It is evening and you are just starting to relax after a hectic day. Like you, you hear the unmistakable high-pitched whine of a circling mosquito. It’s something most of us in Australia are used to, but does it feel like it’s happening more often this spring? Or that there are more of those bloodsuckers hunting for a meal?
We asked three experts from the University of Melbourne – PhD student Véronique Paris, Nick Bell and Professor Ary Hoffmann from the Faculty of Science’s Pest and Environmental Adaptation research group to explain exactly what’s going on.
Q) Are there actually more twilight repellents this spring or are we just noticing them more?
Yes, there seem to be more mosquitoes this year, you’re not imagining it. It’s something we’ve noticed ourselves
As part of our research, we are conducting surveillance around our lab space and offices to check for mosquitoes that could infest our research colonies and have recorded many more this year.
As we run a lab where we breed mosquitoes for research, it is important that we make sure these are not escaped mosquitoes. All mosquitoes around our offices have been identified as Culex species, which we do not currently keep in our research insect garden, which tells us that these are not escapees but invaders.
Of course, since our friends and family know we’re researching mosquitoes, we get a lot of messages and calls about the surge they’re noticing in mosquitoes. And, of course, everyone wants advice on how to control them.
As our team does a lot of field training around regional and rural Victoria, we have also noticed this on our field sites.
But it’s not just Victoria where we’re smearing more mosquitoes with insect repellent. New South Wales is seeing an explosive increase in mosquito populations, and local experts expect it to be far from over.
Since we know that these mosquitoes do not come from our labs, we do some genetic screening to determine what species these mosquitoes belong to. Interestingly, we suspect this isn’t a human-biting species, as the team has noted that they don’t try to feed us when we’re in the building.
Q) Why are there more mosquitoes this year?
The heavy rainfall in South East Australia is likely to be a big factor.
All mosquitoes require water to complete their life cycle, with some species specializing to breed in very small containers such as old tires, discarded plastic containers and tree holes. Other species prefer larger bodies of water such as water tanks or ponds.
Many pests breed well in standing water rather than running water, and recent rains and floods have produced much more standing water.
This often includes new ‘cryptic breeding grounds’ – places that can be more difficult to access, such as under houses, in garages and in the bits and pieces of junk many of us have accumulated around the backyard – cleaning up means you’re doing your part in the keeping the mosquito population down.
Most mosquito species develop quickly in warmer conditions, so the combination of heavy rains and rising temperatures as summer approaches provides twilight repellent with ideal conditions for their populations to grow.
Q) Mosquitoes seem bigger, should we be concerned?
Mosquitoes range in size from very small to quite large.
There are over 300 species found in Australia, and about 10 species are quite common around Melbourne. People around the world confuse cranes with extremely large mosquitoes, when in fact they are mosquito predators (commonly known as ‘Mosquito Hawks’ in other countries).
These are good insects that don’t bite you and that you like to have in your home. Spiders are your friends too – if you enjoy having spiders in your home, they will definitely help.
Then you have chironomids, which look very similar to mosquitoes to the untrained eye, but are not so closely related. Their common name – the non-biting mosquito – is a bit of a clue as to why you shouldn’t worry about them. Chironomids, like mosquitoes, breed in stagnant water and can form huge mating swarms.
Some “giant” mosquitoes like Toxorhynchites speciosus, which can grow more than 12 millimeters in size, does not bite humans. In fact, as adults, they are strictly vegetarian. In fact, their larvae (wrigglers) are predatory and eat other mosquito larvae, so these are another natural ally in your fight against mosquitoes.
So, in general, you don’t have to worry about the size of a mosquito. While many types of mosquitoes don’t bite humans at all, it’s still best to avoid them whenever possible, just in case.
Q) What can we do about it?
The most effective strategies are quite simple: avoid being outside at dusk and dawn when the human biting species is most active.
Wear long-sleeved shirts and long pants. Be sure to use insect repellent when you’re outside, and don’t forget to reapply after you’ve been exercising, swimming, or wearing it for a few hours.
On your property, do your best to remove potential breeding grounds – anything can hold standing water.
Some twilight sprays with an insect repellent, such as the common pest Aedes notoscriptuswill happily reproduce in very small amounts of water, so even the drip trays under your plants are a potential breeding ground.
This species can transmit viruses such as the Ross River virus and is also the prime suspect involved in the transmission of the bacteria that causes Buruli ulcer.
To ensure you don’t breed mosquitoes in your yard, remove all debris from your property, such as old cans, tires, and buckets, and empty all containers (such as drip trays under potted plants) after it rains.
Also, make sure you keep your gutters clean and that pipes and screens around your water tanks are intact, repair or replace them if not.
Anything that can hold water is a potential breeding ground, so if you have a water tank, make sure mosquitoes can’t get in and lay eggs. Old septic tanks are also a common breeding ground, so if you live in the countryside or on the outskirts, make sure these are well stocked.
You can also purchase granular products (“insect growth regulators”) to add to standing water that cannot be easily drained.
With climate change bringing more variable conditions, including more intense rainfall periods, it is inevitable that periods when mosquitoes become a problem will become more common.
And areas that didn’t have as many mosquitoes before could see explosive events like we’re seeing now.
In addition, changes in the distribution of mosquito vectors and disease-transmitting hosts mean that diseases are likely to emerge in new areas. We have already seen the recent outbreak of Japanese encephalitis (JEV) in Australia, which has probably been taken over by migratory waterfowl.
Ringworm is one of the mosquito species that can transmit JEV. This species can breed in urban areas as well as natural habitats including bushland and wetland.
It is difficult to accurately unpack the range of effects we can expect from climate change, but we have some examples of mosquito species changing their range in a way that is consistent with climate change.
An important example of this is the Tiger Mosquito (Aedes albopictus), which has recently shifted its habitat exclusively from the tropics and subtropics to have now invaded most of Europe now that conditions are warm and the species appears to be evolving to handle cooler conditions.
Tiger mosquitoes are currently found in the Torres Strait, but have not yet established themselves in mainland Australia.
So yes, parts of Australia are seeing an increase in mosquito numbers this spring, and we’re likely to see more sources of this sort as the real impacts of climate change kick in.