Bug of the Month - Emperor Gum Moth

by Patrick
Publish date
4 January 2013
Comments (35)

The apparent decline of the Emperor Gum Moth (EGM), Opodiphthera eucalypti, around Victoria has been a hot topic of debate amongst entomologists and other EGM fans in the last few years.

Emperor Gum Moth A newly-emerged male Emperor Gum Moth.
Image: Patrick Honan
Source: Patrick Honan

The decline is anecdotal and as yet there is no hard evidence, but theories abound. Many people contact us noting that they don’t see EGM caterpillars anymore, as they did when climbing trees as a kid. Which prompts a question in return: "When did you last climb a tree?"

Emperor Gum Moth Male Emperor Gum Moths have enormous feathery antennae used to detect the presence of females.
Image: Patrick Honan
Source: Museum Victoria

Another possibility is the demise of the introduced Peppercorn Tree (Schinus molle) in Victoria. Originally from the Peruvian Andes, Peppercorns were planted in every Victorian primary school and many parks from the 1880s to the early 1900s. EGM caterpillars, although feeding naturally on eucalypts, will also consume Silver Birch (Betula pendula) and Liquidambar (Liquidambar styraciflua), as well as Peppercorns. Victorians who went to primary school up until the 1970s would be very familiar with EGM caterpillars feeding on Peppercorns, but the trees have gradually died out or been removed until now there are very few left. Peppercorns are now considered an environmental weed.

Children planting trees State school children planting peppercorn trees in Carlton Gardens, just outside the now Melbourne Museum, on Arbor day, 1905.
Source: Reproduced from Carlton in DPCD report by Lovell Chen

Emperor Gum Moth The colour of adult Emperor Gum Moths varies considerably throughout their range.
Image: Patrick Honan
Source: Patrick Honan

Another strong possibility is the intrinsic variation in insect populations. Many species undergo booms and busts, appearing in vast numbers one year then apparently disappearing for several years afterwards, sometimes for a decade or more. These fluctuations are usually climate related, with each species requiring an exact combination of factors (such as a mild winter and a wet summer) in a particular order to afford them a boom year. Perhaps the last couple of decades have not produced the right combination for EGMs, and they’re just waiting for their number to come up.

European Wasp The dreaded European Wasp. Workers tear EGM caterpillars off trees and cut them into small pieces before transporting them back to the nest.
Image: Patrick Honan
Source: Patrick Honan

One of the most popular theories is attack by European Wasps (Vespula germanica) on EGM caterpillars. Caterpillars are a favoured prey of European Wasps, and they can do enormous damage when present in large numbers. However, somewhat ironically, after reaching plague proportions in the 1980s and 90s, wasp populations have dropped dramatically in the last 15 years or so, again for no discernible reason other than a possible combination of environmental factors.

Emperor Gum Moth caterpillar feeding An Emperor Gum Moth caterpillar feeding on Eucalyptus species.
Image: Patrick Honan
Source: Patrick Honan

In the end, any decline of EGMs probably comes down to habitat loss. The number of host gum trees has steadily reduced in urban areas in particular, but also in suburban areas and even rural towns. If fewer trees are available, there will naturally be fewer caterpillars. So if you’re missing these iconic caterpillars, the best strategy is to plant a gum tree.

Young caterpillars Young EGM caterpillars look very different to older caterpillars, but their presence is a possible sign of a healthy local environment.
Image: Patrick Honan
Source: Patrick Honan

But these theories are, at this stage, pure speculation. EGMs are still around, if you know where to look. A Museum Victoria Bioscan at Wilson’s Promontory in 2011 attracted hundreds of EGM adults (as well as the closely related Helena Gum Moth, Opodiphthera helena) to light traps at night. And just last month, a dozen EGM caterpillars were on display in the Forest Gallery at Melbourne Museum. Plans are underway to assess the extent of the EGM decline in Victoria, so stay tuned for further developments.

Further reading:

Coupar, P. & Coupar, M., 1992, Flying Colours – Common Caterpillars, Butterflies and Moths of South-Eastern Australia, NSW University Press, 119pp.

Common, I.F.B., 1990, Moths of Australia, Melbourne University Press, 535pp.

Zborowski, P. & Edwards, T., 2007, A Guide to Australian Moths, CSIRO Publishing, 214pp.

