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The development and application of the sterile insect technique (SIT) for New World screwworm eradication

Theoretical basis of the sterile insect technique
Development of the sterile insect technique
Application of the technique to various insect pests
Application to screwworm
Research and methods development support
Concluding comments

A.M. Van der Vloedt and W. Klassen

This article gives a brief historical background of the sterile insect technique (SIT) and its application to the eradication of the New World screwworm (NWS), Cochliomyia hominivorax. Described are certain biological characteristics of the fly and its mass production in the laboratory, the action on the chromosomes of ionizing radiation and its lethal effect on the sperm. This lethality halts the embryogenesis of the fertile eggs. Consequently the females have no descendants. After the great success of a pilot project on Curaçao, NWS was completely eradicated from the United States and Mexico. To meet the emergency situation in North Africa, air transportation of millions of sterilized flies to the Libyan Arab Jamahiriya began in late 1990. During the preparatory phase, exacting biological studies were carried out to improve effects of transport on quality of adult insects dispersed.

The authors' address is Insect and Pest Control Section, Joint FAD/IAEA Division of Nuclear Techniques in Food and Agriculture PO Box 100, A-1400 Vienna, Austria.

Since ancient times, the New World screwworm (NWS), Cochliomyia hominivorax, has been a serious enemy of warm-blooded animals, including humans, in an area of the Americas extending from the southern United States southward into Argentina. Originally native peoples in infested areas devised and relied upon herbal remedies to treat screwworm-infested wounds. Subsequently, settlers from Europe and their descendants progressively introduced more effective treatments for infested wounds. Only in recent decades has this pest been eradicated from the United States and Mexico, through the use of the sterile insect technique (SIT).

To apply this technique, screwworms are mass reared on an artificial medium. Sexual sterility is induced by exposing the pupae to gamma rays, which damage the chromosomes in the sperm. When the eggs of wild females are fertilized with sperm from irradiated males, cell division is disrupted and the embryo dies. If sufficient numbers of sterile males are released into the wild over several generations, the reproductive success of the wild population can be progressively reduced and extinguished. In order to achieve eradication, a ratio of not less than 10:1 sterile to indigenous screwworms must be maintained throughout the ecosystem.

The recent introduction of NWS into the Libyan Arab Jamahiriya during 1988, and its successful colonization of this new environment far from its original habitat in the Americas, has compelled the governments of the North African countries at risk to initiate emergency control measures of intensive surveillance of the deadly parasite, rigid control of animal movement and treatment of animals at quarantine checkpoints. This action is being undertaken in collaboration with FAO, the International Fund for Agricultural Development (IFAD), the United Nations Development Programme (UNDP) and the International Atomic Energy Agency (IAEA), as well as with bilateral assistance.

A campaign to eradicate this parasite from the 20000 km² of infestation in Libya has been launched. The principal method in the eradication strategy is based on the use of SIT. The efficiency of this technique in eradicating screwworm populations while having no adverse impact on other non-target species has been well proven in the Americas. The use of the technique has consequently been recommended and support has been given by authorities in various countries in the international community.

Theoretical basis of the sterile insect technique

The sterile insect technique is the first involving insect genetics for population control, and it can be applied only to pest species that reproduce by sexual means. It is effective only if the sexually sterile males are aggressive and successfully compete with wild males in searching for and mating with indigenous females. The method should be used only when a thorough knowledge has been acquired of the biology, ecology and behaviour of the target pest species.

The sterile insect technique may be considered as a form of birth control. In its application, the target pest species is mass-reared, sexually sterilized and distributed over the range of the pest population being targeted. If the goal is eradication of the pest population, sexually sterilized males must be released within reasonable proximity of all indigenous females. Thus, release of sterile males should not be limited to selected zones; otherwise these zones would soon be reinvaded by individuals from adjacent untreated areas. To achieve eradication, then, the total pest population in a region must be targeted. Alternatively, segments of populations in large areas can be targeted, provided that the avenues of reinvasion are guarded first by means of quarantine check-points to prevent reinfestation through human activity and second by the release of sterile males to create a barrier zone that is wider than the flight range of the insect pest.

Sexual sterility can be induced in target pest species by means of chemical and physical agents including alkylating agents, antimetabolites, X-rays, gamma rays and neutrons. To date, no chemical sterilants have been discovered that can be used without at least presenting some hazard to workers in mass-rearing factories, nor are chemicals yet available that can be applied to the indigenous pest population without risk to non-target species. Consequently, in current practice, sexual sterility is induced with radiation emitted from radioisotopes such as caesium-137 and cobalt-60. The dosage of radiation applied must have no significant adverse effect on the males' longevity, searching behaviour and mating ability.

