The idea of controlling insects by sterilising technique was first conceived by E.F. Knipling as far back as 1937 in a laboratory in Texas, USA. While studying the biology of the screwworm fly, Cochliomyia hominivorax (Coquerel), he observed that the female fly mated only once (monogamous).
From this, he argued that if the males of this fly could somehow be sterilised, the females would fail to produce viable eggs and the insect’s population could thus, be controlled. While entomologists were looking for sterilising agents to test Knipling’s idea, geneticists were accidently sterilising insects in their own search for chemical mutagens immediately after World War II.
E.F. Knipling conceived a new approach for insect control in which the pest’s natural reproductive processes would be disrupted by physical or chemical means. This concept, as it was applied to the screwworm fly starting in the early 1950s, eventually came to be known as the sterile insect technique (SIT), or the sterile insect release method (SIRM). Interestingly, about the time that Knipling was formulating his concept of insect control, a Russian geneticist, A. Serebrousky, suggested the use of chromosomal translocations to reduce reproduction in harmful species.
Studies of insect reproduction, from the early 1900s to the present time, have demonstrated that the insects treated with certain mutagenic chemicals (chemosterilants) or x-ray or gamma radiation, are unable to produce a normal number of living progeny.
Treated insects (which may be completely or only partially sterile) are released in large number into a field environment and are expected to mate with the normal insects, thus interfering with reproduction. If the matings between treated insects and normal insects are successful, reproduction of the field populations will be disrupted, and the population will decline.
1. Chemosterilants:
Any chemical that can inhibit the growth of gonads or interfere with the reproductive capacity of an insect in any other way (such as prevent copulation and production of viable eggs, induce dominant lethal mutations, inhibit development of progeny at any stage (egg, larval or pupal) is called a chemosterilant. Chemicals that are currently employed as chemosterilants can be grouped as alkylating agents, antimetabolites and miscellaneous compounds.
1. Alkylating Agents:
These compounds were discovered by LaBrecque in 1961 in the USA.
They contain several groups of aziridinyls (aziridines or ethylenimines) and can be classified into three categories- Apholate, aphomide and aphoxide whose examples are described below:
i. Apholate:
Chemically, this compound is 2, 2, 4, 4, 6, 6-hexakis (1-aziridinyl) 2, 2, 4, 4, 6, 6- hexahydro-1, 3, 5, 2, 4, 6-triazatriphosphorine. It is a white odourless and crystalline substance, partially soluble in water and organic solvents (alcohol, chloroform, and acetone), stable in cool dry condition and affected by humidity and high temperature.
ii. Aphomide:
Chemically, this compound is N, N’-ethylenebis, [p, p-bis (1-aziridinyl) N”- methylphosphinic amide],
iii. Aphoxide:
Chemically, this compound is tris (1-aziridinyl) phosphine oxide, also called TEPA. It is a crystalline, colourless and odourless substance, extremely soluble in water, and highly soluble in alcohol, ether and acetone and hygroscopic and unstable. Its other derivatives are METEPA, phosphine oxide, tris (2-methyl 1-aziridinyl) and thio-TEPA, (phosphine sulphide, tris (1-aziridinyl). The former is a straw coloured liquid completely soluble in water and solvents, and the latter, a white crystalline solid relatively less soluble.
2. Antimetabolites:
Antimetabolites are chemicals, which are structurally related to biologically active substances. Due to their structural similarity, the biological system fails to distinguish them from their own natural substances and commits the mistake of utilising them in place of the latter.
For example, if bacteria are grown in a medium containing 5-fluorouracil, the latter will replace a large percentage of the normal metabolite, uracil, in the RNA of the former. Three groups of antimetabolites are more important: purine compounds, pyrimidine compounds and folic acid compounds.
3. Miscellaneous Compounds:
Under this are the chemosterilants which are structurally unrelated.
