Normally, the caterpillars are only active when the oaks begin to green up. The oak processionary moth is an inconspicuous grey moth that can be observed from late July to early September. In August, the females lay up to 150 eggs in the upper crown area of oaks. The embryos survive the winter in the eggs. As soon as it gets warm, the caterpillars hatch from the egg. Before they become moths, they have to go through five to six larval stages of about ten days each.

It becomes dangerous for humans from the third larval stage onwards, because this is when they develop the stinging hairs. The stinging hairs are like hollow pins on the inside, which contain the protein toxin thaumetopoein and trigger allergic reactions. These can be skin irritations or eye irritations, but also fever, dizziness, breathing difficulties and even allergic shock. That is why experts advise people to keep their distance.

Because the caterpillars love warmth, they prefer forest edges or individual trees that are exposed to the sun. Light oak-hornbeam forests and oak-pine forests are among their favourite places. They damage the trees because the caterpillars eat the freshly sprouted leaves. The caterpillars live in family groups and walk together along the tree trunk at night, which has given them their name.

Niesar has been investigating alternative biological control methods against the oak processionary moth with the Centre for Forest and Timber Management. In some areas of Germany, for example, mouse nest boxes are increasingly being installed because they are considered natural predators of the EPS caterpillars. Meanwhile, a trial conducted in Recklinghausen in 2020 was not crowned with success. "That didn't do anything at all," Niesar reports. "Apparently the titmice also had negative experiences. In any case, in our experience they only fed other caterpillars to their offspring." The thesis that titmice can decimate the population of oak processionary moth caterpillars during their brood rearing can thus unfortunately not be confirmed. The cuckoo and hoopoe, on the other hand, would eat the caterpillars, but their populations are far too small to seriously limit the population of the pests.

More promising, however, is the use of a biotechnical method with sex attractants. In this so-called confusion method, so many pheromones are released that the males cannot find their potential sexual partners. In a first trial, the attractants were shot in a close-meshed manner on tree crowns and quite good results were achieved. "The confusion method only works in the latency phase," says Niesar, i.e. shortly before mass reproduction. And before that, many nests in an affected area would have to be vacuumed, then the attractants could be sprayed over a wide area from helicopters. However, this would be investigated over a longer period of time.

Under no circumstances should private individuals who discover such nests on trees on their properties take action themselves. "You should leave that alone, it is a job for professionals," says the expert. The first thing to do is to inform the local public order office. (Original text: Dylan Cem Akalin / Translation: Mareike Graepel)