Sterile insect technique

Sterile insect technique is a method of biological control, whereby millions of sterile insects are released. The insects are normally male as it is the female that causes the damage, usually by laying eggs in the crop. The sterile males compete with the wild males for female insects. If a female mates with a sterile male then it will have no offspring, thus the next generation's population is reduced. Repeated release of insects can eventually wipe out a population. This technique has successfully been used to eradicate the screw worm-fly (Cochliomyia hominivorax) in areas of North America. Insects are sterilised with radiation, which unfortunately weakens the newly sterilized insects making them less able to compete with wild males.

Success stories

• Malaria
• Screw Worm Fly
  • Eradicated from US, Mexico, Guatemala, Belize and most of Panama

Drawbacks

• Repeated treatment is required to exterminate the population.
• Sex separation is difficult.
• Radiation treatment affects health of males, so the sterilized insects are at a disadvantage when competing for females
• Species specific. There are 22 species of Tsetse fly in Africa, for instance, and the technique must be implemented separately for each.
• Many fertile pest insects must be grown before sterilisation which must be housed securely to prevent their escape or release: in Feburary 2003, the irradiation machinery at a plant in Mexico failed and 4 million fertile screw worm were released before the problem was spotted.

Genetic modification

The release of genetically modified insects with "Dominant Lethal" genes, which can only survive in controlled environments and pass on their death gene to the offspring of the individuals they mate with is a modification of the sterile insect technique which utilises recombinant DNA technology to manipulate insect genomes. The technique is sometimes abbriviated RIDL (Release of Insects carrying a Dominant Lethal). The use of this technique has potential to overcome many of the drawbacks of the radiation based sterile insect technique. The advantage to RIDL over the radiation based technique is that modified male insects will not exhibit any of the physical weaknesses that inhibit their ability to compete with the wild male insects. The insects can also be given genetic markers, such as fluorescence that make monitoring the progress of eradication easier. Progress towards applying this technique to mosquitos has been made by researchers at Imperial College London who