Sterile insect technique
The sterile insect technique (SIT) is a method of biological insect control, whereby overwhelming numbers of sterile insects are released into the wild. The released insects are normally male, as the females cause the damage usually by laying eggs in the crop, or, in the case of mosquitoes, taking blood from humans. The sterile males compete with wild males to mate with the females. Females that mate with a sterile male produce no offspring, thus reducing the next generation's population. Repeated release of sterile males can diminish small populations, although success with dense target populations has not been demonstrated.
The technique has successfully been used to eradicate the screw-worm fly (Cochliomyia hominivorax) in areas of North America. Many successes controlled species of fruit flies, most particularly the Mediterranean fruit fly (Ceratitis capitata) and the Mexican fruit fly (Anastrepha ludens).
Sterilization is induced through the effects of irradiation on the reproductive cells of the insects. SIT does not involve the release of insects modified through transgenic (genetic engineering) processes.
Insects are mostly sterilized with radiation, which might weaken them if doses are not correctly applied, reducing their fitness. Other sterilization techniques are under development which would not affect fitness.
Raymond Bushland and Edward Knipling developed the SIT to eliminate screw-worms preying on warm-blooded animals, especially cattle. They exploited the fact that female screw-worms mate only once to attack screw-worm reproduction. The larvae of these flies invade open wounds and eat into animal flesh, killing infected cattle within 10 days. In the 1950s, screw-worms caused annual losses to American meat and dairy supplies that were projected at above $200 million. Screw-worm maggots can also parasitize human flesh.
Bushland and Knipling began searching for an alternative to chemical pesticides began in the late 1930s when they were working at the United States Department of Agriculture Laboratory in Menard, Texas. At that time, the screw-worm was decimating livestock herds across the American South. Red meat and dairy supplies were affected across Mexico, Central America, and South America.
Knipling developed the theory of autocidal control – breaking the pest's reproductive cycle. Bushland's enthusiasm for Knipling's theory sparked the pair to search for a way to rear flies in a "factory" setting, and to find an effective way to sterilize flies.
Their work was interrupted by World War II, but they resumed their efforts in the early 1950s with successful tests on the screw-worm 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 eradicate screw-worms from the 176-square-mile (460 km2) island of Curaçao, off the coast of Venezuela. Screw-worms were eliminated in a span of only seven weeks, saving the domestic goat herds that were a source of meat and milk.
During the 1960s and 1970s, SIT was used to control the screw-worm population in the United States. In the 1980s, Mexico and Belize eliminated their screw-worm problems with SIT. Eradication programs progressed across Central America, with a biological barrier in Panama to prevent reinfestation from the south.
In 1991, Knipling and Bushland's technique halted a serious outbreak in northern Africa. Programs against the Mediterranean fruit fly in Mexico and California use the same principles. The technique was used to eradicate the melon fly from Okinawa and in the fight against the tsetse fly in Africa.
The technique has suppressed insects threatening livestock, fruit, vegetable, and fiber crops. The technique was lauded for its environmental attributes: it leaves no residues and has no (direct) negative effect on nontarget species.
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 1992 World Food Prize. The technique were hailed by former U.S. Secretary of Agriculture Orville Freeman as "the greatest entomological achievement of (the 20th) century."
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 regions of sub-Saharan Africa, covering about 36 countries and 60 million people. An estimated 50,000 - 70,000 people are infected and about 40,000 die every year. The three most recent epidemics occurred in 1896-1906, 1920, and 1970.
Studies of the tsetse fly show that females generally mate only once (occasionally twice). Studies found this process to be effective in preventing the scourge.
- The screw-worm fly (Cochliomyia hominivorax) was eradicated from the United States, Mexico, and Libya.
- The Mexican fruit fly (Anastrepha ludens, Loew) was eradicated from most of northern Mexico.
- The tsetse fly was eradicated from Zanzibar.
- The Mediterranean fruit fly (Medfly, Ceratitis capitata, Wiedemann) was eradicated from the northern part of Chile and southern parts of Peru and Mexico.
- The melon fly (Bactrocera cucurbitae, Coquillett) was eradicated from Okinawa.
- Anopheles mosquito - malaria vector, example Anopheles arabiensis.
