For that, they can thank the scientists who at Maroua conducted the biggest field trial so far of a naturally occurring bacterium, whose spores harbour a toxin lethal to the larva of the mosquito Culex quinquefasciatus. Now they are optimistic about eradicating larvae from other tropical cities.
In many, such as Dar es Salaam, the mosquito is not just a nocturnal nuisance, but is also the carrier of a parasite that causes the debilitating disease filariasis. Urban disease carriers such as the mosquito are posing a growing threat in the tropics, where big cities are booming, and their control requires new ways of thinking now that the use of chemical insecticides is discouraged.
Because these insecticides can accumulate in groundwater, the soil and the food chain, biopesticides - such as the spores of the bacterium Bacillus sphaericus - are emerging as the preferred weapon; they are toxic to certain mosquitoes, but harmless to other insects, animals and humans.
'The bacterial spores contain two proteins that are themselves harmless,' says John Harris, product manager at Novo Nordisk, the Danish biotechnology company that supplied bacteria, free, for the Maroua trials. 'They are converted into a lethal toxin only when they encounter the exact combination of enzymes and pH found in the gut of the larvae of Culex mosquitoes. The toxin produced lacerates the gut wall and kills the larva.'
But showing that biopesticides work in the lab is one thing; success in the field is another. The Maroua trials were conducted by Dr Jean- Marc Hougard's team from the Pasteur centre in Yaounde (part of the French Scientific Research Institute for Development and Co- operation) under the World Health Organisation's tropical disease research programme. Dr Philippe Barbazan, one of the team, says the aim is to develop a 'battle strategy' - for a cheap, effective control method that can be used by local health officials with limited resources.
Maroua, an urban island separated from the nearest town by 200km of savannah, was the ideal testing ground. The mosquito thrives in polluted urban waterways but cannot survive in open savannah, which acted as a barrier and presented a secure environment for the trials.
First, the researchers found out the numbers, distribution and breeding patterns of the mosquitoes; then, last February, a team of 50 drove round Maroua, spraying a suspension of Bacillus sphaericus into the city's 30,000 cesspits and sewers, where Culex lays its eggs.
By starting in the dry season, the researchers gave themselves a head-start: mosquito numbers are at their lowest then, and the biopesticide is less likely to be diluted by rain. The toxin lasts longer than the life-span of the average female mosquito, so even the following generation of larvae can be affected.
The researchers ran a public information campaign in the schools and communities. Dr Barbazan says the people's fears disappeared, and everyone eventually agreed to have their sewers sprayed. When the mosquitoes began to disappear, the researchers became local heroes, with two daily spots on the local radio.
A few weeks into the trial, mosquito numbers had fallen by more than 50 per cent, and by June, more than 90 per cent, compared with the previous year, although they did begin to increase again in August when heavy rains washed away the biopesticide and refilled dried-out cesspits.
This time the researchers are spraying twice: the first, last November, was aimed at reducing mosquito numbers before the dry season; the second is planned for July, just before the start of the wet season.
If this double treatment works, they will hand over the programme to local health officials. Dr Barbazan says that householders may eventually be supplied with bottles of bacteria that they would add to their sewage systems several times a year.
The WHO is also testing the biopesticide in Brazil, Tanzania, Zanzibar, India and Sri Lanka. By eradicating Culex, it hopes to break the life cycle of the parasitic worm that causes filariasis, which affects 80 million people.
In the long term, biopesticides may prove to be more efficient and more environmentally friendly than chemical insecticides. The latter consist of simple, small molecules, and when insects develop resistance to one compound, they become resistant to all compounds in the same class; the former are complex, large molecules, and even if an insect develops resistance to one, a slightly modified molecule remains toxic.
Another biopesticide, Bacillus thuriengensis, is already being used against black fly in West Africa, as part of a WHO programme aimed at curbing river blindness. In rotation with chemical larvicides, the biopesticide slows and offsets the resistance of black flies to chemical insecticides.
Unfortunately, biopesticides are not yet adapted to fighting the Anopheles species of mosquito, which transmits the malaria that affects more than 270 million people worldwide. Anopheles lays its eggs in rain puddles rather than sewers.
Scientists are already trying to introduce the genes of Bacillus toxins into other organisms, such as algae, in an attempt to develop biopesticides with higher toxicity and longer lifespans, capable of attacking more species of insects.