Tweet
The turn of the screw worm: One of the world's worst pests is turning up in unusual places. If it is not stopped, the screw worm fly could devastate the livestock industry in North Africa and Australia
SCREW WORM FLIES are prodigious travellers. And in recent years their movements have taken on a new, and more urgent, significance. Hitching lifts on ships and planes, the American species of the fly has invaded North Africa; its Old World counterpart is just a short hop from Australia. Screw worm flies are not welcome visitors. They are one of the world's most damaging parasites of livestock: the maggots of the New World screw worm fly ate their way through more than $100 million-worth of American livestock a year in the 1950s and 1960s.
For the past 50 years, researchers have made huge efforts to develop ways to control the flies. Control programmes have worked well in North America, where the fly has now almost disappeared, but other countries have not been so lucky. In 1988, the American species of screw worm fly was accidentally introduced to Libya, where it has done enormous damage. During its first year in Libya, the parasite infested more than 200 people and 2000 domestic animals . Such an accidental introduction of the Old World species to Australia, which is only 110 kilometres from infested New Guinea, could devastate the livestock industry there.
Screw worm flies are one of a group of parasites that evolved to fill a particularly nasty niche in the animal world. When warm blooded vertebrates underwent their explosive diversification some 100 to 200 million years ago, many invertebrates evolved in concert, enabling them to exploit this new and growing supply of food. Some of the true flies (Diptera), the bot flies, warble flies and blow flies, developed maggot larvae that were able to invade and feed on the organs, tissues or ingested food of these animals, which provided a warm, sheltered and nutritious mobile meal.
The maggots of many species of these flies may do little or no damage to the host. In some cases the maggots may do good, by removing rotting tissue. But there are some that destroy healthy tissue, eventually killing the host. Among this last group are two species of screw worm flies, Chrysomya bezziana and Cochliomyia hominivorax.
These two species of screw worm flies are obligate parasites, that is, they cannot survive without a living host. The larvae feed only on the living tissues of such a host animal. This lifestyle probably evolved initially from less specialised flies with larvae that fed on decomposing organic matter. Then, in an intermediate evolutionary stage, the larvae probably began to scavenge on skin soiled with faeces and on diseased or ill- smelling wounds, usually in a relatively harmless fashion, cleaning up the rotting tissues within the wound. In the final evolutionary stage, the larvae began to invade healthy tissues, probably those in contact with the rotting ones - and the parasitism began to damage the host.
Adult female screw worm flies deposit batches of between 200 and 300 eggs around the edges of wounds in the skin. Even a tick bite or a small scratch from a thorn may be enough to attract an infestation. When the maggots hatch, they eat into the wound. Their feeding causes extensive tissue damage, typified by the formation of an increasingly cavernous wound, progressive liquefaction of the flesh, necrosis and L haemorrhage.
Wild mammals show some resistance to fly-strike. Domestic animals, especially cattle, are less hardy, which is why screw worm flies are of such importance economically. Humans are not immune to infestation, however, especially where hygiene is poor and livestock are heavily infested. The maggots of Chrysomya bezziana can infest any part of the body if there is a wound, but most often infest mucous tissues and other places where the skin is soft, such as the genitals, the nose, mouth, ears and eyes. In 1883, a Dr Richardson reported a case of human infestation by Cochliomyia hominivorax in the Medical Monthly of Peoria, Illinois. A traveller in Kansas was sleeping when a fly laid its eggs in his nose. The fly was probably attracted by a discharge of mucus. The first symptoms were those of a severe cold. As the maggots cut away through the tissues of the head, the patient became slightly delirious and complained about the intense misery and annoyance in his nose and head. The maggots finally cut through the soft palate, impairing his speech, and then invaded the eustachian tubes. Despite the removal of more than 250 maggots, the patient eventually died.
Cochliomyia hominivorax is known as the New World screw worm fly. Its natural range extends throughout the southern parts of the US and the warmer temperate and subtropical parts of Southern America from Mexico to northern Chile. The huge losses to screw worm in the 1950s prompted scientists in the US to start investigating the feasibility of releasing sterile male flies in an attempt to eradicate the pest. The technique involves exposing male insects to doses of radiation that induce lethal mutations in the flies' sperm, effectively sterilising them. Eggs fertilised by these sperm fail to hatch. If sterile males are released to the wild in large enough numbers, so that they outnumber the wild males, enough sterile flies will mate with wild females to effectively sterilise those females. Continued release of sterile males in large numbers will eventually drive the wild population to extinction.