They are of the following categories:
i. Non-alkylating analogues of aziridinyl compounds:
These chemicals were discovered in an attempt to find safer chemicals. They are HEMPA (hexamethylphos-phoramide), a dimethylamine of TEPA and HEMEL (hexamethylemelamine) (Fig. 10.1), a dimethylamine analogue of tretamine, both being male housefly sterilants.
ii. Organotin Compounds:
Numerous triphenyltin derivatives such as triphenyltin hydroxide, triphenyltin chloride (chlorotriphenyltin), triphenyltin acetate (acetoxy triphenyltin), (Fig. 10.1) bis (triphenyltin) sulfide, alkyltriphenyltin, etc., have found to be reproduction inhibitors in the house flies. They sterilise both males and females, but the females are more sensitive because they are affected at a much lower concentration than the males.
Field Trials:
Agricultural Pests:
Not much effort has been made to control agricultural pests by chemosterilants. Most of the work that has been carried out, relates to cage experiments performed in various laboratories. Such experiments have been tried with lepidopteran insects like cabbage looper, Trichoplusia ni (Hubner); pink cotton bollworm, Pectinophora gossypiella (Saunders), etc.
However, some actual field trials have been attempted in California on Mexican fruit flies (with TEPA) and in Alabama on the cotton boll weevil (with apholate). In spite of the success in the case of the former, chemosterilants were abandoned in favour of irradiation for the sake of safety. The latter could not meet with complete success due to insufficient isolation of the treated areas.
Public Health Pests:
The house flies were the first to be tried with TEPA mixed in food bait and used in a half area refuse dump in Florida in 1962. Nine weekly treatments brought the insect population down. Measurements were done by counting the number of flies landing per minute on a grid 45 × 18 cm kept at ten scattered localities. However, the population soon increased because the area was not isolated.
Attempts to control house flies have also been made in Italy (1964), Sicily (1967) and japan (1968). While some success was achieved, the problem of reinvasion from the nearby untreated areas was always there to annul the effects of the treatment. Evidences that the house flies are getting resistant to chemosterilants are now coming in.
Testse flies were chemosterilised in Zimbabwe and 98 per cent control was achieved but complete eradication could not be possible due to reinvasion from the adjoining areas. Mosquitoes (Aedes, Culex) were also put to chemosterilant treatments. The first field release of chemosterilised mosquitoes was made in Seahorse Key islands off the coast of Florida in 1968, another, in Kenya (1971), and then in India at villages Bamnauli (1971), and Dhulsiras (1972), near Delhi. Controls were only partial again due to the lack of isolation of these places.
2. Ionising Radiations:
Sterilisation of insects by irradiation can be brought about by exposing them to ϒ-radiation (gamma-radiation or alpha and beta particles), X-rays and neutrons. Of these, ϒ-radiation by 60CO with its half-life of 6 years is the most common method of sterilisation. In India, sterile insect technique for red palm weevil, Rhynchophorus ferrugineus (Olivier) has been developed at the Bhaba Atomic Research Centre (BARC), Trombay.
Use of ionizing radiation has been the main method of inducing sterility in mass-reared insects for area-wide IPM programs for the past five decades, because of the mutagenic or teratogenic effects of chemosterilants which leads to human health and environmental issues especially the integrity of ecological food chains, and chances of development of resistance in insects.
Other advantages of the irradiation are: insects can be irradiated inside packaging materials, the sterile insects may be released immediately after the irradiation, radiation does not leave residues that could be harmful to humans or the environment, temperature rise in the irradiation process is usually insignificant.
Millions of sterile insects per week for national Area- Wide Integrated Pest Control programs or making research on SIT against screwworms, fruit flies, moths and tsetse flies are being produced at 41 different facilities worldwide (www(dot)dirsit(dot)iaea(dot)org).
Radiation doses differ for different species and even for different developmental stages of the same species. A high dose will provide complete sterilisation (i.e., all generations produced by the sterilised insects will be sterile), while lower (sub-sterilisation) dose will provide a partial sterility which may not continue beyond the F1 generation.
However, higher doses adversely affect longevity and competitiveness and, therefore, partial rather than complete sterility are preferred (atleast in the cases of pests, affecting costly crops or those of more serious diseases), so that some control (atleast up to Fi generation) could be achieved. Competitiveness being more crucial for males, allowing residual fertility (partial sterility) in males and complete sterility in females, will maximise the impact in a control (not eradication) programme.
