“I can’t actually determine which species it is,” says this researcher. “Within the culm flies, almost all examples in our area can be traced back to the species Thaumatomyia notata. It is actually not visible enough in the photo, it could just as well be fruit flies, but it is difficult to determine.”
“Fruit flies can also occur in large numbers, they mainly reproduce in fermenting things such as rotting fruit,” this fly expert adds. So we now have two options: leaf flies or fruit flies and there are quite a few species of both.
“Most fly species also tend to fly upwards towards the light, which is why they all sit on the window,” Niels-Jan Dek concludes.
Further research brings us to the Animal Pests Knowledge and Advice Center (KAD). This center puts us on the trail of grass flies, which belong to the culm flies. They come together in large swarms in the autumn.
They like to perch in ivy or vine, growing against the facades of houses. They do this with the intention of overwintering in plants or indoors. It also happens that the grass fly chooses high apartment buildings and offices to hibernate, especially in the cavity.
We come into contact with Dick Belgers from KAD. He thinks he can say with a fair degree of certainty that this case is indeed the grass fly. “There are about 150 species of leafflies in the Netherlands,” he says. “Some species are known for their large numbers, including the grass fly.” Look, this takes us further.
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