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3.
Insect-resistant crops
Bacillus
thuringiensis (Bt) is a soil bacterium which produces
a toxin that is highly valued by organic farmers as a safe
and effective bio-pesticide. It targets particular species
and is used in occasional applications, especially in instances
where there is a serious pest infestation. Crop plants have
now been engineered with the gene for the Bt toxin so that
they have an in-built insecticide. These GE 'insect-resistant'
crops were grown on 7.7 million hectares worldwide in 1998
(1). In marked contrast to the occasional
application of the Bt toxin in organic farming, the GE Bt
toxin is produced in the plants all the time they are growing.
This means that insects are continually exposed to the toxin,
and are therefore under constant pressure to develop resistance
(2). The US Environmental Protection Agency
has predicted that most target insects are likely to build
up resistance to Bt within 3 to 5 years as a result (3).
This will permanently destroy the effective use of Bt in organic
agriculture.
In
addition, the genetically engineered Bt is present in form
that may harm a wider range of insects.
A
recent study in Switzerland found that when lacewings (beneficial
insects that prey on crop pests) were fed cornborers raised
on Bt maize, the lacewings suffered from disruption to their
development and increased mortality (4).
The
use of other insect toxins in GE crops, such as a lectin
from the snowdrop, has also shown worrying signs of effects
on the food chain. In a laboratory experiment, female ladybirds
were fed on aphids that had been eating insect-resistant
potatoes. Compared to ladybirds fed on a normal diet, they
laid fewer eggs and lived half as long (5).
References
1.
James C. (1998) Global Review of Commercialised Transgenic
Crops: 1998. ISAAA Briefs No8. ISAAA: Ithaca, NY.ISAAA: Ithaca,
NY.
2. Tabashnik B.E. (1994). Evolution of Resistance
to Bacillus thuringiensis. Annual Review of Entomology 39:47-79.
3. EPA (1994) Pesticide Fact-Sheet 4/98 -
Bacillus thuringiensis Cry IA(b) delta - endotoxin and the
geneic material necessary for its production (Plasmid vector
pclB 4431) in corn. OPPTS.
4. Hilbeck A., Moar W.J., Pusztai-Carey M.,
Filippini A., Zigler F. (1998) Toxicity of Bacillus thuringiensis
CryIAb toxin to the predator Chrysoperla carnea (Neuroptera:
Chrysopidae). Environmental Entomology, Vol. 27, No.4, August
1998.
5. Birch A.N.E., Geoghegan I.E., Majerus M.E.N.,
Hackett C. & Allen J. (1997) Interactions between plant resistance
genes, pest aphid populations and beneficial aphid predators.
Scottish Crop Research Institute, Annual Report 1996/97 pp
68-72. SCRI: Dundee.
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