In every insect species, a radiation dose that does not adversely affect longevity and competitiveness has to be determined separately by experimentation because such a dose differs for different species. The more detailed information on doses of radiation applied for insect disinfection and sterlization of mites and insects are available with International Database on Insect Disinfection and Sterlization (IDIDAS).
Concept of Knipling’s Technique:
The concept of E.F. Knipling’s technique involves two systems, viz. sterile male technique and sterile insect technique. Although both the procedures envisage the common principle of sterility, they greatly differ in the manner in which populations are affected.
1. Sterile Male Technique:
According to this technique, if fully competitive sterile males are released in the natural (i.e., wild) populations, it will reduce the reproductive potential of the natural populations in the same ratio as sterile to fertile. If, for instance, the sterile to fertile (S:F) ratio is 1:1, then according to the law of chances, both sterile and fertile males will have an equal (fifty-fifty) chance of mating with the wild females.
The matings with the sterile males will be infructuous (eggs will be non-viable or sterile), which will mean that the reproductive capacity of the natural population is reduced by 50 per cent. In other words, a control of 50 per cent has been achieved. Thus, if the S: F ratio is increased to 9:1, then the control (atleast theoretically) will be 90 per cent. This technique is now synonymous with the sterile insect release method (SIRM), wherein both males and females, mass reared and sterilised in the laboratory, are released at every generation until the pest is eradicated or controlled.
2. Sterile Insect Technique:
This technique envisages the sterilisation of a portion of the wild population itself (both male and females) by chemosterilants.
The effect then will be two-fold:
(i) The percentage sterilised, cannot reproduce and, in effect, will amount to killing the same proportion of insects (same as in the sterile male technique) and additionally,
(ii) The sterilised males and females of the wild population will, in turn, nullify the reproductive capacity of a proportionate number of the fertile individuals in the population by competing with them. This method, in a way, combines chemical and biological control methods- the effect of the chemosterilant will be chemical, and that of the sterilised insects, biological.
Thus, if 90 per cent of the given population is sterilised, this percentage cannot reproduce, which will amount to 90 per cent killed (first effect). Of the remaining fertile 10 per cent, only 1 per cent be expected to have fertile mating because of the existence of 90 per cent sterilised insects in the population (second or bonus effect, as termed by Knipling).
Thus, a total of 90 + 9 = 99 per cent of the insect population, will fail to have fertile mating, amounting to 99 per cent control. An added effect produced by released insects on the natural population, could be to exert an additional pressure on their struggle for existence by competing with them for food and shelter. Besides, this method does not cause hazards to man and his environment.
Knipling’s SIRM Model:
The SIRM modified model of Knipling has been illustrated in Table 10.2. The assumed number of insects in the wild population is 1000, and that of the sterile insects released in each generation, is 2000.
If for an assumed natural population of 1000 insects (1000 males and 1000 females), 2000 fully competitive males are released in each generation, the ratio of sterile to fertile (S: F) in the population will be 2:1. This means that only one-third of the natural wild females can be expected to mate with the wild fertile males and two-thirds with the released sterile males resulting in 1000 × 2/3 = 66.7 per cent infertile mating.
Thus, only 333 progeny of each sex will be produced instead of 1000. In the next generation, the release of 2000 sterile males will result in a 6:1 sterile to fertile ratio and only one-seventh of the 333 wild fertile females will be expected to mate with the fertile males, producing 47 progeny and six-seventh with the released sterile males resulting in 100 × 6/7 = 85.7 per cent infertile matings.
The release of 2000 sterile males in the third generation will raise the S:F ratio to 42:1, resulting in only 1 progeny which, in the fourth generation, will be eliminated because of its having 2000 times greater chance of mating with the sterile than with the fertile males.
The aforestated model, assumes a constant insect population in each generation, which is not the case in nature. In each generation, the insect population tends to increase. And though the potential rate of increase of most insect populations is very high, casualties due to various factors (such as parasitism, predation, natural hazards), bring the net increase down to a lower rate. According to Knipling’s own estimate, an insect population tends to increase five-fold in each generation.