The US Department of Agriculture began experiments in control in Florida in 1947 and in 1954 mounted the first full- scale operation on the island of Curacao, 80 kilometres off the coast of Venezuela. The plan involved sterilising male flies with X-rays and swamping the local population with about 500 flies per square kilometre each week. The wild flies were eradicated in 6 months. The next step was to try to rid the mainland of screw worm fly, beginning with the Florida peninsula in 1957 and, by stages, driving the flies southwards to the Mexican border.
By 1966 the whole of the US was free of screw worm fly. The Department of Agriculture has continued with its programme, pushing the fly down through Mexico and now trying to drive it from Central America towards the Darien gap in Panama. The breeding of Cochliomyia hominivorax for the sterile male release programme employs some 1500 people, who produce more than 300 million flies each week. There have been some setbacks, with a few new outbreaks, particularly in southern Texas in 1972 and 1976. Screw worm fly was brought under control again in 1979. The outbreaks may have been the result of the intense selection pressure generated by the massive scale of the release of irradiated screw worm flies, leading to rapid evolution of a strain of wild flies that no longer mated readily with the released males. Factory rearing the flies may have favoured the selection of a 'domesticated' strain unable to compete with wild males. Some scientists at the Department of Agriculture think that the series of warm winters in the 1970s made the problem worse. If this is so, the anticipated global warming caused by the greenhouse effect might make it hard to maintain control over the fly in North and Central America. The chances are that the range of the screw worm fly may even expand.
Transatlantic transport
In 1988, C. hominivorax turned up in Libya - the first time it had been seen outside the Americas. Livestock imported from Central or South America may have carried the infestation to Tripoli. Although essentially a subtropical species, the New World screw worm fly has established a breeding population in Libya. There are fears that it could spread across North Africa and through Egypt into Asia, and across the Sahara, via the Nile, into Central Africa. The fly could even reach Europe, although the climate is generally too cool for it to breed there. The warm and humid regions below the Sahara, on the other hand, provide conditions ideal for the fly and impossible for any attempt at control.
The Old World equivalent of C. hominivorax ranges from subSaharan Africa to India, Southeast Asia, Indonesia, the Philippines and the island of New Guinea. In Zimbabwe, it is second only to the tsetse fly as a pest of cattle. The Old World screw worm fly infests all sorts of domestic animals apart from cattle - water buffalo, sheep, goats, donkeys, horses, dogs and camels - as well as people. Yet nowhere is there any large- scale programme to eradicate the fly. Indigenous African cattle do not suffer as much as introduced varieties, because they clean accessible wounds by licking, and wild animals, which have co-existed with the fly throughout their evolution, seem to have some resistance to its effects. No one knows what effect a future meeting of the Old and New World flies in Africa might have, on each other or on their hosts.
So far, C. bezziana has not made its way to Australia, but the livestock industry there is rightly worried about its nearness . Papua New Guinea is separated from Australia by only 110 kilometres at the narrowest part of the Torres Straits. If screw worm became established throughout its potential range in Australia, it could cost the livestock industry as much as Pounds sterling 150 million a year.
The most likely route of accidental introduction to Australia is from ships returning to port after taking cargoes of livestock to areas infested with Chrysomya bezziana. Australia has elaborate quarantine procedures to prevent this. Nevertheless, inadvertent introduction is all too possible. In April 1988, nine dead screw worm flies were found in a light trap in Darwin, on the northern coast. Accidental introduction by humans cannot be ruled out either. In 1984, a 69-year-old Indian woman in Kuala Lumpur, Malaysia, went to her doctor for treatment for her ulcerated foot. The doctor diagnosed a 5- to 6-day-old infestation of Chrysomya bezziana. The woman had travelled on a commercial aircraft from Sri Lanka only 2 days earlier.
1: The alien that threatens two continents
IN THE early summer of 1988, scientists in Libya suspected that the New World screw worm had somehow infected Libyan animals. By July, researchers from the Natural History Museum, London, confirmed their suspicions. Scientists now fear that the parasite may spread from Libya to the Middle East and eventually to Asia, leaving a trail of diseased and dead animals, both domesticated and wild.
The screw worm's entire life cycle can be as short as three weeks; such reproductive efficiency coupled with the fly's prodigious powers of flight (females have been known to fly 200 kilometres in search of suitable animals to attack), makes it a formidable threat.
