Why mosquitoes continue to bite merrily into November

Mosquitoes thrive in warm, humid conditions. In such environments, their tiny bodies reach their optimal operating temperature without risking dehydration. We've become accustomed to seeing them in greater numbers near bodies of water and on mild summer evenings.

But what's the deal if you're still getting bitten in November? Do we have to get used to it because of climate change? There are two answers to that. The first is: Yes, at least in the long term, there are indications that the distribution area of ​​mosquitoes is increasingly expanding northward. This is suggested, at least, by model calculations for tiger and bush mosquitoes.

The second answer is somewhat more nuanced: While it's a plausible assumption that mosquito repellent will be needed regularly until the end of November, Pie Müller from the Swiss Tropical and Public Health Institute in Basel believes it's not that clear-cut. The mosquito specialist points out that occasional mosquito bites in November "have probably always occurred."

Normally, mosquito activity comes to an end when the days become colder and shorter. The male mosquitoes then die off. The females seek out a sheltered place such as a cave or a barn, where they enter a state of torpor – and thus overwinter. The eggs and larvae of mosquitoes are also frost-resistant.

Often, a few warm days are enough for the females to return to active life from this state. Müller has observed several times that they are still looking for fresh blood even in November. "Especially after rainy summers," he says.

Because the generation time of mosquitoes only lasts about two weeks, a mosquito population can adapt very quickly to the prevailing conditions. That's why there are enormous fluctuations from year to year, explains Müller.

Such fluctuations make it very difficult to identify a general trend in the number of mosquito bites in November over the past few decades. This would require systematic data collection. "Unfortunately, that doesn't exist," says the insect researcher.

Gabi Müller, head of the pest prevention unit at the Zurich Environmental and Health Protection Agency, assesses the situation similarly: Even now in November, she is still receiving occasional photos of mosquitoes. These come from members of the public reporting potential tiger mosquitoes in their area.

Müller has been performing her duties for almost thirty years. Initially, however, she had nothing to do with mosquitoes. This only changed with the emergence of the Asian tiger mosquito. Originally from Asia, this species is feared because it can transmit a number of pathogens, such as the dengue, chikungunya, and Zika viruses.

When tiger mosquitoes are discovered anywhere in the city, Müller and her team go to considerable lengths: They set up traps and treat the water in nearby street drains with a biological agent specifically effective against mosquito larvae. The effort is worthwhile, says Müller. "In Wollishofen, we succeeded in eradicating a tiger mosquito population this way."

However, the photos Müller receives these days mostly depict bush mosquitoes. Like the tiger mosquito, they were originally native to the east and are also expanding their range further and further as trade flows increase.

Compared to the Asian tiger mosquito, bush mosquitoes play only a minor role in the transmission of diseases. "We are not taking any action against bush mosquitoes, nor against the native mosquitoes," says Müller.

So, we simply have to put up with these pests. If the mosquitoes make themselves known late in the year, firstly, we can still enjoy the nice weather – and secondly, we can expect that it won't be long before winter begins and puts an end to their troubles.