COCONUT LEAF-MINING BEETLE
Promecotheca reichei Baly -- Chrysomelidae
Native to Fiji, the coconut leaf-mining beetle was originally under natural control by native natural enemies. However, in the 1880's as trade developed and coconuts became extensively cultivated, the beetle gradually became a serious pest (DeBach 1974). Larvae bore within the leaflets and adult beetles feed on the leaves externally. In heavy infestations they caused a reduction of ca. 75% in the leaf surface of most trees in a planting (Taylor 1936, 1937).
Detailed studies of the beetle and its natural enemies were not made until 1929 when sufficient entomological expertise became available. It was found that all severe outbreaks had one unusual feature in common: only one stage of the insect was present at any one time, and there were no overlapping generations. This was considered atypical in the tropical climate of Fiji where reproduction can continue year round. Studies showed that beetle outbreaks were due to the indirect adverse effects of a predatory mite, Pediculoides ventricosus (Newport), on two native parasitic species which were capable of controlling the beetle in its absence. The mite was probably accidentally introduced to Fiji, as it is now known to be cosmopolitan and feeds on a great variety of insects. This mite caused an even-brooded condition of the beetle and this resulted in the native parasitoids being rendered ineffective because they attacked only the larval stages and these were mostly lacking for long periods of time because of the even-brooded situation.
The mite attacked all larval stages and the pupae of the beetle but not adults or eggs. In dry seasons it increased with incredible rapidity and would destroy all the beetles except eggs and adults in a given locality. The adults laid eggs and died and this later resulted in the single-stage condition consisting principally of beetle larvae, but meanwhile the predatory mite population crashed because of food shortage when only adult beetles and eggs were present (DeBach 1974). With onset of the wet season the beetles increased very rapidly in the absence of significant parasitism and because the mite was inactive during wet periods. Dr. T. H. C. Taylor concluded that the mite would have replaced the indigenous parasitoids and done effective control had it not been decimated by wet weather each year. In areas of Fiji with continuous wet weather the beetle seldom became a serious pest, probably because the mite was held down and the native parasitoids operated without interference.
It was then considered that new parasitoids be found, those with a set of quite definite characteristics that could cope with the single-brooded condition. These included mainly (1) the parasitoid should have a more rapid rate of increase relative to the pest than the native parasitoids, and one generation must require no longer than one month, (2) adults of the parasitoid should be able to survive the long periods when no individuals of the pest were in a suitable stage for its attack, and (3) it should parasitize all larval stages as well as pupae of the host so that suitable hosts would be available over a much longer period in each pest generation than for a parasite species which attacked only one stage, and therefore the period during which the new parasitoid would lack suitable hosts would be correspondingly shorter. Also it would be good if the parasitoid were internal, very active and capable of rapid dispersal and tolerance of the climatic conditions in Fiji.
No candidate parasitoids with the above characteristics were known at the time , but in 1930 R. W. Paine's investigations in Java resulted in the discovery of a large number of parasitoids of a related species of Promecotheca (P. nuciferae Maulik). This beetle occurred throughout Java but was never a pest. Taylor followed up with detailed studies of the Javanese parasitoids and concluded that only one of the parasitoids, Pleurotropis parvulus Ferriere, met the conditions desired in a new parasitoid, even though this species was considered to be one of the least important of the complex controlling its host in Java. Pleurotropis parvulus attacks all stages of larvae as well as the pupae of Promecotheca, and many individuals develop in each host individual giving it a high potential rate of increase with respect to the host. The life cycle is only about three weeks long as compared to almost 3 1/2 months for the beetle in Fiji and more importantly, the adult parasitoids live for 5 1/2 weeks which enables them to survive periods when suitable stages of the host are rare or absent.
The parasitoids were shipped from Java to Fiji in six large wire-gauze cages each containing four seedling coconut palms heavily infested with Promecotheca nuciferae of all stages. Parasitoids were included in the cages and were able to reproduce during the voyage. Taylor left Batavia, Java on April 17, 1933 on the S. S. Van Rees which traveled from Singapore to Sydney via Java, New Guinea, New Hebrides and New Caledonia. It was necessary to trans-ship at Noumea, New Caledonia on May 10 to the S. S. Kareta in order to go on to Suva, Fiji, where he arrived on May 14. Before arrival in Noumea (4 days from Fiji) all palms, soil, containers, etc. were thrown overboard to eliminate any potential pest hazards, after all parasitoids and parasitized larvae and pupae of the host had been placed into glass tubes. From Suva it was necessary to proceed at once on a chartered motor launch to the Lau group of islands where beetle problems were severe. They arrived at Nabavatu on Vanua Balavu on May 24 (37 days out of Java), where an insectary were set up. Some 1,200 adults and pupae of P. parvulus arrived in good condition. Taylor also imported the two parasitic species considered most important in Java as an additional precaution. Only one, Dimmockia javanica Ferr., was successfully introduced, cultured and liberated in sufficient numbers, but it failed to establish.
The first liberations of P. parvulus were made on May 26, 1933 and were continued to April 30, 1934, when so many parasitoids were present in the field that insectary rearing was no longer necessary. Within a year it had completely controlled all the severe outbreaks of Promecotheca on Vanua Balavu, Kanacea, Taveuni, Mago, Lakeba and others. Taylor recorded that it literally attained 100% parasitism on all trees, even though in many outbreaks every three over hundreds of acres of land bore about 4,000 beetle individuals. At the first peak of the parasitoid explosion, about 5,000 adult parasitoids were emerging per tree daily and this continued for about 10 days (the parasitoid is gregarious). Not a single Promecotheca individual escaped in many outbreak areas and the parasitoid dispersed so well that it controlled the pest even on isolated coconut trees hidden in the forest (DeBach 1974).
As it turned out the parasitoid had the disadvantage of being too effective. On small islets or very isolated spots of coconuts it would exterminate the pest and then die out itself. This required reintroduction of the parasitoid if the beetle reinvaded. However, this never occurred on larger islands or estates, and today the coconut leaf-mining beetle is rare and of no economic importance. DeBach (1974) considers this biological control project to be unique in the annals of biological control, because preselection concentrated on a parasitic species that was considered of lesser importance in its place of origin.
REFERENCES: [Additional references may be found at: MELVYL Library ]
DeBach, P. 1974. Biological Control by Natural Enemies. Cambridge University Press, London & New York. 323 p.
Taylor, T. H. C. 1936. The biological control of the coconut leaf miner (Promecotheca reichei Baly) in Fiji. Agric. J. Fiji 8: 17-21.
Taylor, T. H. C. 1937. The biological control of an insect in Fiji. An account of the coconut leaf-mining beetle and its parasite complex. Imp. Inst. Ent., London. 239 p.