Bug of the Month - Emperor Gum Moth

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by Patrick
Publish date
4 January 2013
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Comments (14)

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 (14)

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Pen 6 January, 2013 12:42
Hi, are the caterpillars also called spitfires? thanks Pen
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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.

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Noland 7 January, 2013 17:13
This is amazing. Amazing.
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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
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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?
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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.
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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.
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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.
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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 .
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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.
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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.
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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
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