Bug of the month - Steel Blue Sawfly

by Patrick
Publish date
1 July 2012
Comments (28)

If you're out in the bush or a local park during winter, you're likely to happen across a group of 'spitfires' clinging to the branch of a gum tree in the cold. These insects are technically called sawflies, a group of insects closely related to wasps. There are more than 200 species of sawfly in Australia, but the local species is the Steel Blue Sawfly (Perga dorsalis).

sawfly larvae A small clutch of sawfly larvae clinging to a branch.
Image: Patrick Honan
Source: Museum Victoria

The name 'sawfly' derives from a 'sawbench' under the abdomen of the female with which she lays eggs. Female wasps, in contrast, use a pointed ovipositor to lay eggs and in some species this doubles as a sting – adult sawflies do not sting and both adults and larvae are completely harmless.

Patrick Honan Female Steel Blue Sawfly.
Image: Female Steel Blue Sawfly
Source: Museum Victoria

Female sawflies use the sawbench to cut the upper surface of a leaf and deposit 60-70 eggs into the leaf tissue. The larvae hatch and feed on gum leaves, grouping together for protection in a rosette pattern, similar to the head-outwards stance adopted by Bison when under attack. This is known as a 'ring defence', or cycloalexy. As the larvae grow, they collect in larger groups around branches during the day and spread out to feed at night.

sawfly eggs A raft of eggs cut into a gum leaf by a female Steel Blue Sawfly.
Image: Patrick Honan
Source: Museum Victoria

Like their cousins, the ants, bees and wasps, sawflies show some social behaviour but only in a primitive way. When feeding at night, larvae tap the branch to keep in constant communication with each other. If an individual becomes lost, it will tap more rapidly until it receives an answer from the rest of the group – if an individual becomes completely separated it will not survive long on its own.

Detail of sawfly larva abdomen. Sawflies grouped together on a branch. The pale tips of the abdomen are tapped on the branch to keep in touch.
Image: Patrick Honan
Source: Museum Victoria

The activities of a group of sawfly larvae are governed by a few select individuals that become in effect the leaders of the group. They lead the rest out to feed at night and, if they run out of food, lead the group across the ground to other trees. When large numbers of sawfly larvae are present they are able to defoliate small gum trees, but in general are not a major pest.

mass of sawfly larvae A mass of sawflies resting during the day, the result of the merging of several smaller groups.
Image: Patrick Honan
Source: Museum Victoria

When ready to pupate, the larvae leave the host tree and burrow down to make mass cocoons in the soil. Here they sit through spring and summer to emerge in early autumn. Adults have no mouthparts and do not feed, living only for a week or so.

pupating sawfly larvae Sawfly larvae in their pupal cells underground, preparing to pupate.
Image: Patrick Honan
Source: Museum Victoria

Not all emerge, however, as many succumb to parasitic flies. These flies, about the size of a blowfly, will lay eggs in the sawfly larvae and the fly maggot literally eats its host from the inside out, eventually emerging from the sawfly's cocoon.

parasitised sawfly larva An opened pupal cell showing the consumed sawfly larva on the left, and the engorged parasitic fly larva on the right.
Image: Patrick Honan
Source: Museum Victoria

Sawfly larvae have an unusual defensive mechanism that has given them the name 'spitfires'. They store eucalyptus oil in a small sac in their gut, and regurgitate this oil when under threat. Despite their nickname, they are unable to actually spit this fluid and the oil itself is harmless unless eaten (like all eucalyptus oil). In fact it has a very pleasant eucalytpusy smell.

sawfly larva mouthparts A large blob of frothy regurgitate in the mouthparts of a sawfly larva.
Image: Patrick Honan
Source: Museum Victoria

Australia is one of the main strongholds of Symphyta, the suborder of insects to which sawflies belong. The Steel Blue Sawfly is one of the few insect species active in Victoria during winter, so next time you're in the bush take the time to stop and smell the sawflies.