Although Africa has its own species of screw worm, it exists in an equilibrium that is much less damaging to its hosts. The voracity with which the parasite could establish itself in Africa is the same for any pest introduced to a new habitat that contains none of its natural enemies.
The New World screw worm is thought to have reached Libya with livestock imported from South America. According to the FAO, the parasite is confined to about 20 000 square kilometres, 30 kilometres south of Tripoli and 60 kilometres east of the Tunisian border.
The organisation estimates that, if allowed to spread, screw worm could infect 70 million head of livestock in the five North African countries. The cost of controlling the pest would exceed $250 million a year. The Sahara desert forms a natural barrier to the fly but it could be only a matter of time before infected camels, transported animals and meat or migratory wildlife take the parasite to subSaharan Africa.
Screw worm fly would have a devastating effect on the economy of nomadic herders, who have no experience of diagnosing or treating the disease and no resources to control it. Perhaps the greatest danger would be to wildlife. Once wild animals are infested there would be little hope of ever controlling the parasite.
While the parasite is confined to a relatively manageable area of Libya there is a chance of eradicating it using the sterile insect technique. For this to work, the numbers of flies must be brought down to a certain level by spraying insecticides.
But together with treating infected livestock with insecticides, stricter quarantine measures and training of local veterinarians, the technique provides the only hope of eradicating the fly from Africa.
In the middle of last year the governments of a group of Middle Eastern countries approached the International Fund for Agricultural Development for help in controlling the infestation. The FAO offered insecticides and IFAD offered to investigate the possibilities of biological control. With help from IFAD the UN Development Fund has now put together a package worth $2.8 million for a pilot programme to control screw worm in Africa. The pilot programme will work out the best way to package and deliver the sterile flies, which have to be transported to Libya from the Mexico/US Screw Worm Eradication Commission at Tuxtla Gutierrez, Chiapas, southern Mexico.
The FAO wants to eradicate screw worm fly by releasing 100 million sterile male flies a week for 40 weeks spread over two years. It is now looking for the $80 million such a programme will cost.
2: Australia mans the barricades
AUSTRALIA has spent almost 20 years preparing for the worst. It intends to be ready for the screw worm fly when it arrives. In 1973 the CSIRO, the government research organisation, established a unit to rear screw worm flies in Port Moresby, the capital of Papua New Guinea.
With a staff of five, Geoff Clarke and his assistant L Leane Regan can rear as many as 30 million flies a week. If there is an outbreak, the unit will be able to fly large numbers of sterile male flies to Australia to swamp the population of introduced flies and eradicate them as the Americans did with L Cochliomyia hominivorax.
Newly hatched maggots are reared in large aluminium trays containing polyester L matting soaked with a mixture of dried blood, dried milk, dried egg and water. After 6 to 7 days at 37 Degree C the maggots reach what is known as the wandering stage: in the wild they would drop out of the wound onto the ground but in the laboratory they crawl out of the rearing trays and fall into hoppers slung below.
The staff collect the larvae from the hoppers as they begin to pupate. After a week the adult flies emerge from the pupae and, after mating, the female lays eggs in tight, white clusters along the sides of wooden batons placed on top of a five- day-old mixture of blood, milk and egg to simulate the edge of a wound.
In the middle of last year Clarke began testing the effectiveness of releasing sterile males near a remote village called Safia in the Musa Valley, some 160 kilometres east of Port Moresby. The valley is isolated from surrounding areas by high mountain ranges. Around 4000 head of beef cattle graze on the native grassland, and the valley harbours a significant population of screw worm flies.
The experiment involved releasing L newly emerged flies sterilised at the pupa stage by 5000 rads of gamma radiation. Each week about 500 sterile males per square kilometre were dropped from a light aircraft. The team assessed the effectiveness of its programme by monitoring the percentage of eggs that hatch from samples of eggs collected from known cattle in the study area and by assessing the density ofL the adult population using fly traps.
The early results of this experiment looked promising, achieving 30 per cent sterility after releasing only nine batches of sterile males. In September, however, the trial was stopped because of a dispute between local people and the government farm, revolving around land L ownership, possibly inflamed by rumours of gold in the valley. Village elders issued death threats and eventually all scientific work at Safia had to be abandoned.
So far, the unit in Papua New Guinea has never been called into action. But as the outbreak of New World screw worm fly in Libya shows, the threat is very real.
