You don’t have to venture into the depths of Bukit Timah Nature Reserve or have a picnic in the Botanic Gardens to get attacked. You could be in the sanctuary of your own home, windows closed and air-conditioner switched on. Most times, the assault is covert; you might only know when an itchy spot gets your attention.
But by then, the damage is already done. You have been bitten by a mosquito.
In Singapore, concern over mosquito bites go beyond the superficial itch and for good reason, too. More than 6,000 cases of dengue have been reported so far this year – more than the 5,258 cases recorded for the whole of last year, according to the National Environment Agency (NEA).
The number has already hit 941 cases for the week ending Apr 23 – an increase of 28 per cent, or 208 cases, from the previous week’s 733 cases. And this is even before Singapore reaches the traditional peak dengue season from June to October, NEA noted.
With the number of dengue cases rocketing, it certainly pays to be vigilant about weeding out mosquito breeding grounds. To date, over 170 species of mosquitoes have been identified in Singapore by the NEA’s Environmental Health Institute. Of the lot, the three most common groups of mosquito here are Aedes, Culex and Anopheles.
While most of the species are found in rural, forested areas, the infamous Aedes aegypti – the main species that transmits dengue in Singapore – has adapted to urban environments and developed a taste for human blood, according to the NEA.
NEA calls for ‘urgent action’ as dengue cases surpass 6,000 in under 4 months, more than in whole of 2021
Aedes albopictus, which breeds in areas with a lot of greenery, can also transmit dengue but it doesn’t do so as efficiently as Aedes aegypti. Both species can transmit the chikungunya and Zika viruses.
Aedes aegypti and Aedes albopictus bite primarily during the day, said the NEA, but they can also attack at night in well-lit places. After sundown, the Culex and Anopheles take over the bloodsucking. Only female mosquitoes bite and spread diseases; male mosquitoes are quite happy feeding on nectar.
WHAT ATTRACT MOSQUITOES
- Your breath
Your skin’s unique concoction of bacteria and sweat is a magnet that draws in the winged blood suckers, said Dr Cameron Webb, clinical lecturer at The University of Sydney’s Westmead Clinical School, and Institute Of Clinical Pathology And Medical Research.
In a The Conversation article, he wrote that this smelly mix comprising up to 400 chemical compounds on the human skin “is likely to explain why there is substantial variation in how many mozzies we attract”.
“One of the best studied substances contained in sweat is lactic acid,” said Dr Webb. “Research shows it’s a key mosquito attractant, particularly for human-biting species such as Aedes aegypti.”
- The chemicals you emit
The ones that are linked to oestrogen can make you irresistible to mosquitoes, said Dr Jonathan Day, a professor of medical entomology at the University of Florida, in a ABC News article.
Once they hone in, mozzies use your body heat “to very quickly determine where blood is closest to the surface,” he said.
They do this before inserting a needle-like proboscis into your skin to extract the blood, typically from your forehead, wrists, elbows and neck. If you’ve just finished a workout or are feeling warm, you may be more prone to bites as your blood is closer to the surface of the skin throughout your body, he said.
- Your clothes, perfume and maybe your feet
You may prefer to wear dark colours but if you’re going somewhere bright or has light colours, the contrast is akin to painting a big target sign on yourself.
According to Rentokil’s medical entomologist, Dr Chan Hiang Hao, mosquitoes are attracted to colour contrasts. “Certain floral perfumes are also known to attract mosquitoes,” he said, adding that things like Limburger cheese and dirty socks, especially nylon ones, are also mosquito magnets.
However, Dr Webb noted that the mosquito-and-cheese link doesn’t apply to every mosquito species. “The bacteria that gives (Limburger) cheese its distinctive aroma is closely related to germs living between our toes,” he said. That explains why Anopheles gambiae that spreads malaria are attracted to smelly feet.
But replace the malaria-bearing mosquito with another species such as Aedes aegypti and the phenomenon is not repeated, said Dr Webb. “Even pathogens such as malaria may make us more attractive to mosquitoes once we’re infected.”
DO THESE MOZZIE-REPELLING METHODS WORK?
The most basic method of mosquito-proofing yourself is to apply an insect repellent and wear long-sleeved clothes when you’re out and about.
To rid your home of the irritant, the best thing you can do is to remove stagnant water around the house to stop them from breeding. You could also consider installing mosquito screens on the windows or spraying dark corners with an aerosol insecticide, advised the NEA.
But beyond those methods, there are also many other items on the market that promise to shield you from mosquitoes – from insect repellent sprays to patches, wristbands, essential oils, bug zappers and even apps. How do they stack up? Here’s a look.
- Mosquito repellent apps
They work on the principle that certain sound frequencies can stop mosquitoes from biting, so the apps supposedly emit a range of frequencies to accomplish that. Just how effective are these digital deterrents? “These apps are most likely a hoax. There is no strong scientific reason and study to prove that this works,” said Dr Chan.
- Wristbands and patches
These are usually infused with citronella or lemongrass essential oils, which have a very apparent aroma once you remove them from their packaging. But they may simply wind up making you smell good, that’s all.
“Unfortunately, essential oils, including the commonly believed lemongrass, do not work,” said Dr Chan. “Wristbands and patches are not effective due to the limited coverage. They only protect areas where they are stuck on or worn,” he added.
- Topical insect repellents
Repellents containing DEET (N,N-diethyl-m-toluamide), picaridin or IR3535 as the active ingredient are the most effective in repelling mosquitoes. “These products are safe even for use by pregnant women and nursing mothers. Products with less than 10 per cent DEET are safe for young children and infants from two months of age,” said a spokesperson from NEA.
But if you’re chemophobic, you could give natural repellents a shot. However, don’t expect them to work as well as the chemical ones. “Although these can generally provide some protection against biting insects, they are less effective than those containing DEET, picaridin and IR3535,” said the NEA spokesperson.
“The effectiveness of repellents based on plant-based extracts, such as citronella, eucalyptus and other essential oils, varies from person to person, and such products usually require more frequent application.”
- Aerosol insecticides
These sprays can kill mozzies – provided you use the right one. Before you depress the can’s button and release the spray, check the content, said entomologist Jo-Lynn Teh on the Thermacell Singapore website.
“Aerosol insecticides are designed differently for crawling insects (oil-like formulation) and flying insects (water-like formulation). Using the wrong one without reading label not only makes your room oily, you may also expose yourself to unnecessary hazard,” she said.
- Bug zappers
These light devices typically have a lamp that emits ultraviolet or UV light to lure the mosquitoes in. The space in the metal mesh is just right for a mosquito to complete the electric circuit when it tries to fly through, and it gets killed by the current.
But don’t count on the devices working 100 per cent. While they may zap more than 10,000 bugs in one evening, they may not all be mosquitoes. “The UV light devices do catch mosquitoes to a certain extent. However, mosquitoes are more attracted to a combination of human odour and body temperature,” said Dr Chan.
- Mosquito coils
If, for some reason, you’re still using mosquito coils, consider giving them up, said Dr Chan. “They produce smoke and an unpleasant smell, and are a potential fire hazard. They also dirty the area with ashes.”
His recommendation is to use the electric liquid vaporiser instead. “It is more user-friendly with no smoke or smell. It will not cause a fire and is rather long lasting (about one to two months).”