Comments (35)

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Pen 6 January, 2013 12:42
Hi, are the caterpillars also called spitfires? thanks Pen
Discovery Centre 7 January, 2013 13:39

Hi Pen - There are two types of insect larvae that bear the common name of Spitfires, but neither includes Emperor Gum caterpillars.

The insects most popularly known as Spitfires are Sawfly larvae, which form black masses on the branches of gum trees during winter. These develop into stout, wasp-like adults that are rarely seen. Sawfly larvae vomit a brown liquid when disturbed, but are otherwise harmless.

Cupmoth caterpillars are also known as Spitfires. These are small, colourful caterpillars with rosettes of stinging spines at each corner. They feed on gum leaves, sometimes occurring in large numbers during summer, and develop into drab brown moths.

Noland 7 January, 2013 17:13
This is amazing. Amazing.
Juan 14 March, 2013 14:52
In relation to European wasps, I recently observed a European wasp directly on the web of a leaf curling spider, climb up the web and eat the spider. I checked inside the leaf and all that was left was the spider's legs. This amy be why I seldom see leaf curling spiders in Melbourne gardens, or least as many as there used to be. Is the European Wasp having a devastating effect on our native spiders? Juan
Discovery Centre 17 March, 2013 14:15

Hi Juan,

Dr Angus Martin first observed this behaviour in The Victorian Naturalist in 1995, making the point that there are no leaf curling spiders in Europe, so this is a strategy developed by wasps after reaching Australia. They enter the shelter, cut the spider's legs off and carry the body back to the nest to feed the larvae. We have no idea what impact this has on spider populations as no further research has been carried out, but we can assume that in summers where wasp populations are high, the impact would be significant.

Martin, A., 1995, The Wasp and the Spider, The Victorian Naturalist, 112(4), p.117.

Mary Kay Ungarelli 6 August, 2013 04:01
I live in USA State of Illinois, this morning I found two Emperor Gum Moths mating on the grass of my back yard! When they separated I laid down tape measurer to 10 inches, their wing span was 7 inches, they are beautiful. The male flew away, the female clamped onto my finger and I put her on wooden fence, it is now 1:30pm she is still there. How in the world did they get here? My friend who lives about 3 miles from me has also had them, as did a veterinarian friend! Any ideas as how this species has arrived here?
Discovery Centre 6 August, 2013 12:08
Hi Mary, we have had a number of people contact us saying they have found Emperor Gum Moths in the U.S. The Emperor Gum Moth is a species that is native to Australia and has been introduced to New Zealand. We are not aware of it being officially recognised as being present in America. If you are able to take some good quality images please feel free to send them to discoverycentre@museum.vic.gov.au and we will have a look at them for you. Alternately as there are some large moths in America which can look similar to the Emperor Gum you may want to contact your local Museum who should have a good knowledge of your local moth fauna.
Lauren 7 October, 2013 11:47
I had the privilege of seeing a male Emperor Gum Moth on the weekend. They are beautiful. I live right next to Enfield State Forest in Smythesdale, Victoria. It was hanging around my porch light. We got some great photos and I have emailed them to you.
Jane 20 February, 2014 16:20
Thanks very much for this information. It answers the question I ask myself every time I pass a group of old peppercorn trees near my house - what happened to all the emperor gum caterpillars I remember seeing as a kid. Glad they are still around.
Marg Tuininga 22 September, 2014 08:15
My husband has been asking where are all the emperor moth caterpillars these days? Thanks for the blog, now think the European wasps have eaten them! We have lots of gum trees so will keep a look out for them.
Edward W. 24 September, 2014 01:09
As boys In Coburg we used to have no trouble collecting lots of EGM catterpillars till about 1957. I only saw them once on gum trees, all the rest on Peppercorn trees without the mauve berries. You did not have to climb the trees to find them, they could be seen easily from the ground. Every time I pass a Peppercorn Tree I look up, and have been doing this for 50 years. No results. Could it be that Peppercorn Trees made them sterile. In Tasmania where I spent three years I only ever saw about six very small Peppercorns in one location at Launceston .
Dale Morgan 20 October, 2014 22:49
Just id this moth as we had one on our window tonight. Just delightful. We are in Badger Creek in Victoria on the edge of the Yarra ranges national park and we have no European wasps here.
Janet 21 October, 2014 13:57
We found a moth recently on my mulberry tree but do have lots of gum trees nearby. We live in Lake Macquarie near Newcastle NSW.
peter hocking 20 November, 2014 22:52
hi i found some EGM last tuesday 18/11/2014 near carum got about 50 of gum trees and still have cocoons from last season
mark 3 December, 2014 18:08
Hi peter.like to take a photo of a mature emporer gum catterpillar. What is the exact location of the place you saw the EGM ...any caterpillars? Reg. Mark You mentioned
Glenn 12 December, 2014 11:16
We have 2 young peppercorn trees in our backyard and we currently have about 15-20 mature caterpillars on them. Can't wait for our annual flurry of EGMs at our back door. They are still around and amazing the next generation.
mark 13 December, 2014 14:54
Hi Glenn. Very keen to get a photo of an emporer gum catterpillar. Are you around melbourne somewhere; may I visit?
Mark 13 December, 2014 15:00
Hi Glenn would greatly appreciate if I could take a photo of one of the empoerer gum caterpillars you have? I am in Melbourne
Silvana 18 February, 2015 13:10
We have just discovered two of our backyard gum trees are filled with the EGM caterpillars, some very large/mature. I recall seeing the EG moth in our garage some time ago so perhaps that was their origin. The are fast movers and noisy eaters! We live in the South East Green Wedge in Melbourne.
Howard Connor 7 June, 2015 19:48
Our garden is full of Eucalyptus species, silver birch and peppercorns but the EGMs have disappeared over the last 24 years. I found the article about their decline rather facile. More research is needed to prevent further decline and also determine whether they may be a sentinel species alerting us to a more deep rooted problem. In our area regular spraying of mosquitoes with a BT derived toxin occurs
Discovery Centre 13 June, 2015 10:33
Hi Howard,