Therefore, to offset (neutralise) this increase, a constant number of sterile insects has to be released every generation. Since the control brought about by the sterile insect is more than the annual increase, a time comes when the sterile to fertile ratio becomes so disproportionate that, there may not be any fertile matings, resulting in complete control or eradication.
The significant feature of the SIRM is that, each release of sterile insects achieves an increasingly higher sterile to fertile ratio and thus becomes progressively more efficient, which is an advantage over the insecticides, where the kill in each generation remains constant. The results shown in Tables 10.3 and 10.4 will bear this out.
When both the tables are compared, it becomes obvious that while it takes eight generations to eradicate the insects by the insecticide control system, it takes only four by the SIRM, the latter getting progressively more efficient.
Eradication of Screwworm Fly:
The screwworm fly, Cochliomyia hominivorax (Coquerel) was the first insect pest that was put to this method of control. This is a muscid fly parasitising on warm-blooded animals from the southern parts of the USA to South America and certain West Indian islands.
The females of this insect lay eggs in skin lesions where the larvae hatch and feed for 5-6 days before falling off to the ground for pupation in the soil. Due to the severity of infestation, the animals die. Before 1958, the estimated loss due to this pest used to be to the tune of $ 12 million a year.
The monogamous habit of this insect led Knipling in 1937 to develop his sterile male and sterile insect techniques. The first successful attempt to control this insect by the release of irradiated insects was made in 1952 on a small island named Sanibel, 3.2 km off the west coast of Florida (USA).
Irradiated flies at the rate of 200 (100 of each sex)/2.59 km2 (each 2.59 km2 is estimated to have 10 insects) were released once a week. After two weeks, 80 per cent of the egg masses sampled proved to be sterile and after three months, the fly population was almost zero. However, the fly could not be eradicated completely because of the island’s proximity to the uncontrolled mainland.
Subsequently, spectacular success was achieved in the eradication of screwworm on the 440.3 km2 Island of Curacao during 1954-55. The males sterilised at a dose of 2500r were released at the rate of 400 per 2.59 km2 per week. By concerted efforts, screwworms were eradicated from Florida (eastern coast) by 1960, from Texas and New Mexico (mid-mainland) by 1964, and from Arizona and California (west mainland) by 1965. To neutralise the effect of invasion from untreated neighbouring areas, barrier releases (i.e., additional releases) between the treated and untreated zones were carried out.
In 1972, the population of the screw worm fly increased beyond the effective management by the sterile fly releases.
Some of the reasons could be:
i. Unusually favourable conditions for winter survival of screwworms in northern Mexico, south of the sterile-fly release area and in the sterile fly barrier.
ii. More favourable weather conditions in the spring and summer in the south western United States.
iii. Relaxation in animal husbandry practices which result in the breeding of animals virtually the year round (earlier, livestock owners followed breeding practices to assure the birth of most calves, lambs and other livestock when screwworms were absent or scarce).
iv. Failure of livestock owners to follow other animal management practices such as minimal surgical operations and careful surveillance of animals during the fly season to look for and treat infested animals before larvae can mature.
v. A general increase in deer populations throughout the normal range of the screwworm.
vi. An upsurge in the Gulf Coast tick, Amblyomma maculatum Koch, which is one of the principal natural predisposing causes of screwworm cases in livestock.
vii. Changes in the behaviour and competitiveness of the released sterile flies because of genetic deterioration.
viii. Changes in the competitiveness of flies due to modifications in the rearing procedures.
ix. Changes in the behaviour of native population through genetic selection pressure making the adults prone to avoid matings with the released strain.
The situation, however, improved in 1973 to 1975 because the sterile fly production rate was doubled to about 200 million per week. New strains of screwworms were established to release flies that were not maintained as laboratory cultures for long. Other modifications of the programme were also made including better distribution of sterile flies.
