Sterile insect technique

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, or, in the case of mosquitoes, taking a bloodmeal from humans. 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 Screw-worm 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), and the painted apple moth.

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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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."

Sterile atomic fly

The sterile atomic fly is an innovative solution to the problem of sleeping sickness, and is being developed by the United Nations and the International Atomic Energy Agency, building on their experience of similar programs over past decades against the fruit fly in Australia and Africa.

Sleeping sickness or African trypanosomiasis is a parasitic disease in humans. Caused by protozoa of genus Trypanosoma and transmitted by the Tsetse fly, the disease is endemic in certain regions of Sub-Saharan Africa, covering about 36 countries and 60 million people. It is estimated that 300,000 - 500,000 people are infected, and about 40,000 die every year. Three major epidemics have occurred in the past hundred years, in 1896 - 1906, 1920, and 1970.

Studies of the tsetse fly show that females generally only mate once in their lifetimes and very rarely mate a second time. Once a female fly has mated, she can then produce continual offspring throughout her short life.

Using this information, the International Atomic Energy Agency has developed a process of irradiating male Tsetse flies that have been specially bred. This process of irradiation sterilizes the male. These sterilized male flies are then released into areas where sleeping sickness is prevalent, and then mate with the females. Because the male is sterile, and the females mate only once, the population of Tsetse flies in the affected area will drop. Studies have shown that this process has been very effective in preventing Sleeping sickness in people who live in the area.

Since sleeping sickness is fatal without treatment and infected people can be without symptoms for months, the release of sterile atomic flies into affected areas leads to greater levels of health and economic activity.

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.

Current Targets

• Anopheles mosquito - Malaria vector
• Tsetse fly (Glossina spp) - sleeping sickness vector.
• Painted Apple Moth (Lep: Lymantriidae) in Auckland, New Zealand
• Aedes mosquitoes, vectors for filariasis, Dengue and yellow fever

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, for instance: there are 22 species of Tsetse fly in Africa, and the technique must be implemented separately for each.
• Many fertile pest insects must be grown before sterilization 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.
• Application to large areas is unlikely to be long lasting as migration of insects from outside the control area will repopulate.
• The major drawback to this technique is the often prohibitive cost of breeding and maintaining such a large number of insects. Costs often far surpass the abilities of the poor countries that need the technology the most.

Genetic modification

A method using recombinant DNA technology to create genetically modified insects called RIDL^TM (Release of Insects carrying a Dominant Lethal) is under development by a company called Oxitec. The method works by introducing a repressible "Dominant Lethal" gene into the insects, this gene kills the insects but it can be repressed by an external additive, this allows the insects to be reared in manufacturing facilities. This external additive is commonly administered orally, and so can be an additive to the insect food. The insects can also be given genetic markers, such as fluorescence, that make monitoring the progress of eradication easier.

There are potentially several types of RIDL^TM (Oxitec ltd), but the more advanced forms have a female-specific dominant lethal gene. This avoids the need for a separate sex separation step, as the repressor can be withdrawn from the final stage of rearing, leaving only males.

These males are then released in large numbers into the affected region. 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, causing the overall population to decline.

Using RIDL 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.