There are many reasons for species decline, particularly in urban areas - loss of habitat, competition from introduced species, excessive predators, lack of suitable food, chemicals, and so on. Where the conditions are suitable, Emperor Gum Moths are still common but these localities are much fewer than in previous years, most notably due to the loss of the introduced Peppercorns (Schinus molle) from around Victoria. More research is certainly needed into the changing populations of this and other native animal species.

Mal cosgriff 19 July, 2015 22:54
In 1993 we had 8 mature eg catapillars on our gums late one afternoon in March I witnessed several wasps insert and lay eggs under the skin half way down spine then sealed the piercing. Next week the catapillars changed to pink and spun their cocoons All seemed normal but in October all 8 had 6 or 7 newborn wasps hat bout of larvae destroying it and 7 small holes in cocoon. This ended 40 years I had witnessed the cycle in Carnegie in Melbourne Another amazing memory was the female moth on a warm January evening attracting males with ferromones on the mild warm northerly wind normally about 2 to 3 males on the right night.
Christopher Harrington 7 November, 2015 16:10
After reading this thread about EGM's, i put on my shoes, went outside and did a little bit of pottering around the Inner Melbourne wilderness. I quickly found a number of EGM cocoons on Peppercorn trees, but I have had no luck, as of yet, with finding EGMs on their natural host-trees, that is in Inner Melbourne. Interestingly, all the EGM cocoons i found in my adventure had been fed on by some type of parasite which, to the best of my knowledge, had placed its eggs next to the cocoon. The EGM cocoons contain a small 2mm by 2mm hole, and have been completely eaten internally, as the insides are hollow. I think the little hive structure that has developed next to the cocoon may have belonged to some type of parasitic wasp, but i am not an entomologist. However, i do have relic of the cocoon featuring the small hive if anyone wants to examine it. Not too long ago I found a number of healthy EGM cocoons on my Grandmothers property in Cranbourne South, right on the green wedge. They were located on some established liquid amber trees, and they had not been devoured by wasps or any other type of parasitic insect. I have also found a number of EGM's in Warburton, a one hours drive from Melbourne, and they were in very healthy conditions, probably because the elevated climate, which is nice and chill, tends to reject parasitic monsters like the Euro Wasp.
Discovery Centre 13 November, 2015 10:48

Hi Christopher,

Emperor Gum Moths (Opodiphthera eucalypti), like most moth caterpillars, are heavily parasitised by parasitic wasps and flies. Adult parasites lay their eggs on the caterpillars and the eggs hatch into larvae which feed on the caterpillars from the inside. The caterpillar continues to develop and eventually forms a cocoon, and it’s only then that the parasites themselves pupate inside the EGM and emerge as adult wasps and flies. EGM caterpillars can often be seen with small white dots just behind the head – the eggs of the parasites. European Wasps (Vespula germanica) are a social species and do not parasite caterpillars, although they do prey on them and can apparently take a heavy toll.

