Steel-blue Sawflies: 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 (Perga dorsalis). The adults are magnificent steel-blue wasps with patches of bright yellow on the thorax.
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. 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.
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.
The cluster in the image was resting on a eucalypt trunk in a very built-up area of Sydney.

Wildlife