Investigations for the biological control in Fiji of the coconut stick-insect Graeffea crouanii (Le Guillou)

Abstract

The stick-insect Graeffea crouanii (Le Guillou) is a pest of coconuts of local and sporadic importance in the south Pacific and there have been recent outbreaks on Taveuni Island, in the Fiji group. As there appear to be virtually no parasites of the nymphal stages, a preliminary search was made in 1960 for parasites of other palm-feeding Phasmatids in Melanesia. This revealed the presence of Tachinidae parasitising species of the genera Ophicrania and Megacrania, and in 1963–64 these were studied in the Solomon Islands. The Tachinidae comprise at least two species of Mycteromyiella: M. laetifica (Mesnil) attacking both O. leveri Günth. and a species of Megacrania in the western Solomons, and M. phasmatophaga Crosskey attacking 0. leveri and some other Phasmatid hosts on Guadalcanal. The early stages of both species of Mycteromyiella are briefly described and compared, with notes on their bionomics. There was no evidence of any egg parasite attacking Ophicrania in the Solomons.O. leveri, which is very closely related to G. crouanii, has never caused significant damage to coconuts in the Solomon Islands, except on the small island of Savo, from which its Tachinid parasites appear to be absent. It is concluded that Mycteromyiella, especially M. laetifica, which appears to be fairly specific, may be an important factor in the control of O. leveri in the Solomons.The principal hosts of O. leveri are the sago palm (Metroxylon salomonense) and species of arecoid palms in the forest, on all of which the insect has better scope for concealment than on coconuts. Collections of nearly 6,000 examples of O. leveri from sago palm on Kolombangara island, in the western Solomons, in August 1963-February 1964 showed an average parasitism by Mycteromyiella laetifica of 28 per cent. Eggs of the Tachinid are laid on nymphs of all instars as well as on adults. The host-survival rate was about 30 per cent, for nymphs and 50 per cent, for adults.A small number of parasitised specimens of O. leveri from Kolombangara were released on Savo, but there was no evidence six months later that M. laetifica had become established there.Breeding trials at Honiara showed that O. leveri could be reared successfully in captivity but not M. laetifica, which shows reluctance to mate in cages and has a narrow range of environmental tolerance in the pupal stage, in which a mortality of at least 70 per cent, seems unavoidable under the conditions practicable for transportation of this stage by air.Between October 1963 and March 1964 nearly 960 puparia of M. laetifica were sent to Fiji. About half of them were used for breeding trials, which showed that the Tachinid could be reared through G. crouanii in captivity but could not be maintained. The rest were released on Taveuni, but a further outbreak of G. crouanii during 1965 yielded no evidence that M. laetifica had become established.Material of M. phasmatophaga, which has a more restricted choice of environment than M. laetifica, but also a somewhat greater potentiality for killing its host, was obtained by exposing O. leveri on seedling coconut palms planted in the forest at Honiara. Quantities were insufficient for transmission to Fiji; 150 parasitised hosts were released on Savo but samples of O. leveri collected there six months later gave no indication of its establishment.Despite this initial failure, it is considered that Mycteromyiella could bring about the control of G. crouanii in Fiji and other affected Pacific islands, and the means by which this might be achieved are discussed.