ISLAND CONSERVATION-The menace of the coconut whitefly |16 July 2007
It was some time in May this year that I realised something was seriously wrong. “I’ve cut off the lower leaves of our koko rouz palm and burnt them,” my father announced when I visited him at La Rosière. “Something very nasty has been attacking them, and the palm seems to have just seized up – it’s no longer giving any coconuts at all.”
I rushed outside to see for myself. There was a naked look about the palm, now that its lower fronds had been lopped off. But even the remaining upper leaves had a scorched look about them – they were coated with the black sooty mould that I had seen on banana and other plants hit by the spiralling whitefly. I wondered if that could be the culprit. But various pressing commitments prevented me from investigating the matter further.
A week or so later, on May 26, while I was on Silhouette attending a meeting of the board of the Island Conservation Society (ICS), a fellow board member who works in the Ministry of Environment broke the bad news: “We have a new problem,” he announced. “The coconut whitefly is now on Mahé.”
I was stunned. So that was it. We were dealing not with a case of the spiralling whitefly adapting to coconut palms as a new host, but with a new species of bigay blan that specifically targeted palms! With endemic palms almost everywhere I looked on Silhouette, I was acutely aware of what could be the consequences if the coconut whitefly started feeding on our lantannyen, palmis and kokodmer palms as well. I remembered reading that the insect, Aleurotrachelus atratus to scientists, had been described and named from specimens collected on coconut palms in Brazil at the beginning of the 20th century and was now spreading around the tropics and subtropics, from southern and central America, Florida in the United States and the Caribbean to Uganda in Africa and Samoa in the Pacific.
By 1996 it had reached Réunion, where very soon it began infesting the endemic palmistes as well as coconut palms. Some time close to 2000 it got to the Comoros, causing so much damage to coconut palms there that a team of French scientists reported: “Les cocotiers observés en Grande Comore dans la région proche de Moroni révèlent un état sanitaire très préoccupant. Les arbres portent des couronnes de feuilles très sombres, les feuilles n’ont plus leur aspect brillant, la charge en noix est très réduite et on l’estime au quart de la production normale (10 noix/couronne/arbre au lieu de 40 noix/couronne/arbre).” The coconut whitefly is now known to have spread to Mayotte, Mauritius and Madagascar as well; last year it was reported for the first time in Hawaii.
Just over a month after the first reports of this new ecological threat in Seychelles, there was a public appeal from the Ministry of Environment and Natural Resources. A preliminary survey had shown that the coconut whitefly appeared confined to Mahé, but apart from coconut palms it had also been observed on the endemic stilt palm or lantannyen lat. As a French expert from Réunion embarked on a more detailed study, the Ministry called on everyone to “be on the lookout for the coconut whitefly, as it is potentially a very serious threat to the coco de mer and other endemic palms”. It appealed to the public “to be careful when transferring plants in Seychelles and to avoid, wherever possible, the movement of palms”.
A few days later the French entomologist, Nicolas Boroweic, confirmed our worst fears: “La population d’aleurode du cocotier est en augmentation et cet insecte nuisible qui s’attaque à nos palmiers, le coco de mer inclus, risque d’infester d’autres îles,” the Seychelles Nation reported. “Selon le consultant, il y a une forte infestation d’aleurodes à Mahé, surtout sur la côte ouest et dans la nord … A Praslin… un foyer a été localisé vers la jetée de Baie Ste Anne.”
What to do now? The use of insecticide is, of course, out of the question – it would be difficult, costly and probably not effective, and would be too dangerous for the natural environment and for human health. Another approach could be biological control: the introduction of a natural enemy that destroys the coconut whitefly by feeding on it or parasitising it. In Réunion Nicolas Boroweic and his colleagues have been studying two tiny wasps whose larvae feed on the immature stages of the whitefly. A similar “parasitoid” wasp has been found in Hawaii. Before any such agent of biological control is introduced we have to be quite sure that there is very little risk of the wasp itself having any harmful impact on our biodiversity. Or should we restrict the movement of plants and plant products from one island to another – to prevent the pest from getting to the outer islands, for instance? Some very difficult decisions for the authorities to take!
It is claimed that, in the Comoros, “the economic incidence (of coconut whitefly damage) is substantial, with lost earnings for producers estimated at 3 to 5 million euros”. One is reminded of the situation in Seychelles in the 1920s and 30s, when the coconut plantations were hit by an infestation of bernik or scale insects. The British colonial authorities brought in the biologist Desmond Vesey-Fitzgerald, who introduced several species of koksinel or ladybird, including the common black one, to feed on the scales.
“There is every reason”, Vesey-Fitzgerald felt confident enough to write in 1953, “to suppose that the established predators will act as an insurance against any other coconut-feeding scale that may in the future become accidentally established in the Colony reaching epidemic status.”
How could he have known, back then, that the next onslaught on our pye koko – no longer a major economic crop but a vital tourism emblem – would be by an alien invasive species belonging to a different group of insects, against which the koksinel would have no effect?
The Island Conservation Society promotes the conservation and restoration of island ecosystems.