Dr Edward F. Knipling and Dr Raymond C. Bushland were awarded the 1992 World Food Prize for developing eco-friendly SIT to control or eradicate insect pests that threaten vast sources of food, especially livestock and wild life populations.
The following requirements must be met before developing and applying SIT for pest suppression:
i. Practical procedures must be developed for rearing enough insects to overflood the natural population.
ii. Methods of inducing sterility or strains possessing appropriate genetic defects must be available.
iii. Reasonably accurate estimates of absolute numbers of the insects in the natural population are essential to determine how many insects must be reared for release, considering the relative competitiveness of the released and native insects.
iv. The released insects must be distributed so that they will be in reasonable spatial competition with the natural population for mating.
v. Information on the normal rate of increase of the natural population is desirable as a guide to the rate of over flooding required to achieve the necessary results.
vi. The degree of infiltration of the target pest and its impact on the effectiveness of the technique must be considered.
vii. The numbers of released insects required for control or elimination must not be unduly hazardous to crops, animals or man.
viii. A critical analysis of the candidate insect pests, including costs, effectiveness and ecological effects of available alternative control methods, is essential in appraising the value of SIT as a replacement or supplement for other methods of control.
3. Hybrid Sterility:
Hybrid sterility refers to sterility that occurs when certain strains, races, or closely related species are crossed and either one or both sexes of F1 progeny are viable but cannot produce viable progeny. In hybrid sterility, mating is induced between closely related species under laboratory conditions.
If such mating leads to fertilization, the hybrid individuals may die in the embryonic stage, in one of the pre-adult stages or early in the adult stage. Alternatively, apparently normal adults produced may show partial or complete sterility in one or both the sexes.
The various defects in the mating between such insects is either an imperfect copulation due to structural differences in their genitalia or if the copulation is perfect, it may lack insemination and if inseminated, the fertilization may not take place and even if fertilized, the individuals may die before hatching, any time between larval and adult stages or they may even survive, but only as hybrid adults.
The hybrid adults may look quite normal externally or may even show heterosis (i.e., hybrid vigour with increased longevity, increased sexual aggressiveness, increased competitiveness, etc.), but one or both sexes will be partially or completely sterile or will have defective gametogenesis resulting in premature death of the progeny at any stage of their development.
This phenomenon has been investigated and used in the management of several pest species, (e.g., between species of the tsetse flies, Glossina morsitans Westwood and G. swynnertoni Austen, between races of the gypsy moth, Lymantria dispar (Linnaeus), and between the moth species Heliothis virescens (Fabricius) and H. subflexa (Guenee).
The hybridisation of H. virescens and H. subflexa, stimulated interest in reducing field populations of H. virescens, using released backcross insects. In an experiment on St. Croix, US. Virgin Islands, using both male and female backcross insects, sterility were infused into a field population with release ratios of 20 sterile backcross: 1 feral insect.
The frequency of sterile male progeny increased for one generation after release, and the distribution of backcross frequencies became homogeneous throughout the population. During a 6-week period, 94 per cent of trapped males were sterile progeny from released or field-reared backcross females and the native males. Isolated populations of H. virescens, probably can be eradicated, using this method, given a sufficient number of released hybrid individuals.
Many attempts have been made to produce sterile hybrids from other pest species. For example, Helicoverpa zea (Boddie) has been mated with Helicoverpa armigera (Hubner) from Australia, Russia, and China and H. assulta (Guenee) from Pakistan and Thailand. These hydridisation attempts failed to produce a sterile hybrid.
Similarly, no measurable hybrid sterility has been found in crosses between the pink bollworm from areas within the United States, Mexico, Puerto Rico, or St. Croix. There was no incompatibility between a strain of the pink bollworm from southern India and two strains (one long-term laboratory strain, the other a newly colonised strain) from Arizona.
Success of SIT in Genetic Control:
The SIT and other genetic methods are species specific and have no negative impact on the environment. But their complexity puts them beyond the reach of individual farmers and the programmes have to be planned, organized and implemented by the government agencies. Perhaps due to this reason, hardly any SIT programmes have been undertaken in Asian countries where farms are very small and large-scale, centralized pest control strategies are difficult to implement.