Comments (28)

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ivan 6 September, 2012 11:49
thanks for the article. I live west of melbourne in derrimut the spitfires are on every gum tree, heaps of them! my son asked if they are native animals, i dont know, are they?
Discovery Centre 8 September, 2012 12:11

Hi Ivan, spitfires are definitely native. They belong to a group called Sawflies, which are primitive harmless wasps.

zoe 19 March, 2013 12:22
is this species also in qld?
Discovery Centre 22 March, 2013 15:33
Hi Zoe, This species is found around Brisbane but there are many other species found further north in Queensland, different in appearance and habits but members of the same group of insects.
Jai 23 September, 2013 17:04
I've just found a huge bunch of these ugly creepy things in my back yard. I have two young kids, a dog and a cat. Will they hurt the kids if they touch them and will the hurt the dog if he eats them. He's not too bright to begin with. How can I remove them?? Where should I take them to?
Discovery Centre 26 September, 2013 11:14

Hi Jai,

Steelblue Sawflies are the larvae (grubs) of wasp-like creatures that are stingless and not related to the social wasps you would be more familiar with.

Sawflies are harmless to humans and animals, but they do defend themselves with eucalyptus oils regurgitated from their crops (stomachs). These oils give off a pleasant smell and are harmless unless eaten – even then they are unlikely to cause any harm.

The larvae feed on leaves and will remain on a single tree throughout their lives as long as enough food is available, so your kids and pets shouldn’t need to encounter them.

The best option is to leave them in place, but if they really need to be moved you can cut the branch they are on and hang it in the branch of another gum tree far enough away. In the evening, the larvae will climb off the old branch onto the new tree.

Nic 29 September, 2013 15:38
Thanks for the information. We watched a group of these guys mass migrate from the base of a turpentine tree at sunset last night along the ground and 20 hours later make their way into a rabbit warren. Why are they going into the warren?
Discovery Centre 30 September, 2013 10:42
Hi Nic, when the sawfly larvae are fully fed, they burrow into the ground and make communal cocoons in which they pupate, eventually emerging as adults. This species is out and about as larvae during winter, and pupate in spring – they are probably the only insect species reliably found during the winter months in Victoria. The adults emerge from the cocoons in autumn and lay their eggs on gum leaves to continue the cycle.

Sawfly larvae always move as a group, so the mass migration you witnessed was the insects searching for a good pupation site – and a rabbit warren would be as good as any.

david 3 February, 2014 11:11
Although the larval stage may be considered a pest in ones garden is there any benefit attributed to the adult wasp ie predation on other bugs?
Discovery Centre 7 February, 2014 12:45
Hi David - The larvae are very minor pests, particularly in suburban situations, and the trees generally recover quite quickly. Adults don’t feed at all, so they are neither friend nor foe to humans, they just are.
john 14 July, 2014 19:34
I have these on some of my eucalypts and over the last years they have actually stripped some small trees bare. What harm am I doing to the environment if I destroy all the caterpillars.
Discovery Centre 16 July, 2014 14:50
Hi John - Strictly speaking, there would be no harm to the environment if you destroyed the sawfly larvae. They are a common species and are widespread across southeastern Australia. Generally their feeding doesn't do severe or long-standing damage, but in certain circumstances they can defoliate small trees, although the trees generally recover. If you'd like an alternative to destroying the larvae, one option is to pick them off the tree and move them to a larger tree nearby. The larvae produce a strong-smelling eucalyptus-based fluid, but are harmless to humans.
Kathryn 10 September, 2014 15:36
Amazing! All my life I was told to be careful of "spitfires" and 30 plus years later I find they are harmless :) great read and now I also know what those grubs I found in the soil are when gardening. Thank you for such an informative article.
Gabriella 16 September, 2014 07:46
Fascinating article! Thanks also to Kathryn for making the connection—I've been wondering about those fat white things too which I'd come across when gardening and feed to my chooks.
Don 8 January, 2015 11:30
These little blighters may be harmless, but here in south east Qld - near Kingaroy, they strip the leaves from gum and ironbark trees. Some gums are totally stripped of leaves. The lack of rain means that the trees take a long time to recover. I wish I could at least reduce their numbers by half or more and let the trees come on a bit. The gum trees also suffer from mistletoe which I am unable to get up and remove. Some of my old gums are now dying. They are 20 and 30+ years old, one was a metre thick. Just another harmless pest?
Sarah 2 March, 2015 19:59
I found a large cluster of these at the bottom of a gum today. Is it odd that they are still in their larval form at this time of year? Also, could you tell me why everyone says they are dangerous? Where did such a notion come from? I assume maybe if you got the oil in your eye it may be painful...but I'm surprised that they are largely safe!
Sandy 4 April, 2015 21:18
Fantastic article, much better by far than anything else on the internet. Thanks for confirming some info I figured out on my own by watching my group of caterpillars, and informing me of the other stuff I didn't know! Keep it up!
nicolr 10 May, 2015 23:18
Hello, I swear that when I was in primary school I was attacked by a group of spitfireS. I ended up with swollen skin and it was itchy as. I still feel I'll when I see them, we have some around our trees at the moment. . good to read thank you.
Jenny Toon 19 October, 2015 17:48
My lemon scented gum has been attacked severely, and with drought coming not sure it will recover. Will a Confidor drenching make any difference in early Autumn?
Alison Sharpe 10 November, 2015 14:47
Hi, just reading this article as we have had some of these spitfires on our gums. The gums are very young and they have stripped the canopies completely....is there a natural way to get rid of them? spraying them with citronella or something like that? Thank you for your help in advance. Alison :)
Discovery Centre 13 November, 2015 16:39
Hi Alison,

