The Heat Is Gone. Why Are Seoul's Mosquitoes Still Biting?

Summer finally seems to be leaving Seoul.

The unbearable heat has eased. Nights are cooler. People are starting to think about long sleeves, autumn clothes, and sleeping with the windows open again.

And then the mosquitoes came back.

At least, that's what it feels like.

One person recently described spending the night hunting mosquitoes.

First, there was a faint buzzing sound.

One mosquito was caught.

Then another.

Then another.

Eventually, the person woke up in the middle of the night and got out of bed.

By morning, there was a serious question to consider:

Should I buy one of those small mosquito nets that cover a single bed?

But what if the mosquitoes suddenly disappear next week?

Then I'd have bought a mosquito net for nothing.

That seems to be the kind of conversation people in Seoul are having these days.

Where Did the Mosquitoes Go in Summer?

This year's mosquitoes seem to have confused everyone.

During the hottest part of the summer, some people barely noticed them.

Then the weather cooled down.

Suddenly, they were back.

And not just outdoors.

People have been reporting mosquitoes in apartment hallways, near elevators, inside libraries, and in their homes.

One person even wondered whether mosquitoes were somehow following people through the front door.

The idea isn't completely as strange as it sounds.

Mosquitoes use several cues to find humans, including carbon dioxide from our breath, body heat, and human odors. They combine these signals as they search for a host.

So when you open the front door and a mosquito happens to be nearby, it may get a very useful invitation.

The door opens.

You walk in.

The mosquito comes in too.

You close the door.

And now you have a roommate.

“Maybe Mosquitoes Like Me More”

Another conversation comes up whenever mosquitoes become a problem:

Why do they always bite me?

Some people blame their blood type.

“I've heard mosquitoes like type O blood.”

Someone else says:

“I'm type A, and they bite me constantly.”

And then there is the family version of the mystery.

A husband, wife, and daughter are all in the same house.

But the mosquitoes seem to have chosen the husband.

When he isn't around, they move on to the daughter.

So maybe it isn't blood type after all.

There is actually some scientific evidence that blood type can influence mosquito attraction, but the story is not as simple as “mosquitoes love type O.”

One study found that Aedes albopictus mosquitoes landed more often on people with type O blood than on people with other blood types. But the differences were not equally strong across all comparisons, and blood type is only one possible factor.

Mosquitoes also respond to cues such as carbon dioxide, heat, moisture, and chemicals associated with human skin and breath.

So if one person in a family seems to be the mosquito's favorite, there may be more going on than their blood type.

Sweat Might Be Part of the Problem

People in Seoul have their own theories about this too.

Some say that people who sweat more get bitten more often.

Others say that people with warmer skin attract more mosquitoes.

There is some science behind parts of this idea.

Mosquitoes can detect heat, and they use several human-associated cues when searching for a blood meal.

That doesn't mean you can identify the mosquito magnet in your family simply by looking at who sweats the most.

But it does help explain why mosquitoes don't simply fly around randomly.

They are looking for us.

And they are surprisingly good at it.

“Have Seoul's Mosquitoes Evolved?”

This may be my favorite complaint.

One person wondered whether mosquitoes had actually evolved this year.

They seemed smaller.

They didn't make the familiar buzzing sound.

They approached quietly.

They disappeared before being noticed.

And instead of feeling a mosquito bite immediately, the person sometimes noticed only later, after the area had become itchy and swollen.

The description was perfect:

Stealth-fighter mosquitoes.

Have they really evolved into tiny stealth aircraft?

Probably not.

At least, there is no reason from these observations alone to think that Seoul's mosquitoes suddenly evolved into a new breed this year.

A mosquito that is difficult to hear may simply be harder to notice in a particular environment. Different mosquito species also behave differently.

But from the point of view of someone lying awake at 2 a.m., the distinction may not matter very much.

Silent mosquito.

Loud mosquito.

Small mosquito.

Large mosquito.

They are all equally annoying when they're looking for you.

Why Now?

There is actually a good reason the timing feels strange.

We tend to think of mosquitoes as a midsummer problem.

But mosquito populations do not simply rise as temperatures rise forever.

Research on mosquitoes in Seoul has found that temperature and precipitation both influence mosquito abundance, and mosquito numbers can be high in both summer and autumn. Extremely high temperatures can actually reduce mosquito abundance in some circumstances.

That helps explain a familiar pattern:

It is unbearably hot.

The mosquitoes seem less noticeable.

The weather finally becomes more comfortable.

And suddenly you notice them again.

This year, Seoul's official mosquito monitoring system has reflected some dramatic fluctuations.

On September 22, the city's mosquito activity index reached 100, classified as Level 4, “uncomfortable.” It remained at Level 4 on September 24 and 25 before falling to Level 3 on September 26.

So perhaps the mosquitoes aren't imagining the change.

And neither are the people complaining about them.

The Mosquito Net Debate

Once the mosquitoes become bad enough, people start looking for solutions.

One person considered buying a single-bed mosquito net.

Another said they had already been using one for two years and recommended it, especially because mosquitoes could remain a problem into November.

Someone else had an interesting discovery.

They had set up a tent-like cover over their bed, thinking it was a mosquito net.

It turned out to be a heating tent.

But it still kept the mosquitoes away well enough to let them sleep.

Sometimes you don't need the perfect solution.

You just need to get through the night.

“Why Are They Only Coming to My House?”

This may be the most universal mosquito question of all.

People see mosquitoes near the entrance to their homes.

They see them in elevators and hallways.

They find them inside their apartments.

And after being bitten again and again, it is very easy to start thinking:

Are all the mosquitoes in Seoul coming to my house?

Probably not.

But Seoul's mosquito forecast shows that local conditions can vary, and the city specifically recommends checking window screens, closing openings through which mosquitoes can enter, and removing standing water around homes.

Sometimes the problem may be outside.

Sometimes it may be the tiny gap around a window.

Sometimes it may simply be that one mosquito followed someone through an open door.

And sometimes, unfortunately, you may just be the person the mosquito happened to find first.

Summer Is Leaving. The Mosquitoes Didn't Get the Memo.

There is something very Seoul about the way people talk about mosquitoes.

They compare notes.

They ask what blood type someone has.

They recommend mosquito nets.

They complain about being bitten dozens of times.

They wonder whether the mosquitoes have changed.

They blame the weather.

They blame their house.

And occasionally, they blame their spouse for somehow attracting all the mosquitoes.

Meanwhile, the mosquito simply keeps flying.

The heat is finally fading.

The evenings are becoming pleasant again.

People are getting ready to enjoy autumn.

But before you open that window, you might want to check the screen.

Because summer may be almost over.

The mosquitoes apparently didn't get the memo.