Agriculture Reference
In-Depth Information
the advent of genetically modified organisms. There are now 500 species
of insect, mite or tick that are resistant to one or more compounds,
together with more than 400 herbicide-resistant weed biotypes, and 150
resistant fungi and bacteria.
12
Evolution of resistance can occur in the
context of genetically modified crops that express an insecticidal product
(eg
B.t.
), leading to insect resistance, or through overuse of herbicides on
genetically modified crops, causing weed resistance. At first, the potential
problem of insect resistance went unrecognized. Now, though, there
are mandatory rules in the US to reduce the selection pressure on pests
through integrated resistance management for
B.t.
genetically modified
crops. These mandate that a proportion of the cropped area must be
devoted to refuges of non-genetically modified crops, that rotations must
be used, and that
B.t.
maize should not be grown where pest pressure is
low. The guidance indicates that 20 per cent of farmland must be devoted
to refuges within 800 metres of a
B.t.
crop field, with varying rules for
refuge size depending upon the proportion of a parish under the same
genetically modified crop. For
B.t.
maize grown in a cotton area, the
stipulation is a 50 per cent non-
B.t.
maize refuge in order to minimize corn
earworm and cotton bollworm resistance. The aim is to provide sufficient
susceptible adult insects to mate with potential
B.t.
-resistant adult insects
in order to dilute the frequency of resistance genes. But there is still
controversy over the size, structure and deployment of non-
B.t.
crop
refuges, how they should be implemented at a regional scale, and the
difficulty of enforcing or encouraging farmers to adopt them.
Recombination of viruses and bacteria to produce
new pathogens
A third risk relates to the potential for viruses or bacteria to incorporate
transgenes into their genomes, leading to the expression of novel and
possibly undesirable traits. In addition, viral transgenes that are incorp-
orated into the genetically modified crop could, in theory, recombine to
produce viruses with high fitness. However, such recombination has not
yet been shown to occur.
13
In theory, viral genes could affect humans, too,
by surviving passage through the human gut and entering gut bacteria and
human body cells. Once inside cells, DNA could insert itself into the
genome to change the basic structure and functions. This could lead to
the emergence of new diseases. However, this would necessitate the highly
improbable integration of whole sequences of DNA into the human
genome.
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