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.

Intrasexual Aggression in the Stick InsectsDiapheromera Veliei and D. Covilleae andSexual Dimorphism in the Phasmatodea

“…In 9 other phasmids known to exist consistently or occasionally at high densities (numerous enough to cause defoliation of trees or at least 5 individuals occupying a bush or refuge), the mean male length/female length ratio is .814 (Wattenwyl and Redtenbacher 1908;Bedford 1975;Lea 1916;Gurney 1947;Wegner 1956;Stockard 1908;Hetrick 1949;Severin 1911;Key 1970;Paine 1968;Wilkins and Breland 1951). Over 3/4 of the complete sample is more sexually dimorphic (p .2201) (see Fig.…”

The worldwide status of phasmids (Insecta: Phasmida) as pests of agriculture and forestry, with a generalised theory of phasmid outbreaks

“…This observation was confirmed by Paine [26]: "Outbreaks seem to occur on certain islands, in certain situations, and not always very frequently, so that while over the whole range of this insect the loss of copra [the dried kernel of the coconut used to extract oil] resulting from its ravages is inconsiderable, losses to individual planters and village communities can be serious". This slow spread is also reported for Didymuria violescens outbreaks in Australia [27].…”

“…Paine [26] gives details of an outbreak on the island of Taveuni (Fiji group) which "began to reach serious dimensions in 1958, and by April 1959 100 acres of coconuts were affected. By the end of 1961 the outbreak had extended over 500 acres in which the older fronds were at least 50 per cent defoliated and nearly 400 palms had been killed".…”

“…The Fijian Ministry of Primary Industries recommends a number of biological and mechanical methods of controlling G. crouanii in Fiji [46]. Recommended biological controls are the egg parasite wasps Paranastatus nigriscutellatus Eady, 1956 and P. verticalis Eady, 1956. Paine (1968 gives the level of parasitism by these species from unpublished reports from Hinckley (10 %) and Pillai (52 %).…”

“…Paine [26] also reports of an unsuccessful attempt to control this species by establishing populations of the tachinids Mycteromyiella letifica and M. phasmatophaga. One of the tachinid flies (Diptera) thought to control Graeffea leveri, Mycteromyiella laetifica on the Solomon Islands were imported to Fiji to try and control this species.…”