How many flies can you breed in a summer?
Start with a single pair of flies, which breed in Spring.
Suppose 150 of the larvae live, grow into flies, pair off, and breed.
Suppose this goes on all Summer, with each pair producing 150 larvae.
By the end of the summer, the original pair would grow to 8.56x1020 flies (865 million, million, million).
That’s enough flies to bury all of Australia to a depth of 11 metres (packing them in at 10 flies per cubic centimetre).
Blow flies
In cities, blow flies are less of a problem then bush flies.
But one type of blow fly is a big problem for sheep farmers.
These blow flies can weaken a sheep, even kill it.
Or rather their larvae can.
The blow fly L. Cuprina lays eggs in a sheep’s wool.
The eggs hatch out larvae, and these dig at the sheep’s skin (blow-fly ‘strike’).
The little wounds begin to ooze pus.
The unfortunate sheep is almost eaten alive.
The farmers fight back with chemical sprays and by burying carcasses (because blow flies breed in rotting carcasses, as well as on live sheep).
But recently, blow flies have become more resistant to chemical sprays.
So the hunt is on for other ways to control them.
So far there has been no success in finding other insects to attack blow-fly larvae that feed on live sheep.
But there is some hope for using complex genetic control methods.
The blow fly breeds in carrion or on live sheep.
There are predators which attack blow-fly larvae in carrion, but these predators aren’t genetically programmed to look for larvae on live sheep.
For this reason, research on biological control agents has come to nothing, and few hold much hope for the future.
But the clever people at the CSIRO are investigating genetic control methods.
The idea is to develop in the laboratory strains of flies which have genetic defects, then release them.
When they breed, their genetic defects will pass to the general blow-fly population and reduce their fertility, or ability to survive.
The prospects for this line of attack seem fairly bright.
Sheep blowflies came into Australia as uninvited guests in about 1880.
The guess is that they came from Africa, as stowaways.
The records of blow-fly strike begin in the late 1880s.
By 1915, L Cuprina had become a serious pest to the sheep-farming industry in eastern Australia.
By the late 1930s blow flies had spread to Western Australia.
They hit Tasmania as a big problem first in 1957.
Now blow flies are everywhere.
Stable flies
You aren’t likely to experience these unless you live on a farm or near a dairy.
They breed in hay that’s soaked in urine and dung.
And they bite.
Mostly they go after cattle, feeding on their lower legs and making them kick and stamp.
The stable flies follow the cows to pasture, then return with them into the dairy at milking time.
Here they often take a few bites at the person who runs the milking machine.
Stable flies look almost the same as house flies.
The main difference is their mouth parts.
The house fly has a sucking part, ending in a spongy disc.
The stable fly has a kind of needle for piercing skin and sucking blood.
March flies
March flies tend to be bigger than house flies.
Some are grey, others are brown.
And as your probably know, they bite hard, suck blood, and hurt.
Their larvae grow up in mud or in decaying organic matter.
They are very seasonal and are only around for a short time
Their scientific name is Tabanidae.
How bush flies get into the cities
In the winter, bush flies die out in the southern part of Australia where it’s too wet and cold for them.
But they keep breeding in the warmer north and the drier inland.
The population rises and falls with rain fall and temperature fluctuations.
From August to November, warm winds blow from the north. (pretty regularly, about twice a month).
These winds lift clouds of pregnant female bush flies and carry them south.
The flies come south in steps sometimes hundreds of kilometres a day.
And they don’t get blown back when the wind shifts to the north.
The wind acts like a one-way valve.
When the wind blows from the south it’s too cold for the flies to get airborne.
They stay where they are.
They wait for the next warm wind coming from the north.
As the flies come south, they find winter pasture covered with good-quality cow dung.
The females start laying eggs, just as the air temperature is warming up.
It’s the new-bred flies from these rich breeding grounds that swarm into Australia’s southern cities.
When to expect flies around Australia.
Here’s a rough guide city-by-city.
Sydney
Bush flies blow in from the pasture-land breeding areas on hot westerly winds.
This means the bush-fly population in Sydney is erratic, because the winds are.
Melbourne
Bush flies appear as early as September, with the numbers increasing through October and November.
The peak is in December or January.
The house fly population rises in about the same pattern.
The Victorian Fly Suppression Unit describes the problem as follows:
“The bush fly has been identified as the primary outdoor pest, especially at social functions, sporting activities and other pursuits in open areas.
It occurs around homes in the outer suburban areas each summer and can also be a problem in the inner suburbs and the city itself in certain ‘plague’ years.
House flies, stable flies and blow flies are secondary problems in and around homes, particularly at backyard barbeques or other outdoor leisure activities.