Matings of sterile males with wild females do not yield offspring. If sufficient numbers of sterile males are released, so that most indigenous females are mated by them, the total number of individuals in the wild population will be reduced in the next generation. Thus, continued releases of high-quality sterile males in overwhelming numbers over several consecutive generations will progressively reduce the wild population and eventually result in extinction.

Development of the sterile insect technique

The idea that populations of economically important insect species might be controlled, managed or eradicated through genetic manipulation was conceived in the late 1930s by an American entomologist, Dr Edward F. Knipling. A similar concept was published independently by the Soviet geneticist Serebrovsky (1940). Knipling derived his concept while working on the screwworm fly problem as it afflicted livestock in the United States. He and his colleague, Bushland, assessed various ways of coping with the entire screwworm population in the United States (Knipling, 1985). They recognized that the strategy whereby every livestock producer had to check all animals for infested wounds at least twice per week would greatly retard the development of efficient livestock production in the United States. In particular, this approach was ineffective and costly on large ranches where many cattle were not encountered routinely. Knipling and Bushland, aware that the screwworm population in the United States represented only a minute part on the northern border zone of a vast infested area that extended south into Argentina, began to consider ways of suppressing the entire northern fringe of this population (Knipling, 1985).

In 1936, Bushland had developed laboratory techniques for culturing the screwworm on ground meat (Melvin and Bushland, 1936). This was a major breakthrough, since previously no one had been able to rear an obligate parasite except on a live host animal. For the first time, substantial numbers of screwworms could be made available for research.

Knipling in 1937 observed the extreme sexual aggressiveness of the male screwworm, as well as the usual refusal of the female to mate more than once during her lifetime. He realized that if sexual sterility could be induced in the males and if a means could be developed to rear, sterilize and release vast numbers of screwworm flies, the screwworm population in the United States could be reduced to insignificant numbers (Knipling, 1955, 1979). To effect this, he devised a simple mathematical model of this revolutionary concept for insect control. Concurrently, Bushland undertook a search for chemicals that would induce sexual sterility, but these efforts were interrupted by the Second World War.

Unknown to Knipling and Bushland, however, a method to induce sexual sterility had already been devised during the 1920s by Herman J. Muller at the University of Texas. Muller (1928) had used a dentist's X-ray machine to induce mutations in the genes and chromosomes of the vinegar fly, Drosophila melanogaster.

Muller further discovered that X-rays when applied in sufficiently high doses break chromosomes and that two broken ends of chromosomes that contact each other, being sticky, tend to adhere and heal together. Furthermore, two broken ends, each with a site of attachment to a spindle fibre, may heal together to form a "dicentric" chromosome with two sites of attachment to spindle fibres. When the cell begins to divide, the dicentric chromosome may be pulled toward opposite poles of the cell. Often, if the dicentric chromosome fails to break, then the cell cannot divide, and it dies. If the dicentric chromosome breaks at a new place, then one cell will receive a deficiency of genetic information, and the other a duplication of information. In addition, each chromosome piece from the original break that has no site of attachment for a spindle fibre cannot be pulled into a daughter nucleus during cell division. Thus, the daughter cells may not receive all of the genetic information needed to function. These and other chromosomal events culminate in the death of any embryo formed by the fertilization of a normal egg cell with irradiated sperm (see Figure 1).

1. Cytogenetic consequences of radiation-induced chromosome damage - Conséquences cytogénétiques des dégâts causés aux chromosomes par irradiation - Consecuencias citogenéticas de los daños inducidos por la radiación en los cromosomas

2. Holding cages for adult flies at Mexico production plant - Cages contenant des mouches adultes au centre d'élevage de Mexico - Jaulas con moscas adultas en las instalaciones de producción de México

3. NWS eggs on liquid medium - Œufs de lucilie bouchère sur milieu liquide - Huevos de mosca del gusano barrenador en un medio liquido

Fortunately, the induction of dominant lethal mutations in the reproductive cells of insects does not hinder the maturation of these cells into eggs or sperm or prevent the latter from participating in the formation of the zygote or embryo. However, dominant lethal mutations do prevent the zygote or embryo from developing to maturity. Embryonic death usually occurs during early cell division. Consequently, eggs fertilized with sperm bearing dominant lethal mutations do not hatch.

Application of the technique to various insect pests

Considerable progress has been made in developing the use of SIT and other genetic methods to control the insect pests shown in Table 1 (Seligman, Gillen and Utner, 1990). Research and development efforts are under way to make the use of these techniques less costly, more robust and more broadly applicable.

