turn has caused substantial crop losses and a slide into crushing debt.

Given this situation, what should be the response to those suggesting that we apply high doses of toxins over extended periods, irrespective of whether the pests are present? After all, this is what supporters of genetically modified (GM) insect-resistant crops are encouraging farmers to do.

We do not have to look far to find well-established and credible alternatives, namely the use of integrated pest management (IPM), or even non-pesticidal management and organic farming.

These strategies are based on the farmers' own knowledge, management skills and labour, rather than external farm inputs. Their demonstrated effectiveness shows that farmers can manage insect pests successfully and affordably without resorting to chemical pesticides — or to insect-resistant GM crops. [1]

The experience of these farmers suggests that widespread use of such GM crops violates the principles of sound pest management.

Weighing the costs

It is generally accepted that under IPM, insecticides should be applied only when the projected cost of damage from pests is greater than the estimated cost of control measures, and only after all other effective insect-control techniques have been considered.

Furthermore IPM practitioners look at the entire range of pests associated with a crop, rather than individual insect species. They seek to understand all the factors regulating pest populations within a particular context. Finally, they devise and implement strategies to keep the pest population below level at which growing the crops becomes uneconomic — known as the 'economic threshold level' (ETL).

Among the many positive aspects of this combination of strategies is that it effectively prolongs the useful life of a pesticide by ensuring that insects do not rapidly develop resistance to it. Such resistance can develop in two ways.

The first is via 'selection for resistance'. In any natural population of pests there is normal genetic variation, which includes variation in the genes that deal with pesticide resistance. Pesticide use inevitably favours the survival and reproduction of individual pests bearing the genes that confer increased resistance.

The second mechanism is 'induced selection'. Even if the insect population has no naturally resistant insects, high doses of a pesticide causing mutations could increase the probability of resistance emerging.

Both of these are known to occur with chemical pesticides, and it is likely that insect-resistant transgenic plants — such as those producing the Bacillus thuringiensis (Bt) toxin — will have the same effect.

Unlike sprays, however, insect-resistant GM plants maintain constant levels of the Bt toxin over an extended period, regardless of whether the pest population is at economically damaging levels. The selection pressure with insect-resistant GM crops is therefore likely to be much more intense than with pesticide sprays.

Toxin consumption
In order to slow the emergence of insecticide resistance, IPM strategies seek to avoid the use of pesticides altogether, unless the pest population reaches the economic threshold level. If this happens, farmers using IPM try to ensure that pesticides are only applied in doses that are appropriate for the severity of pest problem.

By contrast, insect-resistant GM crops aim to eliminate pests by encouraging them to eat high doses of toxins. Researchers, for example, are now reported to be trying to amplify the expression production of the Bt toxins to 25 times more than is needed to kill the relevant pest. [2]

In practice, the number of pests killed depends on the amount of toxin they consume when feeding on the plant tissue. So producing the toxin in the right dose, at the right time, and in the plant tissues where the pest feeds, becomes crucial.

Unfortunately, reports indicate that levels of the Bt toxin can vary between different Bt varieties, between different parts of individual plants, and over time.

In particular, key parts of the plants' flowers, such as the pollen, anthers, pistils and ageing flower petals, tend to have lower concentrations of the toxin than other parts of the plant. [3]

Admittedly these studies have only looked at the variability of Bt production under controlled conditions, rather than in farmers' fields. But the experience of Indian farmers shows that, in practice, the extent to which Bt cotton resists pests is extremely uneven within a season, as well as across years, hybrids and locations.

Refuges are no solution
Another factor that increases the likelihood that pesticide resistance will develop is that a single gene — the Bt cry1ac gene — has been introduced into all the most widely-used cotton hybrids in India, while the same gene is also being introduced into other crops.

In contrast, rather than relying on one technology or method of pest control, IPM encourages farmers to alternate between chemicals that work in different ways. This so-called 'mortality-source diversification' helps prevent pests from developing resistance as quickly as they would if faced with a single toxin.

Advocates of Bt cotton — and government officials responsible for regulating its use — argue that resistance can be slowed by planting 'refuges' of non-Bt cotton, on the basis that this will encourage the survival of insects that are susceptible to the Bt toxin.

In India, however, it is difficult to impose this requirement, given the small size of many farmers' plots. Furthermore such 'biosafety' measures are also very difficult to monitor and enforce — indeed, there is evidence in India that refuges are not in fact being implemented. [4]

The danger is that the widespread use of Bt varieties and other insect-resistant crops will lead to a rise in the number of resistant pests, which will in turn mean that the environment is subject to an ever-higher volume of spraying, and more poor farmers are driven to despair.

The cost is too high. Insect-resistant GM crops have no place in a rational pest management strategy

G. V. Ramanjaneyulu is executive director of the Centre for Sustainable Agriculture, Hyderabad, India.

[1] Ramanjaneyulu G.V. et al. (2004) No Pesticides…No Pests…. Centre for Sustainable Agriculture, Secunderabad, India (2004)
[2] Benedict J.H., Ring D.R. Transgenic crops expressing Bt proteins: Current status, challenges and outlook. In: Koul O., Dhaliwal G.S., eds. Transgenic Crop Protection: Concepts and strategies.Oxford University Press and IBH Publishing Co, New Delhi (2004)
[3] Kranthi K.R. et al. Temporal and intra-plant variability of Cry1Ac expression in Bt-cotton and its influence on the survival of the cotton bollworm, Helicoverpa armigera (Hübner) (Noctuidae: Lepidoptera). Current Science 89, 2 291-299 (2005)
[4] Central Institute for Cotton Research (CICR). Report on Bt Cotton in India. http://envfor.nic.in (2004)

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