Unlike bush flies they usually do not travel far from their breeding source and large numbers can generally be traced to neglected plant or animal waste in the near vicinity.
Summer breeding takes place throughout pastoral areas of Victoria. In winter, the fly dies out in Victoria, but each spring returns from states to the north of us, carried on the northerly winds.”
Adelaide
No one has countered bush flies here, or seems to worry about them much.
The reason is that they’re only a problem now and then, when they come in on a hot northerly wind.
Perth
Bush flies are first noticeable in October.
The numbers rise steeply in November.
They peak in December (when people and food can be covered with bush flies at December barbecues and outdoor functions).
After the peak, the numbers fall off fast.
By the end of January, almost all the bush flies are gone.
This sharp peak and sharp drop-off only happens in the south-west of Australia.
In the south-east, the bush flies linger until March.
The pattern in Perth is very regular.
To the north-east, there’s a large amount of agricultural land with a climate where bush flies survive the winter and breed all year, with the breeding rate depending on the temperature.
By September, breeding increases rapidly because of higher temperatures and drier weather.
Temperatures are also high enough for large numbers of pregnant female flies to move south-west on north-easterly winds.
They then breed in the rich cow dung in the southern pasture-lands.
Brisbane
There isn’t much of a bushfly problem here.
The pasture-lands around Brisbane are poor for breeding in the summer because it’s too wet.
A few bush flies come in on the wind but these stays aren’t a great nuisance.
Canberra
Some bush flies start appearing in October and November but they aren’t usually a problem until December.
Then they reach a peak, and the numbers only decline very slowly.
There are still bush flies around in March.
Alice Springs
Bush flies are always around, but not normally in great numbers.
The fly population normally rises quickly after heavy rainfall.
The rain causes a spurt in plant growth, and this leads to better-quality animal dung.
The flies breed fast in the dung, then spread out looking for other good breeding sites.
But if the weather remains dry, in a month or two the fly numbers are back to the average (low) figure.
although there aren’t great numbers of bush flies in Alice Springs (compared with Perth in December) they tend to be small, persistent and very annoying.
Also, there’s the ‘desert effect’ mentioned earlier:
There may not be many flies, but there aren’t too many targets for them either.
Any targets there are (visitors to Alice, for example) can get pestered a great deal.
Tasmania
Sometimes Tasmania gets no bush flies at all for several years.
When they do appear, it’s only in the warmer months.
Subtropical areas
Bush flies are active in the northern subtropics all year round.
After the summer rains start, the numbers build to a peak in autumn. The numbers usually stay high over winter, but then drop quickly during spring.
In the drier tropical areas, especially the north-west, the main build-up comes in late autumn and early winter.
There’s a rapid decline during spring and there are hardly any flies at all during the summer ‘wet’.
Temperature and other things that affect how many flies pester you
Yesterday the bush flies were all over you, up your nose, in your eyes, all over your toast and jam.
But today there’s none to be seen.
Why?
But Some of the factors have been studied closely, and give some interesting clues include:
Temperature. Below 12°C, bush flies don’t fly. They usually keep still, and prefer warm surfaces.
(If you notice some that have been still for a long time.
They might be dead.
Low temperatures can kill them).
When the temperature goes up just a bit – to about 12.5°C – some of them fly again.
But only short flights.
As the temperature rises, they get more and more active, and bother you more and more.
At 27.5°C they’re at their worst.
At higher temperatures, they fly less, and pester you less.
And at 35°C, they head for the shade, like the rest of us.
They just sit in the trees, or maybe inside a house (the only time they are likely to go inside).
Humidity
This hardly affects flies at all.
It doesn’t matter much whether it’s a sticky, humid day, or a very dry one.
Some have the theory that flies get more annoying on dry days.
If so, it’s not the humidity itself that’s directly affecting their behaviour.
But low humidity can have an indirect effect: if you notice more pesky flies on dry days, it may just mean they’re thirsty and want a drink from your eyes.
Wind speed.
Strong winds don’t make bush flies less pesky.
They bother you almost as much on a windy day as on a still one.
Wind speeds of 36km/hr and more were common in one set of experiments, but it didn’t keep the flies grounded – or off people.
Although bush flies take to the air on a windy day, they get blown downwind.
Relative to the air, they fly pretty slowly (about 8km/hr, top speed).
Solar Radiation
Bush flies get more active as the solar radiation increases.
If the air temperature and everything else is the same, flies are more of a nuisance, the more solar radiation there is.
But solar radiation only affects fly activity about one-sixth as much temperature does.
This has been determined by observing the addictive regression relationship between weather variables and logarithms of fly catches). For one thing, this means bush flies slow down around sunset – even if the temperature is their favourite 27.5°C.