Bug of the month - Steel Blue Sawfly

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by Patrick
Publish date
1 July 2012
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Comments (10)

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 (10)

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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?
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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.
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?
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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.

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?
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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.

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.
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zoe 19 March, 2013 12:22
is this species also in qld?
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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.

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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?
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