Bulletin No. 25

The phasmatid problem in mountain ash forests of the central highlands of Victoria.  F.G. Neumann, J.A. Harris and C.H. Wood.  1977.  Forests Commission, Victoria.  43 pp.


Since the late 1950's the Forests Commission has been concerned about a build-up in extensive areas of mountain forest of large populations of the phasmatid Didymuria violescens, a serious primary insect defoliator of eucalypts in south-eastern Australia. Between 1968 and 1973 a study of the biology of this native insect was carried out in valuable stands of even-aged Eucalyptus regnans of 1939 origin, growing at 300 to 900 m elevation within the Victorian central highlands. The study revealed that D. violescens produces predominantly only one generation every two years because of dormancy (diapause) of the egg-embryo at an early nuclear stage and when it is almost fully grown. The eggs usually incubate for 18 to 20 months, and hatch mainly during November and December. The five nymphal instars and the adults of both sexes reside and feed high up in tree crowns from where the eggs are dropped, principally. between February and April, onto forest litter. The adults, particularly the ovipositing females, are much more destructive than the nymphs. As defoliation peaks in late summer and autumn, when starch concentrations in sapwood of the hosts have fallen to low levels following the flush of growth in early summer, regeneration of foliage tends to be difficult. Up to 40 per cent of trees, mainly subdominant, have died following a single complete defoliation, and over 80 per cent as a result of two severe defoliations within three years. Diameter growth in surviving trees has been significantly reduced or eliminated for at least two years after the second defoliation.

The first outbreak of the pest occurred near Powelltown in summer 1960/61. Since then, a non-contiguous outbreak pattern has developed, with infestations scattered in patches up to 500 ha throughout the E regnans resource. The infestations were most widespread in 1971, when 6 400 ha of forest was likely to be defoliated if control measures were not undertaken.

Early predictions of population levels of feeding stages in summer and autumn have been based on estimates during the previous two winters of population densities of viable eggs in litter. The maximum tolerable egg population density above which control should be implemented corresponds to about 108 000 feeding insects per hectare.

Control of post hatching stages has taken the form of aerial spraying of malathion, an organophosphate of short toxicological persistence. Research has indicated that in years with typical climatic conditions, control spraying should be completed within a five week period, commencing during the first week of January. Spraying at a later time would risk significant defoliation and allow tile insect to sustain its outbreak potential. For each control program the Victorian Pesticide Review Committee has approved the prescriptions for operational spraying. Total spraying costs have averaged $6.85/ha during the past five years.

An analysis of egg population densities has indicated a significant decline since 1971 of pest populations in sprayed stands, but not in unsprayed stands. The population of the 1976/77 generation is at a level where no control spraying is necessary. The effects of malathion on the non-target environment has been reviewed. It appears that at the low application rate prescribed, mammals, birds and plants are not endangered, and studies of the insect fauna below tree canopy have indicated no long-term detrimental effects on diversity and abundance.