Richard Wall is an ecologist and Jamie Stevens a parasitologist at the Department of Veterinary Medicine and the Tsetse Research Laboratory of the University of Bristol.
SCREW WORM FLIES are prodigious travellers. And in recent years their movements have taken on a new, and more urgent, significance. Hitching lifts on ships and planes, the American species of the fly has invaded North Africa; its Old World counterpart is just a short hop from Australia. Screw worm flies are not welcome visitors. They are one of the world's most damaging parasites of livestock: the maggots of the New World screw worm fly ate their way through more than $100 million-worth of American livestock a year in the 1950s and 1960s.
For the past 50 years, researchers have made huge efforts to develop ways to control the flies. Control programmes have worked well in North America, where the fly has now almost disappeared, but other countries have not been so lucky. In 1988, the American species of screw worm fly was accidentally introduced to Libya, where it has done enormous damage. During its first year in Libya, the parasite infested more than 200 people and 2000 domestic animals . Such an accidental introduction of the Old World species to Australia, which is only 110 kilometres from infested New Guinea, could devastate the livestock industry there.
Screw worm flies are one of a group of parasites that evolved to fill a particularly nasty niche in the animal world. When warm blooded vertebrates underwent their explosive diversification some 100 to 200 million years ago, many invertebrates evolved in concert, enabling them to exploit this new and growing supply of food. Some of the true flies (Diptera), the bot flies, warble flies and blow flies, developed maggot larvae that were able to invade and feed on the organs, tissues or ingested food of these animals, which provided a warm, sheltered and nutritious mobile meal.
The maggots of many species of these flies may do little or no damage to the host. In some cases the maggots may do good, by removing rotting tissue. But there are some that destroy healthy tissue, eventually killing the host. Among this last group are two species of screw worm flies, Chrysomya bezziana and Cochliomyia hominivorax.
These two species of screw worm flies are obligate parasites, that is, they cannot survive without a living host. The larvae feed only on the living tissues of such a host animal. This lifestyle probably evolved initially from less specialised flies with larvae that fed on decomposing organic matter. Then, in an intermediate evolutionary stage, the larvae probably began to scavenge on skin soiled with faeces and on diseased or ill- smelling wounds, usually in a relatively harmless fashion, cleaning up the rotting tissues within the wound. In the final evolutionary stage, the larvae began to invade healthy tissues, probably those in contact with the rotting ones - and the parasitism began to damage the host.
Adult female screw worm flies deposit batches of between 200 and 300 eggs around the edges of wounds in the skin. Even a tick bite or a small scratch from a thorn may be enough to attract an infestation. When the maggots hatch, they eat into the wound. Their feeding causes extensive tissue damage, typified by the formation of an increasingly cavernous wound, progressive liquefaction of the flesh, necrosis and L haemorrhage.
Wild mammals show some resistance to fly-strike. Domestic animals, especially cattle, are less hardy, which is why screw worm flies are of such importance economically. Humans are not immune to infestation, however, especially where hygiene is poor and livestock are heavily infested. The maggots of Chrysomya bezziana can infest any part of the body if there is a wound, but most often infest mucous tissues and other places where the skin is soft, such as the genitals, the nose, mouth, ears and eyes. In 1883, a Dr Richardson reported a case of human infestation by Cochliomyia hominivorax in the Medical Monthly of Peoria, Illinois. A traveller in Kansas was sleeping when a fly laid its eggs in his nose. The fly was probably attracted by a discharge of mucus. The first symptoms were those of a severe cold. As the maggots cut away through the tissues of the head, the patient became slightly delirious and complained about the intense misery and annoyance in his nose and head. The maggots finally cut through the soft palate, impairing his speech, and then invaded the eustachian tubes. Despite the removal of more than 250 maggots, the patient eventually died.
Cochliomyia hominivorax is known as the New World screw worm fly. Its natural range extends throughout the southern parts of the US and the warmer temperate and subtropical parts of Southern America from Mexico to northern Chile. The huge losses to screw worm in the 1950s prompted scientists in the US to start investigating the feasibility of releasing sterile male flies in an attempt to eradicate the pest. The technique involves exposing male insects to doses of radiation that induce lethal mutations in the flies' sperm, effectively sterilising them. Eggs fertilised by these sperm fail to hatch. If sterile males are released to the wild in large enough numbers, so that they outnumber the wild males, enough sterile flies will mate with wild females to effectively sterilise those females. Continued release of sterile males in large numbers will eventually drive the wild population to extinction.