Insect pests for which SIT is being used or is being developed

Insectes ravageurs pour lesquels la méthode de l'insecte stérile est en cours d'utilisation ou de mise au point

Plagas de insectos para las cuales se utiliza o se está poniendo a punto la técnica de los insectos estériles


Previous sites

Current sites


Curaçao, USA, Mexico, Puerto Rico, US Virgin Islands

Guatemala, Belize, Libya

Mediterranean fruit fly

Italy*, Peru*, Mexico, USA (California), Israel*

Guatemala, USA (Hawaii)

Caribbean fruit fly

USA (Florida)*

USA (Florida) fly-free zone

Melon fly


Japan, Brazil

Oriental fruit fly

Rota, Hawaii*

Onion fly


Netherlands (control)

Mexican fruit fly


USA/Mexico (quarantine + fly-free zone)

Cherry fruit fly


Pink bollworm


USA (quarantine)

Codling moth

Canada*, USA*

Canada (control)

Gypsy moth


USA (quarantine)

Tsetse flies

United Republic of Tanzania*, Nigeria*, Nigeria, Zanzibar

(Four species)

Burkina Faso*

Tobacco budworm


Boll weevil


Sheep blowfly



El Salvador*

Stable fly

St Croix, USA*

Tobacco hornworm

St Croix, USA*

Cattle fevertick

St Croix, USA*

St Croix. USA

Note: The table shows insect pests for which SIT or a related genetic control method is being used, has been used, or is being developed. The objective is eradication unless otherwise noted.

* Experimental pilot test.

Application to screwworm

After the Second World War, Muller became concerned that the radioactive fallout from the testing of nuclear transportation weapons might induce harmful mutations in human populations and published a general article on the detrimental effects of radiation on genetic material (Muller, 1950). From this article, Lindquist perceived the potential of Muller's findings in regard to the screwworm problem and recommended that experiments be conducted to determine whether X-rays might induce a useful form of sterility in the screwworm.

Husman irradiator for treatment of pupae - Irradiateur de Husman pour le traitement des pupes - Irradiador Husman para el tratamiento de las pupas

Packaging of treated pupae - Emballage des pupes irradiées - Envasado de las papas tratadas

Storage of pupae in incubation chamber before long-distance transportation - Stockage des pupes dans des chambres d'incubation avant leur transport sur une longue distance - Almacenamiento de pupas en una cámara de incubación antes del transporte a larga distancia

Bushland and Hopkins were asked to investigate the effect of radiation on the biology of the screwworm and found the males to be quite sensitive and the females to be more resistant. Sexual sterility could be induced in the males with 25 Gray (2500 reds) and in females with 65 Gray. Five- or six-day-old pupae withstood sterilizing doses of radiation almost as well as adults. Since pupae are much easier to handle than adults, they were selected as the life stage for irradiation in subsequent experiments as well as in operational programmes. By placing sexually sterile males and untreated males in various ratios in cages to compete for females, Bushland and Hopkins (1953) were able to demonstrate the essential validity of Knipling's mathematical model. For example, when sterile males outnumbered fertile males in a 9:1 ratio, only 10 percent of the females produced fertile egg masses.

The next step was to repeat such experiments in the field. For this experiment, Sanibel Island (36 km²), 5 km off the west coast of Florida, was chosen. Bushland and his colleagues released 38 sterile males per square kilometre per week for several months. Although these releases caused about 80 percent of the egg masses to be sterile, eradication was not achieved. Bushland postulated that indigenous females that had already been mated were migrating from the mainland to the island and laying eggs on the wounds of animals.

An opportunity arose to conduct a decisive experiment when the governor of Curaçao, a Dutch island of 440 km² 64 km off the coast of Venezuela, appealed for assistance in moderating the effects of the screwworm on dairy cattle and other livestock. Flies mass-reared and irradiated in Florida were released by aircraft over the island. Initially, the release of 78 sterile flies per square kilometre per week had little impact, but by increasing the rate of release to approximately 150 sterile flies per square kilometre per week eradication was achieved in just seven weeks (Lindquist, 1955).

This success inspired livestock owners in Florida to persuade the State Legislature and the United States Congress to appropriate funds to initiate a control programme in 1957 (Lindquist, 1963). By July 1990 the programme had accomplished the eradication of the New World screwworm from the United States and Mexico. Efforts are now under way to eradicate the screwworm from all of Central America to Panama.

Research and methods development support

As documented in the proceedings Symposium on eradication of the screwworm from the United States and Mexico (Graham, 1985), the successes of these screwworm eradication programmes resulted from nearly 50 years of basic and applied research support, summarized as follows.

Research in the biology and physiology of the screw worm and cost-effective mass-rearing. The latter has been achieved through the modification of the larval rearing medium by replacing raw meat with cheaper sources of nutrients, initially using a liquid medium and more recently a gelled diet matrix.

