"FAO is backing a long-term project with national authorities to develop tsetse-free zones in economically productive parts of sub-Saharan Africa," he told Reuters.

But Hendrichs, an entomologist, said it could take international agencies and national authorities decades to wipe out tsetse flies from three economically important areas - the cotton belt in West Africa, the Southern Rift Valley of Ethiopia and Botswana.

Part of this area is composed of fertile land that is left uncultivated - a "green desert" abandoned by humans and cattle.

"Eradicating the tsetse - and with it trypanosomiasis, the disease it carries - would allow rural Africans to reclaim areas of their continent and greatly increase food production," FAO said on its web site http://www.fao.org yesterday.

Hendrichs had no estimate for the increase in food production that could result from eliminating the tsetse flies.

RADIATING FLIES

Male flies are sterilised through radiation and then released into the tsetse-infested area, where they mate, said Hendrichs, based at the International Atomic Energy Agency in Vienna.

But while the sterile males transfer sperm, the eggs in the wild female do not develop. With continuous release of sterile males, the reproductive rate of the whole population rapidly declines, leading to extinction.

The technique was successful in ridding the Tanzanian island of Zanzibar of the tsetse in 1997.

Hendrichs said that after using radiation, areas need to be monitored to ensure the insects do not return, and said nomads moving with herds were a risk for transmission of disease.

The biting tsetse fly transmits a deadly parasite, trypanosome, that attacks the blood and nervous system of its victims. It causes trypanosomiasis, known as "nagana" in livestock and "sleeping sickness" in humans.

Trypanosomiasis is one of the most devastating diseases in sub-Saharan Africa, killing 80 percent of infected victims. It can be treated with drugs, but many poor have no access to them.

Trypanosomiasis keeps people in afflicted regions poor by preventing them from producing food, Hendrichs said.

"Tsetse and trypanosomiasis are major impediments to the development of sustainable agricultural systems in the region, hitting the poorest of the poor," he said.

Of the 37 countries in sub-Saharan Africa infested with tsetse, 32 are among the poorest in the world, according to FAO.

Of 165 million cattle in sub-Saharan Africa, 155 million are in tsetse-free areas, such as the highlands or the semi-arid Sahel zone, leading to overgrazing by animals and overuse of land by people for food production.