I was clearing eucalypt saplings from an area towards the back of the house. They’d grown from seeds, fallen from large trees on the neighbouring property.

We love eucalypts but we don’t want them growing close to the house, in the area that’s designated ‘the garden’. They suck up so much moisture from the soil that they deprive the plants around them. Limbs tend to drop off without warning and they also present a fire hazard in the bushfire season, making them less than ideal trees in a residential setting.

While I was busily cutting back the saplings, I came across a bizarre cluster of what appeared to be caterpillars, tightly entwined on a small, partly defoliated tree. They freaked me out.

After calming down and doing a little research, I discovered that they’re the larvae of the Steel-blue Sawfly and they can cause serious damage to young eucalypts. I suppose I could have left them – thus saving myself any further work – but I thought it a good idea to remove them to a less temporary home.

I gleaned the following about them from the Department of Primary Industries website, ‘Insect Pests – life histories and habits’, as well as from the CSIRO website, and from What Garden Pest or Disease is That? by Judy McMaugh.

One generation of Steel-blue Sawfly larvae is produced per year, with the female laying her eggs in early Autumn. Using her ovipositor, she makes a nest for the eggs.

The larvae emerge from late Autumn onwards, forming clusters during the day, some of them covering large sections of tree. Starting at the top of the tree, the larvae feed at night, consuming whole leaves down to the midrib.

Once the tree is totally defoliated, (and this is the weird bit) the larvae move in a column, usually at night, in search of new trees. Their black bodies covered in short, white, bristly hair, they’re straight out of an insectophobe’s nightmare.

Between late Spring and early Autumn, the mature larvae move from the tree and cover themselves in soil to pupate during Summer. Some of the adult sawflies emerge between early and mid-Autumn to breed, while slow developers appear the following year.

A magnificent steel-blue creature with areas of bright yellow on the thorax, the Steel-blue Sawfly is related to wasps but it’s stockier than most of them. It doesn’t have a wasp-like waist, doesn’t sting and its wingspan is about four centimeters. It’s usually seen around the leaves of its host plant.

While sawflies can have little overall effect on large trees, they can defoliate young saplings and smaller eucalypts.

Larvae are harmless, although when threatened by birds, parasitic wasps, flies and middle-aged women, they raise their heads and eject a thick yellowish fluid from their mouths. It smells like crushed eucalyptus leaves.

When worried, they also collectively tap their tails on leaves or on bark, producing a distinct sound. They’ll have to tap little louder than they did for me, though, if they really want to be taken seriously.

To get rid of the larvae, you can knock or wash them off, or remove them by hand and squash them. Being a brave, Aussie country girl, I carefully cut off the branch that the larvae were clustered on and quickly dropped it into a plastic bag. I left the gang to spray each other with that eucalypt-flavoured juice for a while, and then took them down to the forested area of the block where I hung the open bag on some lopped eucalypt branches. Then I ran away. I checked later and they’d disappeared, no doubt on a mission to find food. 

The Steel-blue Sawfly is not an introduced pest so I guess it plays an important role in the health of the ecosystem here. I just wouldn’t want to encounter a column of its larvae creeping up on me at night.