Genetic characterization of screwworm populations and strain development evaluation. Strains used in mass-production become obsolete because of the inherited changes that are induced by prolonged captivity. The strain used for mass-production, therefore, must be replaced approximately every 18 months with one proven to perform well in the field and readily mass-reared.

Ecology and behaviour of screwworm populations in diverse ecological zones. Such research is essential as programmes advance to new ecological zones.

Bait-toxicant systems. Methods have been applied for population detection, monitoring and suppression.

Sterilization and release methods. Procedures had to be developed for radiation treatment, transportation of irradiated pupae, handling at packaging centres and distribution of sterile flies.

Extension. The effectiveness of releases of sterile males is evaluated by collecting egg masses from sentinel animals, and livestock owners have been taught methods of treating infested animals that can be used on a self-help basis.

Although based on experience gained in the Americas, the programme for the eradication of the New World screwworm in North Africa raises a number of unique questions and concerns. First, it is necessary to determine the factors influencing the marked seasonal fluctuation in population densities. During 1990, 95 myiasis cases were reported in January, 87 in February, 156 in March and progressively more each month, with 2915 recorded in October. From these figures it seemed likely that by midsummer 1991 the indigenous population could have increased to 20 million fertile flies. Since the number of sterile screwworms supplied from the rearing facility in Mexico was in the order of 40 to 100 million per week, it was essential that the campaign be functioning well by February 1991, when the wild population would be under the greatest seasonal stress and the highest ratio of sterile to indigenous flies could be achieved. Additionally, pupae from the first generation of adults would be emerging in synchrony after overwintering in the soil. This presented an opportunity to reduce the population much more sharply than during later periods, when all life stages of the generations normally overlap. Because of the significant reproductive potential of the pest, the chances of rapid success would have been reduced if the ratio of sterile to fertile flies were allowed to drop below about 10:1, and more intensive supplementary suppression efforts would have been needed to support the sequential release of sterile insects.

Failure to establish control over the screwworm population in the spring of 1991 would have greatly reduced the prospects for early eradication, and the programme would then have had to be prolonged until the unfavourable climate of the next winter had again suppressed the population. Were the pest to invade regions that do not experience prolonged winter temperatures of 10 to 15°C or lower, then the difficulty in achieving eradication would be exacerbated greatly. Accordingly, every effort must be made to achieve eradication in 1991, before the pest can expand its range significantly.

The major short-term research and development imperatives were identified by an FAD/IAEA workshop in Vienna in December 1990, and corresponding investigations were immediately undertaken (FAD/IAEA, 1990). These are as follows:

· Research was initiated to determine the influence of temperature, humidity, vibration and jarring through transatlantic shipment from Mexico on the quality of the insects received for dispersal in Libya and to determine the actions necessary to minimize or eliminate any deleterious effects recorded. The possibility that insects may suffer from the "jet lag" experienced by humans or from the time difference between the two continents is also being investigated. Regimens of chilling and oxygen deprivation are being investigated as possible means of prolonging the storage time of irradiated pupae and adults without reducing longevity and the ability of males to perform in the field. Simpler methods for packaging flies for release have also been investigated.

· Supplementary means of reducing adult populations developed in the United States and Mexico may need modification. For this reason, bait stations (controlled-release swormlure plus toxicant mixtures) will be evaluated in North Africa. Investigations are being undertaken to determine whether treatment of livestock with new formulations of insecticides can be used concurrently with the release of sterile males.

· Coumaphos is the only chemical compound currently recommended for use in North Africa. Additional cost-effective insecticides that do not repel adult screwworms should be tested against both wound-feeding larvae and gravid females.

As water is very scarce in North Africa, particularly during the winter months, dust formulations may be more appropriate than liquid preparations. Also dusts would not present the same problems of clean-up and disposal.

An injectable formulation of a systemic insecticide would be of great benefit, particularly for use at quarantine stations.

· To pre-empt any problems that may develop during the eradication campaign, it would be desirable to determine the degree of genetic and behavioural heterogeneity within the indigenous population. This information would assist in ensuring that variants are available among the sterile flies brought from Mexico that would have good mating compatibility with variants in the North African population.

Concluding comments

The application of the sterile male technique in attempting the eradication of a very mobile pest with a high reproductive potential requires all measures to be implemented with great thoroughness and timeliness. Margins for error are very small. For this reason, all involved in these operations must fully understand their duties, carry them out competently and efficiently, and feel responsible for the final outcome.

At least 95 percent of the cases of myiasis in the infested zone in the Libyan Arab Jamahiriya are caused by the New World screwworm. The remaining cases are caused mainly by the Old World screwworms Lucilia and Wohlfahrtia spp. These data support the view that the New World screwworm has presented an extraordinary danger to the people, livestock and wildlife of Africa.


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