Destruction of crops by insect pests is a huge problem throughout the world, and the use of chemical insecticides is only partially successful in countering it. The drawbacks to the use of such chemicals are manifold:
• they are often non-specific, so beneficial insects as well as harmful ones are killed
• they are often non-biodegradable, so they can have lasting environmental effects
• aerial spraying only reaches upper leaf surfaces
• resistance develops with continued use.
A form of natural insecticide does exist; it is a crystalline protein produced during sporulation by Bacillus thuringiensis. This is highly toxic to insects when converted to its active form by the enzymes of the gut. This S-endotoxin is relatively selective; different strains of B. thuringiensis produce different forms of the toxin, which are effective against the larvae of different insects. Surprising as it may seem, the use of the S-endotoxin as an insecticide was patented a hundred years ago; however, the success of it has been limited due to a variety of practical considerations.
The development of genetic engineering techniques has meant that attention has turned in more recent times to introducing the genes for the S-endotoxin into the crops themselves, so that they synthesise their own insecticide. This has been achieved with some success, but the problem of the insects building up resistance remains.
As a spin-off from the Human Genome Project, the genetic make-up of many microorganisms has been elucidated. One of these is Photorhabdus luminescens, which encodes a toxin lethal to the two species of mosquito responsible for the spread of malaria, and it is hoped that determining the sequence of the gene will lead to applications in insect control.
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