- Tsetse fly (Glossina spp.) - sleeping sickness vector
- Painted apple moth in Auckland, New Zealand
- Codling moth (Cydia pomonella) in British Columbia, Canada .
- Aedes mosquitoes, vectors for filariasis, dengue, yellow fever, and the Zika virus
- Fruit fly species, including Medfly, Caribbean and Mexican fruit fly (Americas), Queensland fruit fly in Australia (Bactrocera tryoni), and several other Bactrocera spp. across Australia, Asia, and Oceania: The IAEA lists 41 different SIT facilities (DIR-SIT) across the globe, aiding the retrieval of information on mass production and on radiation doses (IDIDAS). It includes data on both the radiation dose required for the disinfestation of generic commodity groups, and the radiation dose used to induce sterility for pest control through the SIT.
- Repeated pesticide treatment is sometimes required to suppress populations before the use of sterile insects.
- Sex separation can be difficult, though this can be easily performed on Medfly and screw-worm larvae.
- Radiation treatment can reduce male mating fitness.
- The technique is species-specific. For instance, the technique must be implemented separately for each of the 22 tsetse fly species.
- Mass rearing and irradiation require precision processes. Failures have occurred when unexpectedly fertile breeding males were released.
- Migration of wild insects from outside the control area could recreate the problem.
- The cost of producing sufficient sterile insects can be prohibitive in some locations.
Using recombinant DNA technology to create genetically modified insects called RIDL ("release of insects carrying a dominant lethal") is under development by UK company Oxford Insect Technologies (Oxitec). They introduce a repressible "dominant lethal" gene into the insects. This gene kills the insects, but can be inhibited by an external additive (tetracycline) that allows the insects to be cultivated in bulk. This external additive can be administered orally via insect food. The insects can be given marker genes, such as DsRED fluorescence, that enable monitoring of the eradication progress under field conditions.
Among several approaches to RIDL, the more advanced forms employ a female-specific dominant lethal gene. This avoids the need for a separate sex separation step, as the inhibitor can be withdrawn from the final stage of rearing, leaving only males.
Males are then released into the target region. The released males are not sterile, but female offspring express the dominant lethal gene, reducing survival rates to some 5%.
Since the males are not sterilized by radiation before release, they have equivalent fitness to wild males. Progress towards applying this technique to mosquitoes 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 CSIRO in the hope of eradicating introduced carp from the Murray River system. As of 2005[update], it was undergoing safety tests.
Conclusion and perspectives
Biotechnological approaches based on genetically modified organism (transgenic organisms) are still under development. However, since no legal framework exists to authorize the release of such organisms in the nature, sterilization by irradiation remains the most used technique. A meeting was held at FAO headquarters in Rome, 8 to 12 April 2002 on "Status and Risk Assessment of the Use of Transgenic Arthropods in Plant Protection". The resulting proceedings of the meeting have been used by the North American Plant Protection Organization (NAPPO) to develop NAPPO Regional Standard No. 27 on "Guidelines for Importation and Confined Field release of Transgenic Arthropods", which might provide the basis for the rational development of the use of transgenic arthropods.
Economic benefits have been demonstrated. The direct benefits of screwworm eradication to the North and Central American livestock industries are estimated to be over $1.5 billion/year, compared with an investment over half a century around $1 billion. Mexico protects a fruit and vegetable export market of over $3 billion/year through an annual investment around $25 million. Medfly-free status has been estimated to have opened markets for Chile’s fruit exports up to $500 million. Eradication of tsetse has resulted in major socioeconomic benefits for Zanzibar. When implemented on an area-wide basis and a scaled rearing process, SIT is cost-competitive with conventional control, in addition to its environmental benefits.
- Genetically modified insect
- Inherited sterility in insects
- The Screwfly Solution by James Tiptree, Jr.— a classic science fiction horror story based on SIT
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- A Genetically Engineered Swat
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- World-Wide Directory of SIT Facilities
- International Database on Insect Disinfestation and Sterilization
- FAO/IAEA/USDA-2003-Manual for Product Quality Control and Shipping Procedures for Sterile Mass-Reared Tephritid Fruit Flies, Version 5.0. International Atomic Energy Agency, Vienna, Austria. 85pp.
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- Oxitec Ltd How the self-limiting gene works Retrieved 14 February 2016
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