Spurlegged Phasmatids in Victoria’s forests

Forest Information Sheet - Forest Health June 2006

Factors Associated with Outbreaks Outbreaks of Spurlegged Phasmatids tend to occur in higher locations, even though the also occurs in Background lower elevation forests of mixed eucalypt species. The Spurlegged Phasmatid violescens The eggs of the insect are incubated in the moist litter (Leach) is a native phasmatid (stick insect) found of the forest floor. The deeper and wetter the litter is, mainly in the mountain forests of south-eastern such as at higher elevations, the greater the rate of Australia. While it depends entirely on the foliage of egg survival as they are protected from drying out eucalypt trees for nutrition and water, preferably and from foraging predators, such as birds and ants. Mountain Ash () and Alpine Ash A cool autumn and cold winter with heavy rainfall and (Eucalyptus delegatensis), it generally causes little fog drip, along with frequent frosts and occasional long-term damage or harm to trees or forests. Large snow cover, provide optimum survival rates of eggs outbreaks, however, occasionally occur that can by keeping the forest litter moist during most of the completely defoliate trees and pose a serious threat to year. The absence of fire over a period of years also Victoria’s mountain forests. contributes to greater egg survival. Mountain Ash is the world’s tallest flowering plant, Outbreaks of Spurlegged Phasmatids and the reaching heights of more than 100 metres, and Alpine subsequent extensive defoliation of eucalypts have Ash is the world’s second tallest. In Victoria, they are generally occurred in even-aged forests of between found in the Central Highlands region, in Gippsland, twenty and sixty years. and the Otway and Strezlecki Ranges. Mountain Ash and Alpine Ash play a vital role in the biodiversity of Victoria’s natural assets, providing essential habitat to important birds and mammals such as the Lyrebird, and the State’s emblem, the Leadbeater’s Possum. Both species can produce epicormic growth (sprouting of new growth from the trunk or branches) when under stress or defoliated. Both, however, only regenerate from seed, not from coppice or lignotubers (new shoots from stumps or roots). The potential Figure 1: Didymuria violescens - the Spurlegged Phasmatid impact of damage from Spurlegged Phasmatids in large outbreaks is therefore great, as the trees have limited ability to recover. Spurlegged Phasmatids were first described in 1814, Biology of Spurlegged Phasmatids though little historical data exists on population The lifecycle of Spurlegged Phasmatids is usually outbreaks and their impact on Mountain and Alpine biennial (two years) although this can vary between Ash forests since. The first significant outbreaks were one and four years (Figure 2). recorded in the 1950’s in south-eastern New South This biennial lifecycle produces large (peak) Wales and in the Kiewa Valley of north-eastern populations alternating with small (trough) Victoria. More extensive outbreaks followed in populations. In certain instances, two distinct peak Victoria’s Central Highlands region during the 1960’s and trough populations may exist in the same area, and early 1970’s. leading to situations where a peak population in one group is followed in the next year by a peak More recently, small outbreaks have been observed population in the second group. This leads to around Tumut in southern New South Wales, and in continual defoliation of trees over a prolonged period, north-east Victoria during 2001, when forests which means they may be unable to recover, and will containing Mountain Ash and other species of die over the next few years. eucalypts in the Mount Pinnibar, Upper Ovens and Kiewa catchments were defoliated (Figure 5). Forest Information Sheet - Forest Health June 2006 Spurlegged Phasmatids in Victoria’s forests

The female usually lays her eggs in the litter of the forest floor from late summer through to late autumn, with peak egg production occurring in early autumn. Mating is not necessary, and unfertilised eggs will always produce female Spurlegged Phasmatids. The eggs have a significant incubation time of 18 to 20 months. They hatch between spring and early summer, and the are initially light green and soft bodied. Immediately after hatching, Spurlegged Phasmatids nstinctively begin moving across the forest floor and climb the first vertical object in their path. To survive, they must reach eucalypt foliage within one to three days. Most spend their entire lives in the first eucalypt canopy they reach, although some males may glide down in search of a mate. Figure 3: Dorsal view of a male adult (A) and a female adult (B) Spurlegged Phasmatid. Note the wider abdomen and shorter antennae of the female compared with the male (Neumann et al. 1977)

