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Uraba lugens
Number 16
Revised August 1992
By Charlma Phillips, Principal Forest Health Scientist

The Gum-leaf skeletonizer is an important pest of eucalypts. It was first recorded as defoliating large areas of forest in New South Wales in 1900 and since then has been recorded from all over Australia (except in the Northern Territory), there have been many severe outbreaks.

The insect gets its common name from the activities of the young larval or caterpillar stages which feed by grazing or skeletonizing the surface of the leaves and giving infested trees a brown or burnt appearance.

A wide range of eucalypts are attacked including Eucalyptus globulus, E. viminalis, E. obliqua, E. delegatensis and E. camaldulensis.

There are three distinct geographical forms: a highland form, a coastal form and an inland form. These forms differ in several aspects including the number of generations per year.

Gum-leaf skeletonizer caterpillars are extremely hairy and may cause severe skin irritation in humans if handled.

Description

Gum-leaf skeletonizer moths are a dull grey/brown in colour. The forewings have darker grey markings while the hindwings are paler grey. The wingspan is approximately 25-30mm.

Eggs are small, approximately 1mm in diameter and cylindrical in shape. They are yellowy-green when first laid, turning brown as they develop.

Caterpillars are extremely hairy with yellow and brown markings. Mature caterpillars are approximately 20-25mm in length. A characteristic of these caterpillars (which makes them very easy to identify) is the retention of head capsules when they moult. The head capsules are pushed up to resemble a horn on top of the head.

Pupae are enclosed in elongated, silken cocoons which incorporate hairs from the body of the caterpillar.

Life Cycle

Adult moths fly at night. They are poor fliers and consequently lay their eggs on the lowest available foliage. Eggs are laid in masses of 200 or more on both upper and lower leaf surfaces and on both adult and juvenile foliage but the preference is for the lower surfaces of undamaged mature foliage. It is rare to find eggs on damaged leaves. Most eggs are laid in showery weather.

Caterpillars go through several stages or moults as they grow. The average number of stages for Gum-leaf skeletonizer caterpillars is 11 though there may be up to 13. (In most moth species there are only 6 caterpillar stages). The first four stages are gregarious and feed en masse. By the fifth stage the caterpillars have become solitary. When disturbed caterpillars react by dropping off the leaf to hang suspended by a silken thread.

Cocoons are found on the bark or amongst leaf litter on the ground. They are very well camouflaged.

The inland and coastal geographical forms of the gum-leaf skeletonizer moth usually have two generations per year. The summer generation adults emerge in November/December and lay eggs. Caterpillars are present from December to March and pupation occurs from February onwards with adults emerging again from early March through to May. These adults produce the winter generation of caterpillars. Eggs are laid and caterpillars are present from April/May through until October/November. These times are approximate as the life-cycle depends very much on the temperature and weather conditions.

The highland form has only one generation per year. Adults emerge in January and begin laying eggs. Caterpillars are present from June/July onwards and pupation occurs in December. Development is very slow at high altitudes as temperatures are much lower than in lowland regions.

Damage

Young caterpillars feed in groups on the leaf surface, skeletonizing it and causing wilting and browning. Older caterpillars consume all the leaf tissue, leaving only the midrib.

Unless there is a definite outbreak, damage is usually slight and is unsightly rather than harmful. However, attacks on young trees, especially if there is repeated defoliation, may result in reduced growth rates and death.

Most damage occurs in the late caterpillar stages so it is important to be aware of this and check the amount of damage and numbers of caterpillars in early summer and late autumn. If large numbers are present at these times considerably more damage will occur later as the caterpillars mature if steps are not taken to control them.

Control

Chemical control:
Spray with maldison mixed with a wetting agent to help penetrate the hairs on the caterpillars.

Natural control:
Caterpillars are parasitised by several species of small wasps and by parasitic flies. They are also killed by fungal and viral diseases. Many predators are deterred by the hairs on the caterpillars but the predatory shield bug, Oechalia schellenbergii (see Forest Health Information Fact sheet No.10) is often seen feeding on them.

Climatic factors over winter (temperatures, wind and rainfall) have a significant effect on the mortality of caterpillars.

Summary

When to look:  Look for caterpillars over summer and again in winter/spring - it is particularly important to look for caterpillars in early summer and late autumn.

Where to look: Look on lower foliage - both on adult and juvenile leaves.

What to look for:  Look for skeletonised leaves or chewed leaves. Look for very hairy caterpillars with head capsules stacked up on top of the head.

For further information contact PIRSA Forestry

Disclaimer: While this publication may be of assistance to you, the government of South Australia and its officers do not guarantee that it is without flaw of any kind or is wholly appropriate for your particular purpose. The Government therefore disclaims all liability for any error, loss or other consequence that may arise from you relying on any information in this publication.