Tiny black bugs slipping through window screens in Japan: what they are

If you've found tiny black bugs crawling around your window the morning after rain, you're not alone. In late October, people in Japan began posting online about them, and one wrote, "I really can't take this."

The culprit passes straight through the mesh of window screens. Here's what local authorities say about it.

What are these bugs?

The bug is called kurobane kinokobae, a type of dark-winged fungus gnat. According to the city of Iwaki in Fukushima Prefecture, it measures only 1 to 2 millimetres. Its body is black to dark brown.

It can appear all year round, especially during the rainy season. Iwaki says it has received many complaints about black bugs getting indoors through screen doors.

Gifu Prefecture has also warned that the bugs can show up in autumn. They become more active when the temperature climbs above 25°C, and they crawl over hands, feet and faces.

Why now?

The bugs tend to appear in large numbers when a sunny day follows rain. The city says they come in through gaps in screens from dawn into the morning.

It rained in the Kanto region (the area around Tokyo) on 4 and 5 November. A high of 25°C was forecast for central Tokyo on 6 November. That is exactly the kind of weather the bugs like.

On social media, some people said they can't open their windows because the bugs get through the screens. One person wrote that hundreds of them are lying by the window every morning. Another said rain had caused a huge outbreak.

How do you keep them out?

Iwaki and Gifu both say the basic rule is to stop them getting in. Their advice:

• Keep windows closed.
• Open and close doors as little as possible.
• Avoid using ventilation fans.
• Don't bring planters or flowerpots indoors.
• Seal gaps around window frames with gap-sealing tape.

It's a small bug, but a stubborn one. A few simple steps before the next sunny morning after rain might save you the trouble.