Sterile insect technique

The screwworm was the first pest successfully eliminated from an area through the sterile insect technique, by the use of an area-wide approach.

The sterile insect technique is a method of biological control, whereby overwhelming numbers 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 blood 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 reducing the next generation's population. Repeated release of insects can diminish small populations, though it could be impossible to eradicate it and is not efficient against dense insect populations.

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 Mexican fruit fly (Anastrepha ludens).

Insects are mostly sterilized with radiation, which might weaken the newly sterilized insects if doses are not correctly applied, making them less able to compete with wild males. However, other sterilization techniques are under development which would not affect the insects' ability to compete for a 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.

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. Since a female screwworm mates only once in her lifetime, this physiological phenomenon has been exploited by biologists in breaking its life cycle. After mating with a sterile male, the screwworm female will not mate again nor lay any fertile eggs.

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 importantly, 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 (460 km²) 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 leaves no residues, and has no negative 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 fly for African trypanosomiasis

Sleeping sickness or the 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 50,000 - 70,000 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.

The sterile fly is an innovative solution to the problem of the African trypanosomiasis. Specially bred male Tsetse flies are sterilized through irradiation process. 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 flies into affected areas leads to greater levels of health and economic activity.

Successful trials

Main article: List of sterile insect technique trials

• Screwworm fly (Cochliomyia hominivorax) eradicated from the United States, Mexico, and Libya.
• Mexican fruit fly (Anastrepha ludens, Loew) eradicated from most of northern Mexico.
• Tsetse fly eradicated from Zanzibar.
• Medfly (Ceratitis capitata, Wiedemann) eradicated from northern part of Chile and southern part of Peru and southern part of Mexico.
• Melon fly (Bactrocera cucurbitae, Coquillett) eradicated from Okinawa, Japan.

Current targets

• Anopheles mosquito - Malaria vector, example Anopheles arabiensis.
• Tsetse fly (Glossina spp) - sleeping sickness vector.
• Painted Apple Moth (Lepidoptera: Lymantriidae) in Auckland, New Zealand.
• Codling moth (Cydia pomonella) (Lepidoptera: Tortricidae) in British Columbia, Canada [1].
• Aedes mosquitoes, vectors for filariasis, dengue and yellow fever.
• Continuing use across the world against various fruit fly species, including Medfly, Caribbean and Mexican fruit fly (Nth, Sth and Central America), Queensland fruit fly in Australia (Bactrocera tryoni) and several other Bactrocera sp. 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 of sterile pest insect and also provides information on the doses of radiation (IDIDAS) used in the control of pest insects and mites of agricultural, commercial or quarantine significance. It includes data on both the radiation dose required for the disinfestation of generic commodity groups, fresh and durable, and also the radiation dose used to induce sterility for pest control through the sterile insect technique.

Drawbacks

• As with insecticide treatment, repeated treatment is sometimes required to suppress the population before the use of sterile insects.
• Sex separation could be difficult for some species, though this can be easily performed on Medfly and screwworm, 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.
• Standard operating procedures of mass rearing and irradiation do not leave room for mistakes. Since the fifties, when SIT was first used as a means for pest control, several failures have occurred in different places around the world where non-sterilized artificial produced insects were released before the problem was spotted.
• Application to large areas should be long lasting, otherwise migration of wild insects from outside the control area could repopulate.
• The major drawback to this technique is that the cost of producing such a large number of sterile insects is often prohibitive in poorer countries.

Genetic modification

See also: Genetically modified insect

A method using recombinant DNA technology to create genetically modified insects called RIDL ("release of insects carrying a dominant lethal") is under development by a company called Oxford Insect Technologies (Oxitec), UK. 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 (tetracycline), which 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 DsRED fluorescence, that make monitoring the progress of eradication easier, preferably under the field conditions.

There are potentially several types of RIDL, 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 have a high probability to die; but about 5% of the descendants survive.

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 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 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.

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.

SIT programs will benefit tremendously if genetic methods can be developed that enable only male insects to be reared as has already been done for the medfly. In addition, more appropriate artificial diets for larvae, and hormonal, nutritional, microbiological, and semiochemical treatments for adults, could make major contributions through improved economy and insect quality.

Economic benefits of SIT has been demonstrated in various cases. For example, direct benefits of screwworm eradication to the North and Central American livestock industries are estimated to be over $ 1.5 billion/ year, compared with a total investment over half a century of close to $ 1 billion. Mexico protects a fruit and vegetable export market of over $ 3 billion/year through an annual investment of ca. $ 25 million, and medfly-free status has been estimated to have opened markets for Chile’s fruit exports of up to $ 500 million. Eradication of tsetse has resulted in major socio-economic benefits for Zanzibar. When implemented on an area-wide basis and with economies of scale in the mass rearing process, the use of SIT for suppression is cost competitive with conventional control, in addition to its environmental benefits.

See also

• Inherited sterility in insects
• Genetically modified insect
• The Screwfly Solution by James Tiptree, Jr.— a classic science fiction horror story based on the Sterile Insect Technique

External links

• Mosquito birth control
• BBC Online article
• Plan to Eradicate the Tsetse Fly
• IPC-SIT webpage
• Improving Sterile Male Performance in Fruit Fly SIT programmes
• BBC article
• SIT Book Review
• A General-Purpose Species-Eradication Method