Effects on Forests Spurlegged Phasmatids cut characteristic crescent- shaped indentations into leaf margins when feeding. Young Spurlegged Phasmatids feed only on soft immature foliage and new shoots, and tend to spend time eating the entire leaf before moving on. In contrast, adults can feed on much tougher mature leaves and tend to be more wasteful, cutting off large leaf pieces that drop the ground. This means when outbreaks occur, the immature outer periphery of trees are eaten first, with older foliage following.

Figure 2: One and two year life cycles of Spurlegged Phasmatid (Neumann et al. 1977)

It takes just two weeks for Spurlegged Phasmatids to move through five distinct instars (growth stages) before reaching adult stage. This is when they can become most destructive (more on that later). Adult Spurlegged Phasmatids usually grow to around 80mm long (excluding antennae), with the males coloured brownish-green and the females leaf-green. Females have a wider abdomen and shorter antenna than Figure 4: Leaf cut characteristics of Spurlegged Phasmatid males, and while both have wings only the male of the species can fly, as the wings of females are too small (Figure 3). The effects of Spurlegged Phasmatid outbreaks in Victoria’s forests depend on the number of trees affected, the amount of foliage eaten, the length of time over which outbreaks and defoliation occurs, the age and species of affected trees, and their ability to recover. Recorded outbreaks that have harmed or killed trees have occurred in areas measuring 50 hectares to hundreds of hectares of forest. Outbreaks of Spurlegged Phasmatids tend to be greatest between late summer and late autumn,

Forest Health 2 Forest Information Sheet - Forest Health June 2006 Spurlegged Phasmatids in Victoria’s forests

