London (13.10.2005)
– Biology is nothing if not opportunistic.
Take great care when seeding a bacterium onto a Petri dish because lots of
other organisms will grow there equally happily if you are not sufficiently
careful to keep them out.
Plant rows of crops in a field on a good soil and watch the weeds doing their
best to compete. Then, as the crops themselves begin to grow, along come the
insects, the fungi and the other pests that prey upon them. And prey more
effectively on the crops than on the weeds because one of the consequences
of domesticating our crop plants has been to deprive them of some of their
own pesticides, protective agents which the wild weeds retain in greater quantities.
Controlling pests with chemical pesticides (weedkillers, insecticides and
the rest) kills the susceptible organisms and can thus leave the field clear
for the occasional resistant mutant. With no competition from its fellows,
the mutant may flourish so that the pest which started out largely susceptible
to the pesticide becomes resistant through selection and survival of the fittest.
In this case “survival of the fittest” means those best able to
resist the chemicals.
Spraying with chemicals is, however, an intermittent activity; the pests are
not subject to pesticides all the time. Building the pesticide into the crop
plant, as in insect-resistant maize and cotton, will expose the insect population
continuously, giving rise to fears that, within a few years. they would become
resistant and so difficult or impossible to control.
In spite of those fears, few if any resistant insects have shown up as a result
of cultivating GM insect-resistant maize in well-managed plantations. Now
comes news that cotton genetically engineered to be toxic to pests remains
effective after nearly a decade in the field; there has been no increase of
resistance in the major insect pest (pink bollworm) across the southern United
States. Genes for Bt resistance are no more common than in 1997 when commercial
cultivation really got going.
One reason seems to be good management: US regulations require farmers to
plant at least 5% of their crop interspersed with 'refuge' zones of non-Bt
cotton. The idea is that the intermixing of susceptible insects from within
the refuges, and which do not gain an advantage from the resistance genes,
with insects from the GM crops dilutes the concentration of any resistance
genes which might arise. That seems to work well.
But concern remains and nothing is for ever. Some scientists originally expected
resistance among insects after three or four years; now they are looking a
further five years ahead. Engineering two resistance genes, rather than just
one, into the crop plant might delay matters a lot longer because it is less
likely that pests could evolve to be resistant to both toxins simultaneously.
Biology may be opportunistic but then so are biologists.
Source:
Tom Simonite (10.10.2005). Bollworm pest remains beaten. Nature Online
(http://www.nature.com/news/2005/051010/full/051010-5.html)
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