Sawfly larvae (Perga species) are hardy insects and difficult to remove from plants other than by physical means. Natural-type remedies such as citronella and garlic sprays don’t appear to have much effect on them. Keep in mind that eucalypts are regularly defoliated by sawflies and under normal circumstances are adapted to coping with defoliation.

Nathan 28 November, 2015 20:11
Hi there, Thanks for a great article! I was just wondering if you had (or could direct me to) more information regarding spitfire locomotion. You mention that a few members of the group become 'leaders' - how is this determined? Also, is the hierarchy dynamic or static in its construction? Many thanks.
Discovery Centre 1 December, 2015 12:28
Hi Nathan, 

Most of the research done on sawflies was by Philip Broughton Carne at CSIRO in the 1960s and 1970s. P.B. Carne published a series of papers on sawfly life history, ecology and behaviour in the Australian Journal of Zoology – The characteristics and behaviour of the saw-fly Perga affinis (Hymenoptera), Aust.J.Zool., 1962, 10:1-34; Distribution of the eucalypt-defoliating sawfly Perga affinis affinis (Hymenoptera), 1965, 13:593-612; On the population dynamics of the eucalypt-defoliating sawfly Perga affinis affinis (Hymenoptera), 1969, 17:113-141. A good summary of Carne’s work is outlined in R.D. Hughes 1974 book ‘Living Insects’ (1974, William Collins Australia Ltd, pp.161-167). CSIRO did much research on eucalypt-defoliating insects back then and it remains the best source of detailed information. 
Deborah 30 December, 2015 10:54
I have just found sawfly slugs on my quince tree, Are these the same as the spitfires I see in the bush. When you say spitfires give off a pleasant smell, what does it smell like? Cheers, Deb
Discovery Centre 30 December, 2015 16:37

The sawflies on your Quince will be Pear and Cherry Slugs (Caliroa cerasi). This is an introduced species that attacks fruit trees and can do considerable damage when present in high numbers. They are more slug-like than other sawfly species and adult females are able to reproduce without males (no males have yet been recorded in Australia). This species can be controlled without chemicals by picking them off and squashing them, or hosing them off. Alternatively, dust them with wood ash or chalk, kitchen flour or powdered clay. Steel-blue Sawflies and related species give off a pleasant eucalypt smell when disturbed.

Warwick Grace 10 April, 2016 07:31
Thank you so much for your information. Most insects or life fascinates me. I'm amazed how we humans can prune a tree to nearly nothing but when something else does we feel we have to kill it. Today I'll be looking at Eleocarpus eumundii which is being defoliated.I will try to convey to those worried that everything will recover. Thanks again.
Discovery Centre 23 April, 2016 15:08

Thanks for your comments. You make an excellent point about pruning – most native insects do minimal damage to plants but their presence excites many gardeners to untold and unnecessary levels of chemical use. In the long run, almost all native plants recover from attacks from almost all native insects, and the presence of the latter only adds more interest to the average backyard.

Dani 20 March, 2017 12:09
Are Perga dorsalis similar to perga affinis?
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