which coincides with starch reserves in the sapwood Monitoring and Managing Outbreaks (the soft wood just beneath the bark that contains Controlling outbreaks of Spurlegged Phasmatids living tissue) dropping to low levels following flushes depends upon predicting where and when they might of tree growth in spring and early summer. occur, along with developing suitable methods to address them. Trees defoliated during this time may struggle to survive as they attempt to regenerate foliage, and a Predictions are based on two key activities: analysing tree may stop growing or die as it completely the number and incubation period of eggs in forest exhausts its starch reserves. Trees that do survive litter, and aerial surveying to assess the level of may die in subsequent years if repeated defoliation defoliation in forest canopies. occurs that fully deplete their starch reserves. Litter samples to soil level on an area measuring The effects on forests are highlighted by a number of approximately 0.4m2 are taken during winter months case studies published in forest research journals: at fixed sites where the potential for outbreaks exist. Samples are dried for around ten days below 25oC • Artificial defoliation experiments showed that before being sieved. The eggs are then hand sorted eucalypt seedlings can be killed if Spurlegged from this material, and bleached to reveal their Phasmatids consume all their foliage between contents. January and June (Cremer, 1960 and 1973). This implies that forests regenerating after a fire or Eggs that are uniformly yellow inside are classified as timber harvesting are more susceptible to viable and undeveloped, requiring around 15 damage from outbreaks. additional months of incubation. Eggs that contain • All of the 24 year old Mountain Ash trees in the distinct embryos (pharate nymphs) featuring Tarago-Tin Creek area south of Powelltown in conspicuous black compound eyes are classified as Victoria’s Central Highlands region which were viable and developed. These eggs are expected to completely defoliated by Spurlegged Phasmatids hatch in the next spring/summer period. Control during the summer and autumn months of 1962 options are considered when four viable eggs are 2 and 1963 had died by the winter of 1965. Trees found per 0.4m litter sample. that were partially defoliated in the area The timing of the control activities depends on the recovered over the same period.(Neumann, age of eggs. If most eggs found in the litter are due Harris, and Wood, 1977). to hatch in the coming spring and summer months, • Up to 40% of trees in 27 year old stands of more immediate action may be taken than for those Mountain Ash that were completely defoliated by not due to hatch in the second summer. Spurlegged Phasmatids in Victoria’s Central Methods for controlling outbreaks of Spurlegged Highlands region during 1966 and 1967 died, Phasmatids have changed since the 1960’s, when along with 20% of trees that were partially insecticide was routinely sprayed over affected areas defoliated (these were mainly younger trees). at ultra low volumes. While this effectively reduced (Neumann, Harris, and Wood, 1977). adult feeding populations and, as a consequence, the • Over 80% of 22 year old Mountain Ash that number of eggs subsequently laid and incubating in suffered two severe defoliations over three years the forest litter, this control method is unlikely to be in Victoria’s Central Highlands died within two used today. years of the second defoliation. Diameter growth Fire is an alternative way of controlling outbreaks, of surviving trees was also significantly reduced although it has limited application. While burning litter or eliminated for at least two years after. on the forest floor does not affect adult populations (Mazanec, 1967). already eating the crowns of trees, it does kill incubating eggs. Reducing the amount of forest litter also makes eggs laid in subsequent years more susceptible to drying out and to being eaten by predators. The use of fire to control Spurlegged Phasmatids is, however, limited because the forest floor in stands of Mountain Ash and Alpine Ash are usually damp, and only a wildfire can burn and reduce the litter. Prescribed burns are also often difficult as tree stands are usually located in rugged areas with limited access. In 2003, wildfire burnt some 150,000ha of Alpine Ash forest to varying degrees of intensity in Victoria’s north east (DSE, 2003), and may have • reduced the risk of Spurlegged Phasmatid outbreaks occurring in these forests. Figure 5: Defoliation caused by Didymuria violescens in high elevation eucalypt forests in the Upper Ovens and Kiewa Further research and evaluation is needed before Catchments of north-east Victoria in summer, 2001 prescribed fire can be used to control outbreaks of (Photo: M. McCormick) Spurlegged Phasmatids.

Forest Health 3 Forest Information Sheet - Forest Health June 2006 Spurlegged Phasmatids in Victoria’s forests

Acknowledgements Messrs Neumann, Harris and Wood, 1977 Forests Commission Victoria Bulletin No. 25 (see Further Reading) formed the basis for information and illustrations contained within this Forest Information Sheet.

Further Reading Neumann, F.G., Harris, J.A. and Wood, C.H. (1977). The Phasmatid Problem in Mountain Ash Forests of the Central Highlands of Victoria. Bulletin No. 25, Forests Commission, Victoria.

Newman, R.L. and Endacott, N.D. (1962). The Control of a Phasmatid Insect Plague in the Forested Catchment of the Kiewa Hydro-Electric Scheme. Australian Forestry, Vol.26, No.1, 6-21.

References Cremer, K.W. (1960). Problems of eucalypt regeneration in the Florentine Valley. Appita, Melbourne, 14: 71-78.

Cremer, K.W. (1973). Ability of Eucalyptus regnans and associated evergreen hardwoods to recover from cutting or complete defoliation in different seasons. Australian Forestry Research, 6: 9-22.

DSE (2003) website reference http://www.dse.vic.gov.au/forests/maps&facts/index_ alpinefires2003.htm Department of Sustainability and Environment, Victoria

Mazanec, Z. (1967). Mortality and diameter growth in mountain ash defoliated by phasmatids. Australian Forestry, 31: 221-223.

Forest Information Sheet - Phasmatid in Victorian forests: Nick Collett, Forest Science Centre, University of Melbourne/DSE Didymuria violescens – the Spurlegged Phasmatid in Victorian forests Nick Collett, Forest Science Centre, DSE Banner photo courtesy Alex Lau

Published by the Victorian Government Department of Sustainability and Environment Melbourne, June 2006

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