Insects

I find the number of species of insects and other invertebrates in the bush rather intimidating and do not really know where to start.  But every now and then, their great ecological importance becomes apparent to me:

Value adding: the mighty meat ant – the importance of meat ant to echidnas
Moth matters
– the dramatic emergence of swift moths


Value adding: the mighty meat ant  –  July 2013

While we are ignorant of the vast majority of insects in this world, most Australians can recognise a meat ant colony.  It’s hard not to when one innocently stands on what seems like a nice clear patch of ground, and it isn’t.

A couple of years ago, an evening supper was provided for Frogwatch volunteers at Mulligans Flat.  The table, spread with tempting food, was mistakenly set out in the dark over a meat ant colony.  Within 15 minutes, with much slapping and trouser leg shaking the whole party disbanded and delicious eatables had been packed away.  I felt very sorry for the organizers who had carefully selected some flat, open ground for the supper – perfect meat ant habitat.

It was a bit of sport for us kids to stomp all over a gravelly meat ant mound.  The challenge was to stay sufficiently long to set the colony a-boiling, but not long enough for them to attach to your shoes, socks or worse.  Actually we were not as brave as it sounds as meat ants have no sting, and defend themselves with a bite.

My ant-friend, Alan Andersen coined the  paralysing term “Dominant Dolichoderinae” to describe the general group of ants to which meat ants belong.  Dominant “Dolly-cod-er-ine-ee with a flat intonation” according to Alan.  The group includes abundant, highly active and aggressive species that favour hot and open habitats, and exert a strong competitive influence.

Not surprisingly, when meat ants get established, they compete with other ants, but they also increase the total ant biomass by an enormous amount.  They seem to be much better at foraging on bare areas and experiments have shown that they collect food from bare rock areas which other ants don’t seem to collect.

Pick an open piece of ground, along a road or where the grass is thin, and that is where the meat ants settle.  Their mounds drain away rainfall and are warmed by the sun.  The colonies can be a significant landscape feature, with cleared highways leading off in all directions, enabling  the workers to quickly transport food back to the colony.

We had a nest at Gang Gang  that was huge – over three  metres long and a couple of metres wide.  I observed  the extent to which they kept the ground bare in the vicinity, preventing the recovery of our previously heavily grazed grassland.  We tried discouraging them by covering the nest mound with thick piles of tussock grass that we had cut from densely grassed areas.  It was an interesting experiment – it really did set them back, but I note a few years later that the (much smaller) colony was still there – they had moved their entrances a few metres over to a nearby track.

I am more warmly disposed towards meat ants since I learnt about their relationship with echidnas.  I had seen what looked like echidna diggings around Gang Gang, but only recently discovered that meat ants form the major part of the diet of these appealing creatures.  Echidnas rip into the colonies with their strong claws to obtain the nutritious virgin queens and the energy-rich eggs.  The worker ants must be too indigestible.

In winter, the ants live closer to the surface where the sun warms their chambers more effectively.  I guess this makes the echidna’s meals easier to access in winter, although observations indicate that they dig up colonies throughout the year.  Echidnas appear to be methodical beasts – one study watched 140 meat ant colonies over two years, and in that time 138 of them were attacked by echidnas.  Luckily for the meat ants (and the echidnas), the damage to the nest chambers can be repaired, and after 3 months, nests often show little sign of damage.

So now I understand why the cute, and apparently vulnerable echidna survives so well in farmland.  I thought that they mainly ate termites and were dependent on the increasingly scarce fallen timber that people snaffle for firewood.  Maybe they would prefer termites, but have had to switch to meat ants.  In any case,  I have renewed respect for the echidna and its ability to broach a meat ant nest, and for the mighty meat ant which has given the echidna a means of survival.



Moth matters – April 2013

If you’ve ever tried to pin a biologist down regarding a specific technical matter, you will find them frustratingly vague, sprouting sentences studded with “maybe”, “possibly”, “sometimes” and commonly “I dunno”.

