Sterile insect technique
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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.
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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
A method using recombinant DNA technology to create genetically modified insects called RIDL (Release of Insects carrying a Dominant Lethal) is under development. The method works by introducing a "Dominant Lethal" gene into the insects in such a way that the gene is expressed only in females, and the gene's effect can be countered in the controlled insect manufacturing environments, fx by giving a food additive. The insects can also be given genetic markers, such as fluorescence, that make monitoring the progress of eradication easier.
The released males are not sterile, but any female offspring their mates produce will have the dominant lethal gene expressed, and so will die. The number of females in the wild population will therefore decline.
The advantages of the RIDL technique are that the male insects can be seperated from the females for release simply by withdrawing the factor in the controlled manufacturing environment that kept the females alive, fx removing a food additive. Using RIDL also means that the males will not have to be sterilized by radiation before release, making the males more healthy when they need to compete with the wild males for mates.
Progress towards applying this technique to mosquitos has been made by researchers at Imperial College London who created the world's first transgenic malaria mosquito.
