About ten years ago, the Asian tiger mosquito Aedes albopictus arrived in Germany. It was first discovered in 2007, when researchers from Freiburg found the first samples in an insect trap near a freeway at Weil am Rhein, close to Germany's border with Switzerland and France.

Since then, the insect has spread rapidly in southwestern Germany.

"In 2015, we saw the first massive proliferation in Freiburg," recalls Norbert Becker, a parasitologist. "People got stung massively in a small garden complex. They couldn't spend their days outside anymore. Unlike our common mosquitos, tiger mosquitos are active during the day."

Since then, the invading insects have arrived in many more southwestern German cities: Heidelberg, Loerrach, Karlsruhe and Sinsheim. "They keep moving north," the professor says.

And it's not just that they sting. Tiger mosquitos transmit dangerous viruses, like Zika, dengue or chikungunya.

While these viruses have not been transmitted inside Germany, the mosquitos are already here.

And with them, the probability of an infection, while still low, is rising.

Traveling - with cargo

There have, however, been cases of such viral transmissions around the Mediterranean, whether in Greece, southern France or Croatia. 

The tiger mosquito had in fact arrived in Europe by the 1970s. It first appeared in Albania, most likely imported with cargo from China. Due to the political isolation of the country, it stayed there for some time.

In 1990, the tiger mosquito arrived at the Italian harbor of Genoa – hitching a ride inside used tires in container ship from the US.

After a rainstorm, larvae hatched inside the tires and rapidly multiplied. At the turn of the millennium, the tiger mosquito spread even further. It traveled on board campers, in cars and trucks - along nearly the entire European Mediterranean coast and all the way to Spain.

High temps, high times

With the mosquitos came viruses: There were incidents of dengue infections along the coast of Croatia - and even on the Atlantic island of Madeira. 

More dramatic, however, was a chikungunya epidemic in the summer of 2017 in Italy.

"We had very high temperatures during the summer months," said doctor/virologist Jonas Schmidt-Chanasit of the Bernhard-Nocht-Institute for Tropical Medicine (BNITM). "That was favorable for the disease. More than 300 people have gotten infected so far."

Mosquitos breed much faster in high temperatures, and the same goes for the viruses they carry. They have no way of controlling their body temperature. So if the mosquito is cold, the virus cools as well.

"There's an optimal temperature for viruses," the doctor says. "More than 30 degrees Celsius [86 Fahrenheit] is great for them. They can multiply much faster than at 18 or 21 degrees."

Extreme weather

For insects and their viruses, a few degrees of average global warming would probably not make much of a difference. But unusual seasonal weather conditions can.

Such events open a window of opportunity for vector-borne diseases - and due to global warming, these windows are opening up more frequently.

"Heatwaves are clearly an advantage to viruses" Schmidt-Chanasit says. "In hot periods, there are more mosquitos and each of the mosquitos carries more viruses."

There doesn't even need to be a lot of rain, since the insects will be able to find the smallest water reservoirs to lay their eggs. Even vessels catching condensed water from some air conditioners may provide a breeding ground for thousands of larvae.

The arrival of the West Nile virus in the US is a textbook case, showing just how important unusual weather conditions can be. The virus first appeared in New York in 1999. Since then it has spread throughout large parts of the US.

"One can say with a fair degree of certainty that the virus arrived with an infected mosquito aboard a plane from Tel Aviv," says Jürgen May, the head of infectious disease epidemiology at the BNITM. 

A chain of specific weather conditions then made the spread of the virus possible:

"There was a mild winter. That enabled the infectious mosquitos to survive until the spring," May said. "There was a warm and dry early summer, which was good for the mosquitos. They stung migratory birds, which then carried the virus further afield. The late summer was wet and lots of new breeding grounds emerged."

But what was essential in spreading the virus? Was it "only" the weather? Or was it climate change as well?

We will never know, epidemiologist May says.

"Nobody will be able to prove whether we might or might not have had just the same situation ten or fifty years earlier."

Pre-existing populations

In the US, the spread of the West Nile virus was not carried out by "immigrant" mosquito species. The mosquitos that did the job were already there.

"These were mosquitos that sting both humans and birds. And that enabled the virus to spread very far in a short timeframe."

However, not every kind of mosquito can carry any kind of virus. An epidemic will only develop if insect and the disease are a match for one another.

Malaria's move to higher grounds

Malaria is not a virus, but a plasmodium, or infectious protozoa. These germs also multiply much better when it's warm. Just a few degrees' change in temperature (Celsius) is enough for the infectious malaria sporozoites to ripen.

"In the case of Plasmodium vivax, which is one of the malaria plasmodia that are dangerous to humans, it takes roughly 38 days if you've got 16 degrees Celsius," May says. "At 20 degrees, it's about 17 days. And at a temperature of 30 degrees, it's only seven days."

He's observed as the disease has begun reaching African large cities that were once built at higher elevations for the express reason that malaria would not reach the people there. This is the case in the Kenyan city of Nairobi as well as in Addis Abeba in Ethiopia, or in Antananarivo in Madagascar.

In this case, the malaria mosquitos are decisive. They are moving to higher grounds.

No intention of leaving

Meanwhile, in southwestern Germany,  Heidelberg parasitologist Norbert Becker is fighting the tiger mosquito with vigor.

"We want to get rid of that tiny creature," he says.

Becker is working with the Kommunale Aktionsgemeinschaft zur Bekämpfung der Schnakenplage (KABS)- a task force that brings together towns, villages and counties along the upper Rhine.

The KABS has been fighting mosquitos since 1976 using environmentally friendly methods. The invasion of the tiger mosquito poses a whole new challenge for them.

The most important thing, he says, is to inform the population through leaflets about their own responsibilities: Cover up rain barrels, don't let your watering cans or other containers fill with standing water.

Where this is not enough, the pest controllers use the active agent of Bacillus thuringiensis israelensis (BTI). It kills mosquito larvae but will not harm other insects or organisms and can be applied in the form of tablets in gullies and drains.

Becker and his colleagues have also had success with specially bred male mosquitos that were then sterilized using gamma rays. When those mosquitos copulate with female mosquitos, there will not be any fertile offspring. 

Will it be possible to drive the tiger mosquito out of Germany that way?

Becker remains skeptical.

"After I saw how the mosquito populations exploded this year, I lost most of my hope. I don't believe we will be able to eradicate that pest," he says. "But at least we're trying."