Steel-blue Sawflies (Perga dorsalis): Eucalypts are attacked by a bewildering array of vegetarian insects. Although thick and leathery, the leaves are the food of many insects. One of the most visible of these leaf-devouring organisms is the Steel-blue Sawfly. The adults are magnificent steel-blue wasps with patches of bright yellow on the thorax and about four centimetres long. The female has a saw-like ovipositor, hence the common name, that fits, like a pocket-knife blade, into a slot in the abdomen. The ovipositor is used to cut slits in Eucalypt leaves where the eggs are deposited.
The larvae cluster on eucalypt trunks and branches during the day and feed at night. The fully-grown larvae are black and sprinkled with short, white, bristly hairs. When threatened, the larvae collectively tap their tails on leaves or bark producing a distinct sound. The tapping may be a means of communication between members of the cluster. Larvae will also raise their heads and eject a thick eucalypt flavoured yellowish fluid from their mouths. This is thought to be a defence against insectivorous birds and parasitic wasps and flies. Even with this deterrent, a large percentage of larvae become victims.
The larvae are sometimes known as Spitfires because of the ejection of the thick fluid. This is not really an apt name because the larvae do not spit out the liquid just dribble it.
When the larvae mature they migrate to the base of the food tree, burrow into the ground and construct barrel shaped cocoons. Some adults emerge in the following spring whilst others are slow to develop and remain in the ground until the following year.
In April 2011 we came across a colony, of about 30 Sawfly larvae, crawling across one of our mown areas (see image). They disappeared into one of our dense shrubberies and had travelled at least five metres from the nearest Eucalypt. The larvae were continually tapping their tails on the ground. This was probably a means of keeping the colony together. They may have been seeking another food source or finding a suitable pupating site.