Why are mosquitoes so bad this summer?

Swarms of mosquitoes are descending on Sydney. It’s been one of the worst starts to summer for pesky mozzies and it’s only going to get worse.

Mosquitoes need water and there’s been plenty of it. Water filling up our wetlands or backyard buckets provides habitat for mosquitoes. Warm humid weather also helps keep mosquitoes alive longer. That’s right, there are more mosquitoes and they’re living longer!


The saltmarsh mosqutio, Aedes vigilax, is closely associated with tidally influenced estuarine wetlands along most of Australia’s coast (image: Stephen Doggett)

Predicting boosts in mosquito numbers isn’t always easy. The reason is that mosquitoes respond to the pattern of rainfall, not just the quantity.

Rain, Rain, Rain

We’ve had over 160mm of rain in Sydney in just the first 10 days of December. The long-term average is 70mm. It is when this rain is falling that makes the difference. If we’d had it all over a couple of days, there would have been one hatch of mosquitoes. Just one generation of adults buzzing about. Problem is we’ve had steady rain. It’s been spread out and so have been the hatches of mosquitoes. It keeps the wetlands flooded too so any wrigglers that have hatched from eggs won’t be stranded in a drying puddle.


Total daily rainfall recorded at Sydney Olympic Park (Data source: Bureau of Meteorology)

The rain is one thing but tides play a role too. While backyard mozzies rely on rainfall, our biggest nuisance biting pest, the saltmarsh mosquito (Aedes vigilax), relies on a combination of rainfall and tidal flooding of local wetlands. Funnily enough, they generally love a hot dry summer.

Swimming with the tide

In a cruel twist of fate, we’ve had tidal flooding of local wetlands at the same time as all the rain. The tides were nothing compared to the “king tides” we experienced last summer but, in combination with the rain, it resulted in complete flooding of our local estuarine wetlands. Maximum occupancy for mosquitoes.

Current monitoring along the east coast, particularly in areas close to estuarine wetlands, are recording above average numbers of mosquitoes. The collections are currently dominated by saltmarsh mosquitoes but there are plenty of “backyard” mozzies too. The current generation of mosquitoes was triggered by environmental factors in mid to late November.

Problem is, we’re going to see more as the next generation is currently wriggling about in our wetlands and backyard habitats. They’ll be emerging over the next few days….

While the current rain and tides are playing their part, something happened way back in October that set the wheels in motion for the marauding mozzies.

Springing into Summer

The hottest spring on record set the scene. Then, around the middle of October, there was about 100mm of rain that followed a few days after a series of higher than expected tides. Historically, regardless of rain, tides or temperature, there is rarely a substantial increase in mosquitoes. This year was different. it was a perfect storm of climatic conditions that “woke up” local mosquito populations a month or so early.

The early start to the season took everyone by surprise. The state-wide mosquito monitoring program generally only kicks off in coastal areas in December and there has never been any mosquito control in local wetlands in October. While I was filming a piece for Chanel 9 News along the banks of the Parramatta River, we were eaten alive by mozzies. I’d never seen so many that early in the season.

henandchickenbay_stormfrontMy experience in the past is that once mosquito populations gain some momentum, only an extended hot and dry period will slow them down. Each mosquito can lay up to (or beyond) 100 eggs. The more mosquitoes, the more eggs. The more eggs, the more mosquitoes. The cycle continues. The short-term forecasts are for  warm and humid weather along with continuing storms. Unfortunately, there may not be a break from mosquitoes for a while yet.

What can you do the beat the bite?

Here are five tips that will help beat the mozzie bites this summer.

1. Tip out, drain or cover any water-holding containers in the backyard. They may be buckets, discarded tyres or a taupaulin covering an old trailer or boat. Anything that collects water can be used by mosquitoes. Make sure your rainwater tank is correctly screened and your roof gutters are clean and free-flowing.

2. Screen your windows. This may not help when you’re outside but it will at least stop them coming inside. If you live near wetlands, give some serious thought to creating a screened outdoor area. There are lots of flexible screening options available. This is common place in some parts of North America and Europe, I don’t know why we don’t do it more of ten here in Australia.

3. Cover up or wear repellent. If you’re outdoors, particularly at dawn or dusk, cover up with long sleeved shirts and long pants. Pale colours tend not to attract so many mosquitoes. Apply repellent to any exposed areas of skin. There are lots of tips on using repellents here and here and here.

4. Sprays and coils and sticks and zappers. There are plenty of products available that contain insecticides. Aerosol “fly sprays” will help get rid of a mozzie buzzing about indoors but the better option are the “plugin” zappers that heat a small reservoir of insecticide. These are very effective and safer to use indoors than burning a mosquito coil. Mozzie coils and sticks should only be burnt outdoors or in sheltered areas. Pick a product that contains an insecticide. The botanical (e.g. citronella) based products will hep reduce the total number of bites but not prevent them all.

5. Listen out for warnings. Stay tuned to local radio, or keep your eye on the website of local health authorities. Mosquito bites can be pretty annoying but there is also concern regarding mosquito-borne diseases, particularly those such as Ross River virus. There is a number of state-based surveillance programs in place and warnings are often issued when elevated levels of pathogen activity is detected in local mosquitoes or there is an increase in human disease reported.

Join the conversation on Twitter. Is this the worst summer for mosquitoes in your part of Australia?