The most common butterfly in the Geelong Botanic Gardens during the early part of summer – apart from the ubiquitous cabbage white butterfly – is the Common Brown Butterfly (Heteronumpha merope).
I first noticed this butterfly in large numbers a couple of years ago when the grass-trees (Xanthorrhoea) in the 21st Century Gardens were in flower. Dozens of male butterflies were feeding together on the floral spikes, making it look as if they were covered in dead leaves. Despite looking at every grass-tree flower spike ever since, I’ve never seen this phenomena again.
Xanthorrhoea flowers produce a lot of nectar, so I imagine that that was what attracted the butterflies. With their modified, tubular mouth parts, butterflies can only sip at nectar to feed. Given that common brown butterflies can live for up to eight months, they must be feeding on something!
The common brown butterfly was first collected by Joseph Banks in 1770 from Botany Bay. I wonder if he picked that the male and female are rather different – or did he think they were 2 separate species like I did when I first encountered the females?
- Male common brown butterflies are a mottled orange-brown with a fairly indistinct eyespot on each wing. When they rest in the grass they almost disappear as they fold their wings vertically and look for all the world like a dead leaf.
- Female common brown butterflies are significantly larger (the forewing is 38 mm across compared with 34mm for males), they are also much more colourful with a completely different wing patination and a much more floppy flight.
Added to these differences, the male butterflies seem to appear much earlier in the season and in larger numbers – at least in the Geelong Botanic Gardens. It is only later that the females start to appear, and now in early January that pairs of butterflies can be seen in their nuptial flight which confirms that, despite their apparent differences, they are in fact the same species.
Not only was the common brown butterfly one of the first Australian butterflies to be collected, but it of was also one of the first to be the subject of modern ecological study (by E.D. Edwards from the CSIRO back in 1973). Female butterflies mate as soon as they emerge, so all those mating dances I have been seeing confirm my feeling that the girls don’t show up in the Gardens until after the boys.
The caterpillars of common browns feed on “rough grass” including species of Poa and Themeda. In southern Australia, these grasses tend to dry out or die back over the hot, dry summer and only provide lush leaves after the first autumn rains. There is not much for a caterpillar to eat over the hottest part of summer in our (normally) hot, dry later summers. Coupled with that, butterflies don’t enjoy the hottest weather of summer – numbers drop off in the Gardens after the first heatwaves of January. So what happens between the peak butterfly season in early summer and peak caterpillar season in the cooler, damper part of the year?
I suspect that the male butterflies actually die once the mating season is over. The females, which can live up to 8 months, aestivate over the hottest part of summer. (Aestivation is the hot weather equivalent of hibernation). The butterflies hide away, lower their metabolic rate and wait out the season until conditions improve and they can lay their eggs on the leaves of their host grasses when the caterpillars have most chance of success.
The larvae of common browns are very variable in colour and pattern and I don’t think I’ve ever really noticed them – but I’m sure the local magpies would know them well as they feed largely on grubs and beetles in the grass. Perhaps to avoid being dinner, the caterpillars feed at night. The pupa forms amongst the leaf litter at the base of the plant which strikes me as lucky as that is also out of the way of our lawn mowers. It’s good to know that the Council mowing practices in Eastern Park are unlikely to be reducing the numbers of this particular butterfly.
So what eats a butterfly? Clearly something as my little collection of butterfly wings attests. This collection includes an entire male common brown which does not seem to have been eaten, and 2 pairs of female forewings which were both found lying close together. Pairs of wings seem to indicate that something is eating them and leaving the non-tasty bit behind. This was probably a bird, but butterflies are also dinner for dragonflies and lizards.
Of course, spiders also feed on butterflies when they get the chance.
Reference: The Butterflies of Australia by Albert Orr and Rodger Kitching (2010)