The only exceptions to this are fruit fly, Bactrocera cucurbitae (Coquillett) eradication programme undertaken extensively in Japan from 1972 onwards and the Bactrocera dorsalis (Hendel) suppression programme in Taiwan from 1975 onwards. On the other hand, Mediterranean fruitfly, Ceratitis capitata (Wiedemann) eradication programme was a huge success in Mexico and other Central American Countries.
For a number of other pests including onion fly, Delia antiqua (Meigen); European cockchafer, Melolontha melolontha (Linnaeus); European cherry fruit fly, Rhagoletis cerasi (Linnaeus); coding moth, Cydia pomonella (Linnaeus) and grape moth, Eupoecilia ambiguella (Hubner), large scale programmes are in operation in a number of European countries.
Many notable trials have been carried out to date and some selected examples are discussed herein:
1. Screwworm Fly:
The first major use of SIT was for the eradication of the primary screwworm from the island of Curacao in 1954. Following this success, the pest was then eradicated from the southeastern United States in 1959 and from Puerto Rico in 1975. Eradication of the screwworm was initiated in the south-western United States in 1962 and completed in 1982.
Because of the extensive damage that the screwworm causes in Mexico, and because it was impossible to prevent reinfestation into the United States, a Mexican-American Commission was created in 1972 to eradicate the pest from northern and western Mexico and to establish a “sterile fly barrier” at the Isthmus of Tehuantepec.
In 1986, the commission extended its eradication activities to the Yucatan Peninsula and countries of Central America. As a result of this programme, Mexico, Belize, and most of Guatemala are now free of the screwworm. Operations are in progress to eradicate the screwworm from Honduras and El Salvador.
The present goals of the programme are to eradicate the pest from Central America and Panama and to establish a sterile fly barrier at the Darien Gap to prevent its reinfestation. Also, efforts will be made to eradiate the screwworm from Caribbean islands which are still infested.
The screwworm invaded North Africa in the 1980s and became established in the Libya. After detecting the screwworm in 1988 and confirming its presence in 1989, a joint decision was made by the Food and Agriculture Organization/International Atomic Energy Agency (FAO/IAEA) and the government of the Libya to initiate an eradication campaign using the sterile insect technique.
Sterile pupae were transported from the Mexico-US Commission production plant at Tuxtla Gutierrez in Mexico, emerged in Libya, and released over a treatment area of 40,000 km2. Between 1990 and 1992, 1300 million sterile insects were shipped from Mexico to Libya for release. The campaign was implemented late in 1990 and completed in October of 1991. North Africa was declared free of the screwworm in June 1992.
2. Fruit Flies:
Genetic control programmes using the SIT have been successful against several species of fruit flies (Diptera: Tephritidae). Limited field application of the SIT has resulted in population suppression or eradication of the Caribbean fruit fly, Anastropha suspensa (Loew) in Florida; cherry fruit fly, Rhagoletis cerasi (Linnaeus), in Switzerland; Oriental fruit fly, Bactrocera dorsalis (Hendel), in the Mariana Islands; Queensland fruit fly, Bactrocera tryoni (Froggatt); and Chinese citrus fly, B. minax (Enderlein), in China.
However, areawide use of the SIT has been successful in the eradication or control of several species of fruit flies. The most spectacular successes have been with eradication of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), from southern Mexico, and of the melon fly, B. cucurbitae (Coquillett) from Japan, and the prevention of Mexican fruit fly from invading California and Texas.
After the first detection of the Mediterranean fruit fly in Mexico, the Ministries of Agriculture of Mexico and Guatemala and the United States Department of Agriculture, formed the Moscamed programme to combat this pest. The objectives of the programme were to stop the northern advance of the pest, to eradicate it from southern Mexico and Gautemala, and in the long-term, to eradicate it from Central America and Panama. By combining the discrete use of Malathion bait spray with the release of sterile flies, the medfly was eradicated from Mexico in 1982. The programme has also been successful in Guatemala.