The parasitic wasps and flies usually emerge from the EGM cocoon rather than beside it, and it’s usually difficult to detect them without opening up the cocoon. The situation you describe may be something different entirely – it may help if you can send photos.

EGM populations vary greatly from year to year, sometimes feeding on their native host plants and sometimes on the introduced ones (such as Peppercorn Trees, Schinus molle). In one year they may be abundant in any particular location and then may be absent for many years, becoming abundant in other areas. The presence of parasites may have something to do with this, but environmental and many other factors probably also play a role.

Jim McArdle 2 July, 2016 09:59
I have had a similar thing happen to the only cocoon I can readily see on the Peppercorn tree opposite my place. There is a hole where the moth would have come out and the cocoon is empty. This happened not long after the cocoon was spun. I am very disapointed as I was hoping that a colony of caterpillas would evolve. There are almost certainly other cocoons on the tree so hopefully they are O.K. My original comment is below.
Jim McArdle 26 November, 2015 19:38
I, My Sister & a friend (Frank Gardner) looked for Emperor Gum Moth Caterpillars for about 20 years (during the recent extended drought in Victoria) without success. However over the past couple of years I have seen quite a few around Rye on the Mornington Peninsula, Where I now live. I posted this on Google+ yesterday. https://plus.google.com/photos/photo/117317862143428883675/6220851727895730690
Rose Steiner 13 December, 2015 12:42
I'm on the Mornington Peninsula and we have about half a dozen of these caterpillars on one of our flowering gums. They are beautiful!
Meredith Schaap 17 December, 2015 18:18
We have a few caterpillars on two young eucalyptus leucoxylons in our garden in Alphington. After reading the above entries we will watch them and not get rid of them!
Lloyd 22 August, 2016 16:25
As a kid my whole life revolved around caterpillars, tad poles and frogs. Great times!!!! I see frogs and tad poles all the time but I just want to see An EGC again it's been 35 year since I seen one.
Jane 5 September, 2016 18:52
On my property (Parkesbourne NSW) they are quite numerous in their season (later Spring, after Bogong moth season). They hit the windows as hard as a sparrow. In good years they strip the gum trees ragged - like Christmas beetles. I think native creatures are just avoiding suburbia - but on a block, we see many moths, butterflies, parrots, echidnas, kookaburras, wombats, kangaroos, eagles, etc, etc.
Cynthia 3 November, 2016 08:29
Just discovered one of these beauties in NE Victoria, wingspand of about 13cm. Simply stunning!
Belinda House 5 November, 2016 01:26
Just seen one this evening. Visiting my Mum in Enfield by the state forest. Wonderful to see as I lve in the UK now.I was quite impressed I remembered what it was!
Dave Orr 31 December, 2016 18:58
When I was a youngster growing up in Beaumaris,Vic, we had lots of egc/egm's,but not so many Indian myna birds.Forty years on the opposite seems to be the case.I wonder if these intelligent but destructive feral birds predate on them?
Discovery Centre 2 January, 2017 13:11

Hi Dave, good question, as noted in the article above there are a number of possibilities as to why Emperor Gum Moth populations may be declining in some areas. We couldn't find any direct studies of the introduced Common or Indian Myna predating on the Emperor Gum Moth. The Birds in Backyards website says 'Common Mynas are accomplished scavengers, feeding on almost anything, including insects, fruits and vegetables, scraps, pets' food and even fledgling sparrows.' If this pest species can feed on fledgling sparrows I can't see that an Emperor Gum Moth would pose much of a problem.

Matt White 14 May, 2017 22:17
There is no evidence for a decline in Peppercorns in Victoria, on the contrary it is far more widespread and common than it ever was in the past.
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