This is why I was so astonished last year when an entomologist flatly told me that “Oxycanus emerges with the first rainfall after the 10th April”.

I think that this is possibly (but only possibly) the most explicit prediction I have ever heard from a specialist. The conversation was in late May last year. I had brought in to the insect collection in Canberra, some moths that we had become interested in. They were noticed above the myriad of other species that occur in the bush, due to the very large numbers that had suddenly appeared around the house and on the roads one wet night.

There were actually two types, and the entomologist told me they were both swift moths (also known as ghost moths). They were both large – their bodies about 4 cm long with a wingspan of 6-7 cm. One was greyish brown with irregular markings (Oxycanus antipoda) and the other a rather attractive apricot orange (Oxycanus dirempta). As they are not well known enough to have common names, let’s call them respectively the grey and orange swift moths.

Grey swift moths emerge from the ground in late autumn to early winter, and set about mating straight away. The males emerge first and fly around low to the ground while the females emerge, climb onto grass stems and sit quietly for a while. All it takes to get a mate is for the females to shuffle their feet and feebly flap their wings, and down come the males. How easy is that? The main problem confronting the males is the distraction of house and car headlights, to which they are strongly attracted, thus potentially missing out on their 10 minutes of connubial bliss.

There is a short pause after mating, presumably to allow fertilization of the eggs, after which the eggs are laid on the ground or grass stems. The eggs seem to take 100-200 days to hatch, so the little larvae (caterpillars) would be hatching in spring. The larvae are quite industrious and create their own silk-lined tunnels, at first on the surface and, as they get bigger, they dig burrows in the grassland where they sleep in the day and crawl out at night to eat native grasses, sometimes dragging material down the hole to consume at leisure. By late winter the tunnels are about 25 cm deep and may have several entrances. The caterpillars pupate (build cocoons) in the same hole. About two years after that romantic night of flight, and feeble wing flapping, new moths are ready to emerge with the first rain after the 10th of April.

The orange swift moths probably have a similar lifecycle, but differ from the grey in excavating their burrows under wattle trees (Acacias), and feeding on the leaf litter.

The number of moths that emerge appears to be related to the amount of rain, and presumably the ease at which the matured pupae can emerge from the ground as adult moths. I note that in 2012 at Gang Gang there were six rain days between the 10th of April and the big swift moth emergence did not occur until May 24th, but falls were all under 10 mm in that two week period. The rain that coincided with the large number of emergent moths on May 24th was over 30 mm. So, was my entomologist colleague at the insect collection correct about the moth predictions? Well perhaps not in the strictest sense – after all, six rain days. But were there moths then that we did not notice? Probably.

There are two things that strike me about the swift moth story. The fact that a handsome moth that lasts for only a few days or weeks is two years in the making. You can only hope that they enjoy their larval lifestyle, sleeping in a silken burrow and eating grass. The second cause of wonderment is that in order to understand some fragments in the life of one insect species, someone spent years, night and day, observing adults and larvae, and culturing eggs and larvae in the lab to observe their development times and habits. In this case, it was an R. J. (Bob) Hardy, from the Department of Agriculture, Tasmania. I raise a toast to him.

Of what value is an insignificant moth such as this? I can think of one thing, apart from their general appeal to the naturalist. These animals are have the role of little ecosystem engineers. Their tunnels are about a centimetre wide, and they bury organic matter by defecating digested grass while in their holes. So rainwater can trickle in, and nutrients are recycled deep into the ground. Having tried to dig even a small hole 25 cm deep in the local stony clay soils, I can tell you it is quite an achievement. These creatures condition the soil, countering the compaction and erosion caused by livestock by enriching, aerating and watering the grasslands. They deserve a toast too.

And as for April 10th perhaps it can be regarded as the opening of the swift moth season. I’ll keep an eye out.