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 Despite Industry Claims, Insect Resistant GE Crops Will Not Reduce Toxic Pesticide Use 
by Organic Consumers Association - 2/8/2006

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.

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Provided by Organic Consumers Association on 2/8/2006
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