The US Department of Agriculture began experiments in control in Florida in 1947 and in 1954 mounted the first full- scale operation on the island of Curacao, 80 kilometres off the coast of Venezuela. The plan involved sterilising male flies with X-rays and swamping the local population with about 500 flies per square kilometre each week. The wild flies were eradicated in 6 months. The next step was to try to rid the mainland of screw worm fly, beginning with the Florida peninsula in 1957 and, by stages, driving the flies southwards to the Mexican border.
By 1966 the whole of the US was free of screw worm fly. The Department of Agriculture has continued with its programme, pushing the fly down through Mexico and now trying to drive it from Central America towards the Darien gap in Panama. The breeding of Cochliomyia hominivorax for the sterile male release programme employs some 1500 people, who produce more than 300 million flies each week. There have been some setbacks, with a few new outbreaks, particularly in southern Texas in 1972 and 1976. Screw worm fly was brought under control again in 1979. The outbreaks may have been the result of the intense selection pressure generated by the massive scale of the release of irradiated screw worm flies, leading to rapid evolution of a strain of wild flies that no longer mated readily with the released males. Factory rearing the flies may have favoured the selection of a 'domesticated' strain unable to compete with wild males. Some scientists at the Department of Agriculture think that the series of warm winters in the 1970s made the problem worse. If this is so, the anticipated global warming caused by the greenhouse effect might make it hard to maintain control over the fly in North and Central America. The chances are that the range of the screw worm fly may even expand.
Transatlantic transport
In 1988, C. hominivorax turned up in Libya - the first time it had been seen outside the Americas. Livestock imported from Central or South America may have carried the infestation to Tripoli. Although essentially a subtropical species, the New World screw worm fly has established a breeding population in Libya. There are fears that it could spread across North Africa and through Egypt into Asia, and across the Sahara, via the Nile, into Central Africa. The fly could even reach Europe, although the climate is generally too cool for it to breed there. The warm and humid regions below the Sahara, on the other hand, provide conditions ideal for the fly and impossible for any attempt at control.
The Old World equivalent of C. hominivorax ranges from subSaharan Africa to India, Southeast Asia, Indonesia, the Philippines and the island of New Guinea. In Zimbabwe, it is second only to the tsetse fly as a pest of cattle. The Old World screw worm fly infests all sorts of domestic animals apart from cattle - water buffalo, sheep, goats, donkeys, horses, dogs and camels - as well as people. Yet nowhere is there any large- scale programme to eradicate the fly. Indigenous African cattle do not suffer as much as introduced varieties, because they clean accessible wounds by licking, and wild animals, which have co-existed with the fly throughout their evolution, seem to have some resistance to its effects. No one knows what effect a future meeting of the Old and New World flies in Africa might have, on each other or on their hosts.
So far, C. bezziana has not made its way to Australia, but the livestock industry there is rightly worried about its nearness . Papua New Guinea is separated from Australia by only 110 kilometres at the narrowest part of the Torres Straits. If screw worm became established throughout its potential range in Australia, it could cost the livestock industry as much as Pounds sterling 150 million a year.
The most likely route of accidental introduction to Australia is from ships returning to port after taking cargoes of livestock to areas infested with Chrysomya bezziana. Australia has elaborate quarantine procedures to prevent this. Nevertheless, inadvertent introduction is all too possible. In April 1988, nine dead screw worm flies were found in a light trap in Darwin, on the northern coast. Accidental introduction by humans cannot be ruled out either. In 1984, a 69-year-old Indian woman in Kuala Lumpur, Malaysia, went to her doctor for treatment for her ulcerated foot. The doctor diagnosed a 5- to 6-day-old infestation of Chrysomya bezziana. The woman had travelled on a commercial aircraft from Sri Lanka only 2 days earlier.
1: The alien that threatens two continents
IN THE early summer of 1988, scientists in Libya suspected that the New World screw worm had somehow infected Libyan animals. By July, researchers from the Natural History Museum, London, confirmed their suspicions. Scientists now fear that the parasite may spread from Libya to the Middle East and eventually to Asia, leaving a trail of diseased and dead animals, both domesticated and wild.
The screw worm's entire life cycle can be as short as three weeks; such reproductive efficiency coupled with the fly's prodigious powers of flight (females have been known to fly 200 kilometres in search of suitable animals to attack), makes it a formidable threat.
