Alongside the traditional helpings of ticks and mosquitoes, people in Switzerland are having to face a far itchier prospect as the summer comes to a close. Thanks to the warm weather, the number of black flies present across Swiss cities and cantons has risen dramatically. Here’s what you need to know about the creature, and how to treat their infamous bites.

Black fly population unexpectedly surges in Switzerland

Niels Verhulst, from the Institute of Parasitology at the University of Zurich, confirmed that significantly more black flies have been reported in Switzerland compared to last year. While their populations are not strictly monitored, large numbers of the flies have been appearing in other measuring devices like mosquito traps.

Speaking to 20 Minuten, Verhulst said that "black flies are actually at their peak in spring." However, he suggested that the warm and humid weather seen in recent weeks “could have led to a second breeding phase.”

What you need to know about the black fly

While they may look harmless, the 33 species of black flies (or blackflies) that call the alpine nation home pack a punch, as they are known for causing severe itching. The flies are between two and six millimetres in size, and as suggested by the name are mainly black and grey in colour. They are mostly active around Swiss rivers, streams and lakes at dawn and dusk.

What happens when black flies bite a human? (squeamish readers skip this part)

When a female black fly bites a human, they use their mouths to saw a small hole into the flesh, creating a mini crater which they then use to suck up blood needed to develop their young. Like mosquitoes, the flies use their saliva to numb the wound. 

The difference is that when the saliva wears off, black fly bites are known to be just as itchy if not more so than mosquitos, with the site of the bite often swelling, blistering and turning red. Further inflammation is caused by scratching, with biologist Sven Klimpel from the Goethe University in Frankfurt noting that their salvia can also trigger an allergic reaction in some people, which can be severe in some limited cases.

Black fly bites are also known to be vectors for disease, though not in Switzerland. However, their bites can become infected if they are scratched excessively and not washed.

How to avoid being bitten by a black fly

Luckily, there are several ways to avoid being bitten by a black fly, the first being to avoid flowing water. If this isn’t possible (or desirable), then wearing long baggy clothing can also help. The flies also don’t tend to bite when you are moving and really do not enjoy closed spaces. Natural and chemical bug sprays are also effective.

What should I do if I am bitten by a black fly?

If you are unlucky enough to be bitten by a black fly, be sure to disinfect the bite as soon as possible. Do not scratch the bite, no matter how itchy it becomes - antiallergic gels can be helpful in this regard, especially if they are cooled in the fridge beforehand.

Sandra Müller from the Stauffacher Pharmacy in Zurich told 20 Minuten that most bites should improve over time. However, “As soon as it swells up a lot, the redness spreads instead of going away,” or the bit area is warm or pulsating, “You should go to the doctor."