The insect attacks the fronds and feeds on the palms' sap, excreting a honeydew on which sooty mould fungi develop. This is what causes the blackish colouring seen on the upper side of the fronds on affected palms. The whitefly is a curse for these islands covered in coconut palms, whose nuts play a vital role in Comoran society.
Until now, there has not been any way of controlling whiteflies in the Comoros. A specific research programme, aimed at identifying and introducing a biological control agent in the region, was launched in 2005 by CIRAD and the Agriculture, Fisheries and Environment Research Institute (INRAPE) in the Comoros*. The programme has just been completed.
Over two years, the researchers met their target: they identified a parasitoid in Réunion that was specific to the coconut whiteflies found in the Comoros. It is a species of Eretmocerus whose taxonomy is currently being described.
Within eight months, whitefly larva densities were cut by between 12 and 73%, depending on the island
After checking that Eretmocerus nr sp. preserved other whitefly species that might be found in the Comoros, notably endemic species, the first parasitoids were released in early 2007 on Ngazidja. Three hundred females collected in Réunion were introduced in a confinement cage, placed on a highly infested coconut palm around two metres tall. The female parasitoids are attracted by their host and lay their eggs under whitefly larvae on the fronds of coconut palms. As they hatch, the parasitoid larvae pierce the testa of the whitefly larvae and eat them from the inside, leaving just an empty mummy behind.
Eight months after the releases, the researchers checked that the Eretmocerus nr sp. had acclimatized to these semi-natural conditions, via the experimental cage on Ngazidja, and to natural conditions on Nzwani. The phytosanitary condition of the coconut groves in the Comoros had improved significantly: whitefly larva densities had been cut by 12% on Ngazidja, 62.5% on Nzwani and 73% on Mwali. Production had increased, which resulted in a drop in coconut prices, to the benefit of consumers.
The main objective set for the programme, viz. to bring whitefly population levels below a harmfulness threshold using a natural parasitoid, was thus achieved. However, the "whitefly-parasitoid" balance needs to be monitored for the first few years after acclimatization, to confirm the success of the operation and measure the increase in coconut palm growth and coconut production on the three islands.
* The biological control programme was conducted within the Crop Protection Network for the Indian Ocean (PRPV).
Helen Burford | alfa
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