Although Africa has its own species of screw worm, it exists in an equilibrium that is much less damaging to its hosts. The voracity with which the parasite could establish itself in Africa is the same for any pest introduced to a new habitat that contains none of its natural enemies.
The New World screw worm is thought to have reached Libya with livestock imported from South America. According to the FAO, the parasite is confined to about 20 000 square kilometres, 30 kilometres south of Tripoli and 60 kilometres east of the Tunisian border.
The organisation estimates that, if allowed to spread, screw worm could infect 70 million head of livestock in the five North African countries. The cost of controlling the pest would exceed $250 million a year. The Sahara desert forms a natural barrier to the fly but it could be only a matter of time before infected camels, transported animals and meat or migratory wildlife take the parasite to subSaharan Africa.
Screw worm fly would have a devastating effect on the economy of nomadic herders, who have no experience of diagnosing or treating the disease and no resources to control it. Perhaps the greatest danger would be to wildlife. Once wild animals are infested there would be little hope of ever controlling the parasite.
While the parasite is confined to a relatively manageable area of Libya there is a chance of eradicating it using the sterile insect technique. For this to work, the numbers of flies must be brought down to a certain level by spraying insecticides.
But together with treating infected livestock with insecticides, stricter quarantine measures and training of local veterinarians, the technique provides the only hope of eradicating the fly from Africa.
In the middle of last year the governments of a group of Middle Eastern countries approached the International Fund for Agricultural Development for help in controlling the infestation. The FAO offered insecticides and IFAD offered to investigate the possibilities of biological control. With help from IFAD the UN Development Fund has now put together a package worth $2.8 million for a pilot programme to control screw worm in Africa. The pilot programme will work out the best way to package and deliver the sterile flies, which have to be transported to Libya from the Mexico/US Screw Worm Eradication Commission at Tuxtla Gutierrez, Chiapas, southern Mexico.
The FAO wants to eradicate screw worm fly by releasing 100 million sterile male flies a week for 40 weeks spread over two years. It is now looking for the $80 million such a programme will cost.
2: Australia mans the barricades
AUSTRALIA has spent almost 20 years preparing for the worst. It intends to be ready for the screw worm fly when it arrives. In 1973 the CSIRO, the government research organisation, established a unit to rear screw worm flies in Port Moresby, the capital of Papua New Guinea.
With a staff of five, Geoff Clarke and his assistant L Leane Regan can rear as many as 30 million flies a week. If there is an outbreak, the unit will be able to fly large numbers of sterile male flies to Australia to swamp the population of introduced flies and eradicate them as the Americans did with L Cochliomyia hominivorax.
Newly hatched maggots are reared in large aluminium trays containing polyester L matting soaked with a mixture of dried blood, dried milk, dried egg and water. After 6 to 7 days at 37 Degree C the maggots reach what is known as the wandering stage: in the wild they would drop out of the wound onto the ground but in the laboratory they crawl out of the rearing trays and fall into hoppers slung below.
The staff collect the larvae from the hoppers as they begin to pupate. After a week the adult flies emerge from the pupae and, after mating, the female lays eggs in tight, white clusters along the sides of wooden batons placed on top of a five- day-old mixture of blood, milk and egg to simulate the edge of a wound.
In the middle of last year Clarke began testing the effectiveness of releasing sterile males near a remote village called Safia in the Musa Valley, some 160 kilometres east of Port Moresby. The valley is isolated from surrounding areas by high mountain ranges. Around 4000 head of beef cattle graze on the native grassland, and the valley harbours a significant population of screw worm flies.
The experiment involved releasing L newly emerged flies sterilised at the pupa stage by 5000 rads of gamma radiation. Each week about 500 sterile males per square kilometre were dropped from a light aircraft. The team assessed the effectiveness of its programme by monitoring the percentage of eggs that hatch from samples of eggs collected from known cattle in the study area and by assessing the density ofL the adult population using fly traps.
The early results of this experiment looked promising, achieving 30 per cent sterility after releasing only nine batches of sterile males. In September, however, the trial was stopped because of a dispute between local people and the government farm, revolving around land L ownership, possibly inflamed by rumours of gold in the valley. Village elders issued death threats and eventually all scientific work at Safia had to be abandoned.
So far, the unit in Papua New Guinea has never been called into action. But as the outbreak of New World screw worm fly in Libya shows, the threat is very real.
Richard Wall is an ecologist and Jamie Stevens a parasitologist at the Department of Veterinary Medicine and the Tsetse Research Laboratory of the University of Bristol.
Comment