Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 254

recalcitrant and consequently their use may lead to a build-up of chemicals
damaging to the environment. This is an increasing problem with the intensive
drive towards more and more cost-effective crop production. Secondly, insects
are known to develop resistance to pesticides and so new and in some ways,
more poisonous chemicals might be introduced to maintain the same level
of effectiveness. Thirdly, chemical pesticides are rarely targeted to specific
problematic species and may kill other organisms of no harm or even of some
benefit to the crop plants. Balanced natural environments have an equilibrium
between assailant and victim, however, it may take a commercially unacceptable
time for this balance to establish, sometimes incurring quite extreme swings in
either direction. For example, one season may see a flourishing of citrus trees
due to an outbreak of disease leading to a dearth of butterflies, the caterpillars
of which feed voraciously on citrus. The lack of insect host reduces the level
of infection, leading to a recovery of the butterfly population the following year
and, consequently, seriously damaged citrus trees. One means by which insect
numbers are controlled in nature is by bacteria which produce toxins killing
the insect which consumes it. Although this may serve to create the balance
described above, it may not be sufficient to satisfy commercial crop production.
Sufficient time may elapse between ingestion of the toxin by the larva and its
ensuing death, for it to have caused considerable damage by feeding on the
crop. Perhaps the best studied pesticidal bacterial toxin is the δ -endotoxin from
Bacillus thuringiensis . This protein, frequently abbreviated to 'Bt toxin', is active
against some members of the Lepidoptera (butterflies and moths), Diptera (flies,
midges and mosquitoes) and Coleoptera (beetles) families and has been used
in its native, unmodified form as a pesticide for many years. There are several
strains of the bacterium each one producing a toxin active against a limited
number of insect species; a relationship which is continuously evolving. Already
successful, it would appear to be the current leading candidate for development
into a more generally useful and effective biopesticide, thus hopefully reducing
the dependence on chemical pesticides.
There are limitations associated with its use, all of which are being addressed
in active research. These include a limited range of insects susceptible to each
toxin, requiring dosing with multiple toxins, insufficient ingestion by the insect
to prove lethal in a usefully short time, stability of the toxin when sprayed
on crops and the development of resistance by the insect. The last stumbling
block has attracted particular interest (Roush 1994, Gould 1994, Bohorova et al .
2001). The genes coding for the toxins have been isolated opening the way to
their alteration and introduction into suitable 'delivery systems', either bacterial
or into the plant itself, offering the plant inbuilt protection, thus attempting to
overcome the various limitations introduced above.
However, even without genetic engineering, Bacillus thuringiensis in its native
form, remains a widely used and successful product for commercial crop pro-
tection especially in 'organic' farming. In practice, the only major developments