Sterile insect technique
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Sterile insect technique is a method of biological control, whereby millions of sterile insects are released. The released 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, though it is often more useful to consider controlling the population rather than eradicating it.
The technique has successfully been used to eradicate the Screwworm fly (Cochliomyia hominivorax) in areas of North America. There have also been many successes in controlling species of fruit flies, most particularly the Medfly (ceratitis capitata).
Insects are mostly sterilized with radiation, which can weaken the newly sterilized insects making them less able to compete with wild males. However, other sterilization techniques in fact boost the insects' ability to mate.
The technique was pioneered in the 1950s by American entomologists Dr. Raymond C. Bushland and Dr. Edward F. Knipling. For their achievement, they jointly received the 1992 World Food Prize.
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[edit] Development of the sterile insect technique
Raymond Bushland and Edward Knipling first developed the technique to eliminate screwworms preying on warm-blooded animals, especially cattle herds. With larvae that invade open wounds and eat into animal flesh, the flies were capable of killing cattle within 10 days of infection. In the 1950s, screwworms caused annual losses to American meat and dairy supplies that were projected at above $200 million. Screwworm maggots are also known to parasitize human flesh.
The quest of Bushland and Knipling to find an alternative to chemical pesticides in controlling the devastation wrought by these insects began in the late 1930s when both scientists were working at the United States Department of Agriculture Laboratory in Menard, Texas. At that time, the screwworm was decimating livestock herds across the American South. Red meat and dairy supplies were also affected across Mexico, Central America, and South America.
While Bushland initially researched chemical treatment of screwworm-infested wounds in cattle, Knipling developed the theory of autocidal control – breaking the life cycle of the pest itself. Bushland's enthusiasm for Knipling's theory sparked both men to intensify the search for a way to rear large numbers of flies in a "factory" setting, and most important, to find an effective way to sterilize flies.
Their work in this area was interrupted by World War II, but Drs. Bushland and Knipling resumed their efforts in the early 1950s with their successful tests on the screwworm population of Sanibel Island, Florida. The sterile insect technique worked; near eradication was achieved using x-ray sterilized flies.
In 1954, the technique was used to completely eradicate screwworms from the 176-square-mile island of Curaçao, off the coast of Venezuela. Screwworms were eliminated in a span of only seven weeks, saving the domestic goat herds that were a source of meat and milk for the island people.
During the 1960s and 1970s, SIT was used to control the screwworm population in the United States. The 1980s saw Mexico and Belize eliminate their screwworm problems through the use of SIT, and eradication programs have progressed through all of Central America, with a biological barrier having been established in Panama to prevent reinfestation from the south. In 1991, Knipling and Bushland's technique halted a serious outbreak in northern Africa. Similar programs against the Mediterranean fruit fly in Mexico and California use the same principles. In addition, the technique was used to eradicate the melon fly from Okinawa and has been used in the fight against the tsetse fly in Africa.
The technique has been able to suppress insects threatening livestock, fruit, vegetable, and fiber crops. The technique has also been lauded for its many environmentally sound attributes: it uses no chemicals, leaves no residues, and has no effect on non-target species.
Proven effective in controlling outbreaks of a wide range of insect pests throughout the world, the technique has been a boon in protecting the agricultural products to feed the world’s human population. Both Bushland and Knipling received worldwide recognition for their leadership and scientific achievements, including the World Food Prize. Their research and the resulting Sterile Insect Technique were hailed by former U.S. Secretary of Agriculture Orville Freeman as "the greatest entomological achievement of (the 20th) century."
[edit] Success stories
- Screwworm fly - Eradicated from the United States, Mexico, Guatemala, Belize and most of Panama.
- Medfly Ceratitis capitata - successful control in Israel, California, Central America etc.
[edit] Current Targets
- Anopheles mosquito - Malaria vector
- Tsetse fly (Glossina spp) - sleeping sickness vector.
- Painted Apple Moth (Lep: Lymantriidae) in Auckland, New Zealand
[edit] Drawbacks
- Repeated treatment is required to exterminate the population.
- Sex separation is difficult for some species (though can be easily performed on Medfly, for example).
- Radiation treatment in some cases affects the health of males, so sterilized insects in such cases are at a disadvantage when competing for females.
- The technique is 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 and must be housed securely to prevent their escape or release: in February 2003, the irradiation machinery at a plant in Mexico failed and 4 million fertile screwworms were released before the problem was spotted.
[edit] 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, for example 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 separated from the females for release simply by withdrawing the factor in the controlled manufacturing environment that kept the females alive, for example by 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.
A similar technique is the daughterless carp, a genetically modified organism produced in Australia by the CSIRO in the hope of eradicating the introduced carp from the Murray River system. As of 2005, it was undergoing tests to assess the risks of releasing it into the wild.
