Insect pests - a SITting target?
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| Credit: FAO |
A cheaper and safer alternative to chemical control has proved highly effective against several key insect pests. The Sterile Insect Technique (SIT), pioneered in the USA and advanced by the Joint FAO/IAEA Division in Vienna, has achieved considerable success in the control of New World Screwworm, tsetse and fruit flies. By appreciating that SIT is a more environmentally friendly way of dealing with insect pests, it is hoped that more government authorities and the private sector will adopt this nuclear technique as a new tool in integrated area-wide eradication campaigns. Even where eradication is not absolute, the reduction in pest populations is usually more effective, often less costly, and the product (in case of fruit, etc.) is more valuable than is the case with current chemical control. And there is no adverse impact on the environment.
The success of the technique lies in the regular release by air of large numbers of sterile male flies into the wild. The males are irradiated with gamma radiation in fly factories, which renders them sterile but otherwise leaves them reproductively active. Sterile males releases are initiated, either when the pest insect population is at its seasonal lowest density, or once the wild population of the pest has been reduced to relatively low numbers by conventional techniques, such as trapping. Then the large numbers of sterile males released out-compete wild males for females and, as no offspring result from these sterile matings, this ultimately brings about a reduction in the insect population. The technique only affects the reproduction of the targeted pest (e.g. tsetse or fruit fly) so this biological control technique, unlike chemical control, is species specific and therefore has no effect on beneficial insects (pollinators) or natural enemies (predators, etc.).
The Mediterranean Fruit Fly (Ceratitis capitata)
The economic consequences of the damage caused by this pest is so great that countries free of medfly (USA, NZ, Chile and Japan) prohibit the import of fresh produce from countries where the pest occurs. SIT has already been used, or is currently being used, to successfully eradicate medfly in the EU, North and Central America, Japan, Chile, Argentina and Australia resulting in substantial savings to the economies of these countries. With advice and assistance from the Joint FAO/IAEA Division, a pilot project has now been launched in the Hex river valley in the Western Cape Province of South Africa - a region that supports a major fruit-based industry.
At least 19 varieties of fruit in the region (including deciduous fruit, table and wine grapes and a number of citrus crops) are attacked by two species of fruit fly: the Medfly (C. capitata) and the Natal fruit fly (C. rosa). Control is currently over 20 million Rand per year but essential: fruit is not only an important part of local diets, especially for subsistence farmers, but the deciduous fruit, wine and citrus industries also provide important employment in the region. A large proportion of fruit, such as table grapes, are also exported and increasingly many cartons of grapes are rejected by EU inspectors finding flies in infested fruit.
By realising that SIT is a more environmentally friendly way of dealing with this insect problem, it is hoped that all growers in the Hex valley area will work together to overcome it. The outcome will then be more effective, and much cheaper, than current chemical control. It is hoped that the project will not only reduce fruit fly populations and resulting damage to crops but also increase opportunities for export to other countries. On this basis, the Joint FAO/IAEA Division is hoping that the project will spread to neighbouring regions of S. Africa and to neighbouring countries so that area-wide control will be achieved.
IAEA and FAO have also supported successful fruit fly SIT projects in Chile, Costa Rica, Guatemala, Mexico, Peru and Portugal. Over the last few months work is progressing on the development of an SIT programme of medfly control in Israel, Jordan and Territories under the Jurisdiction of the Palestinian Authority. IAEA and FAO have also supported the development of more effective SIT technology, in particular genetic methodology that allows the mass production of only sterile males which, when released in the absence of sterile females, are much more effective in competing with wild males for wild females. Such sterile male only releases have successfully been used recently in California and Florida to eradicate medfly outbreaks.
The continuing presence and advancement of tsetse into new agricultural systems limits livestock production in almost two-thirds of sub-Saharan Africa. But, in certain regions, it may now be possible to control or even eradicate this pest using the Sterile Insect Technique, which has achieved notable success in completely eradicating Tsetse in Zanzibar. The Joint FAO/IAEA Division, which provided technical support for the IAEA Model project in Zanzibar, is now confident that tsetse are no longer present on the island. In spite of thorough entomological and veterinary monitoring activities no wild tsetse fly was captured since August 1996 and no trypanosomes were found since September 1997.
Steps are being taken for the introduction of the SIT in the Southern Rift Valley region of Ethiopia - an area of high agricultural potential. The current challenge to further agricultural development in this area is Glossina pallidipes, which has migrated into the area. Moreover, the species is still on the move, and is beginning to adapt to higher altitudes. In Ethiopia, most of the cattle are kept on the high plateau and the Ethiopian government is extremely anxious to prevent G. pallidipes from reaching these higher lands. If tsetse could be eradicated from the region, farmers could move their cattle from the densely populated higher altitudes of the plateau to the valley zone, and thus reduce overgrazing in the environmentally fragile highland areas.
Glossina swynnertoni, another species of tsetse, is also causing problems in mainland Tanzania. With assistance from the European Union, Tanzania is currently establishing a national priority concerning the control/eradication of tsetse. The World Bank also appears to be willing to support the adoption of the existing technology, which was used in Zanzibar, to tackle this particular species in the border area between Kenya and Tanzania. The collection of relevant baseline data which will be instrumental for the planning and decisions of any subsequent intervention strategy is scheduled for 1999.
West and southern Africa also suffer from disease spread by these flies. In Mali, for example, Glossina palpalis gambiensis, a species that previously fed on the blood of crocodiles and lizards has changed its host preference. This tsetse has invaded the newly created irrigation schemes in the peri-urban areas of Mali and it now survives on the blood of humans and animals. Cattle are already suffering from nagana, sleeping sickness in livestock, and it is feared that sleeping sickness disease will soon appear in humans. In southern Africa Glossina morsitans centralis requires the annual investment of US$ 1.5 million, just to keep the status quo of control, and authorities in Botswana are determined to eradicate the species.
However, it is imperative that the SIT is applied as part of an integrated area-wide concept which, contrary to the selective field-by-field control, is a more holistic approach that targets the entire pest insect population within existing natural or sustainable artificial boundaries. Efforts are under way to scale up the system for tsetse fly mass production to a semi-industrial level. In the first half of 1999 prototypes of a new semi-automated Tsetse Production Unit ("TPU-2") are scheduled to be delivered by FAO/IAEA to African insectaries in Addis Ababa (Ethiopia), Bobo-Dioulasso (Burkina-Faso), Muguga-Nairobi (Kenya) and Tanga (Tanzania) for testing and methods adaptation. Industrial scale production has already been achieved for other insects including the Mediterranean fruit fly (see above).
New World Screwworm (Cochliomyia homivorax)
Once endemic throughout the W. Hemisphere, Screwworm has now been eradicated from the USA, Mexico and much of Central America using SIT. In 1988, when the pest was found in Libya, an urgent national and international effort was mounted to prevent its spread to livestock and wildlife in the rest of Africa and the Mediterranean Basin. The SIT campaign was successful in achieving eradication, preventing the enormous losses which would have occurred if the infestation had spread.
Campaigns to eliminate the pest from the rest of Central America and the Caribbean continue. SIT releases will be initiated on Jamaica in mid 1999, and NWS is expected to be eradicated from the island by 2001. Preparations for shifting the SIT eradication operations to neighbouring Caribbean islands, especially Hispaniola and Cuba, are underway.
An international effort to contain and eventually control an outbreak of the Old World Screwworm (Chrysomya bezziana) in the Gulf states, with a focus in Iraq and southern Iran, is currently being launched by AOAD, FAO and IAEA. The feasibility of an SIT component is among the intervention options being explored.
