Bt Coconut Bombs Strike Malaria Vector

Peruvians will just go nuts-even coconuts-over a new low-tech method to control the malaria-carrying Anopheles mosquito. A coconut is the key to this idea, along with the spore-forming bacteria Bacillus thuringiensis var. israelensis H-14 (Bti H-14), which kills mosquito larvae when ingested. The two elements were combined to form a low-technology biocontrol of the mosquito larvae that is inexpensive and suitable for local production and application. This is the work of researchers at the Instituto de Medicina Tropical Alexander von Humboldt in Lima, Peru, led by microbiologist Palmira Ventosilla, and supported by IDRC.

Malaria is a major health concern in most developing countries and poses a formidable threat in Peru. A third of the country is at high risk of contracting the disease, which is considered endemic in most regions, infecting one person in 1,000.

Insecticides have long been the method of mosquito control in Peru. But chemicals are costly and can cause havoc in the local ecosystem. Mosquito resistance to chemical control is growing, as is the incidence of malaria. For economic reasons a country-wide chemical control campaign is no longer feasible.

Bti, which was discovered 20 years ago, is commercially available to control mosquitoes. Bti produces a cytolytic protein during spore formation. The mosquito larvae ingest the toxic protein which destroys the stomach lining, killing them. However the commercial product not appropriate for many situations. Its production is costly, requires trained personnel for application, and its effectiveness in the field is short-lived.

The Lima team began looking for an alternate incubation medium for Bti in 1988. They sought some type of produce or grain that would serve this function, and the coconut was a natural choice. The coconut, in copious supply in Peru, furnishes a virtual mini-lab. Coconut water provides the culture medium, and its hard shell offers both an innovative fermentation vessel and storage container.

The efficacy of Bti depends upon the conditions under which it is cultured. Coconut water is a very effective medium for sporulation. Total growth of the bacteria in coconut water in agitated flasks at 37íC and in coconuts at room temperature is comparable to that in common culture media. The concentration reached is approximately 1.2x109 bacteria per ml, with 6.5% as spores. As a fermentation medium, coconut water was three times more effective than the conventional medium.

The technology involved is fairly simple. A hole is drilled into the coconut shell, Bti is inserted, and the hole is plugged (see photo). This is allowed to ferment for two to three days, while the spores multiply. Then the coconut is broken over a larvae-infected body of water, and the entire contents are thrown in. Two or three coconuts are used, depending on the size of the pond.

In field trials Bti-filled coconuts killed all mosquito larva and stopped all larval growth for up to 45 days. Field trials show that Bti spores are viable in natural ponds for 21 days, and upto 25 days in warmer areas.

Bti incubation in coconuts promises to be an effective, safe, economical and fairly simple vector control, but the difficulty of technology transfer to the village level must not be underestimated. The team has developed a field kit for village use, containing Bti-saturated swabs and cotton plugs. Still, villagers must be trained to use this technique. Ventosilla has applied for a grant to study village sociodynamics, to design a more effective transfer.

Of Mosquitos and Coconuts, IDRC Reports, Vol 19(1) April, 1991. p 17-19.

For more information:

Palmira Ventosilla
Instituto de Medecina Tropical Alexander Von Humboldt
Universidad Peruana Cayetano Heredia
A.P. 5045, Lima 100
Fax: (51-14) 823-404