The Japanese government initiated a project to eradicate the melon fly in 1972. Using the SIT, following one or more treatments of a lure/toxicant, the eradication effort began on Kume Island and was expanded until the melon fly had been eradicated from all the infested islands of the Kagoshima and Okinawa Prefectures. Japan was declared free of the melon fly in 1992. Although the eradication programme required an investment of about $100 million, the benefits from eradication of the melon fly should be over $100 million per year.
3. Pink BoIIworm:
The pink bollworm, Pectinophora gossypiella (Saunders), is a serious pest of cotton in many parts of the world. Although the pink bollworm is occasionally recovered from states east of Texas, this pest is most destructive in Arizona, southern California, and the adjacent northwest Mexican desert.
After its introduction and establishment in central Arizona in the mid 1950s and in the Colorado River Basin of western Arizona, southern California, and northwestern Mexico in 1965, a programme was initiated in 1968 to protect the 500,000 ha of cotton in the San Joaquin Valley.
This programme has prevented the establishment of the pink bollworm through the use of sterile insect- release technology, minor use of pheromones as a mating disruptant, and adequate cultural control. The California cotton industry considers the 27-year old San Joaquin Valley Exclusion Project to be a major success.
4. Codling Moth:
Codling moth, Cydia pomonella (Linnaeus) is the key pest of most apple and pear growing areas in the world. A sterile insect release programme to eradicate codling moth was initiated in 1992 in the Okanagan region of British Columbia, Canada, by the year 2000. The programme was jointly developed and implemented by the British Columbia Fruit Growers’ Association, the provincial government of British Columbia and the federal government of Canada.
The whole programme was divided into three distinct phases, viz.:
(i) A pre-release sanitation phase was designed to reduce wild populations to the maximum possible by using cultural control methods and insecticide applications for two years,
(ii) The second phase involved mass-rearing and release of sterile moths for three years.
(iii) After eradication, the third phase was intended to protect against reinfestation by monitoring for the presence of wild moths, releasing sterile moths at border sites to prevent the invasion of wild moths, and controlling the transfer of infested fruit containers. A mass-rearing facility was built which produces 15 million moths per week. After several years of operation, and inspite of some initial failures, most growers in the programme area report no damage and no longer have to spray against codling moth.
5. Tsetse Flies:
Tsetse flies of the Genus Glossina represent a major threat to agricultural development in Sub-Saharan agriculture as they transmit protozoan parasites of the Genus Trypanosoma, which cause a debilitating sickness in livestock and sleeping sickness in humans.
The first genetic control attempts of any insect were conducted with this species in the 1940s, where hybrid sterility between the three subspecies of G. morsitans Westwood group was used. Over 100,000 field collected G.m. centralis Machado pupae were released into an isolated population of G. swynnertoni Austen over a 7 month period.
The sterility generated led to the replacement of the latter species by the former. Due to the arid conditions, G. m. centralis population rapidly disappeared and the area became tsetse free. Large scale field trials using the release of sterilized males have been successfully carried out in Nigeria and Burkino Faso.
However, in both cases reinvasion occurred when the programmes were terminated and the tsetse free areas were recolonized. Recently, the same technique has been used with complete success to eradicate G. austeni Newstead from Unguja Island, Zubland, and Republic of Tanzania. The island has now been declared tse tse free and is free to develop its agriculture without the threat of trypanosomosis.
6. Onion Fly:
Onion fly, Delia antiqua (Meigen), is the single insect pest of onions in the temperate regions of the world. A small commercial SIT programme started in 1981 is currently operating in the Netherlands for the control of this pest. This is probably the only truly commercially run programme of its kind in the world. About 400 million flies are produced annually and are used for the control of the pest on about 2600 ha of onion, representing about one sixth of the Dutch onion crop.
The programme is technically very successful but suffers from the poor farmer uptake and in some way the selfish behaviour of a minority of farmers who shy to benefit from sterile flies released on their neighbour fields. This situation illustrates the need that all potential beneficiaries participate in such area-wide programmes. Given the right political and social support, the programme could be expanded to cover the whole of the Dutch onion crop.