Oak processionary moth: ​Poison on the trail. Or: When the caterpillars arrive!

The trail is perfect. Sunshine, bends – the tyres hum as they roll over the forest floor. Then, suddenly, your skin starts to burn. Your arms itch. Your eyes water. What looks like an allergic reaction could actually be an encounter with one of Germany’s most unpleasant forest dwellers: the oak processionary moth. This can happen to mountain bikers, gravel bikers and road cyclists alike.

Between May and July, the caterpillars are at their peak. Warm, dry regions with large stands of oak are particularly affected. Fuelled by climate change, the caterpillars’ habitats are expanding ever further north. The insects live in large colonies and often march in single file, forming long lines as they move across tree trunks, footpaths, forest tracks and cycle paths – hence their name. In pine forests, such as those in the Vinschgau Valley, you may encounter the pine processionary moth, a close relative.

The problem is not the caterpillars themselves, but their so-called stinging hairs. From the third larval stage onwards, the caterpillars develop hundreds of thousands of microscopic, hollow hairs that contain a stinging toxin. They are stirred up by the wind, hikers or cyclists. The hairs can be carried through the air for many metres and stick to skin, clothing or equipment.

Itching on two wheels

The consequences range from annoying skin irritation to serious health problems. Typical symptoms include intensely itchy welts, redness and small blisters. If the hairs get into the eyes, they can cause painful inflammation, such as the troublesome conjunctivitis. If they are inhaled, they can cause coughing, breathing difficulties or, in rare cases, even anaphylactic shock.

What makes them particularly treacherous is that the stinging hairs remain active for months. Even old nests on the ground or on embankments can still cause discomfort.

​Not a villain, but a symptom

Despite all the itching, the fact remains that the oak processionary moth is not a pest in the traditional sense, but rather part of our ecosystem. The caterpillar has been part of our native fauna for centuries and has every right to be on the oak tree. It only becomes a problem when its populations explode.

Why is this happening more often these days? One reason is the increasingly hot and dry summers. Added to this is the fact that many natural predators are missing. Bats, cuckoos, parasitic wasps and predatory beetles normally keep their numbers in check. Where these helpers have become scarce, the caterpillars can multiply in huge numbers.

That is why conservationists advise that nests should only be removed where there is a genuine danger to people – for example, near schools, playgrounds, campsites or busy footpaths. In the woods, the creatures are often simply left to be caterpillars.

It is also important to distinguish it from the harmless web moth. Its white webs sometimes cover entire shrubs and trees and look spectacular, though some people find them frightening. Unlike the oak processionary moth, however, the web moth has no poisonous stinging hairs and poses no danger to humans. The webs disappear again after a while.

In short: not every web in the woods poses a health risk. But you should take every warning sign about processionary caterpillars seriously. After all, the caterpillar can’t help being poisonous – but the rash still itches.

Recognising a danger – and then what?

A warning sign at the edge of the forest should be enough to make cyclists change their route. If you look more closely, you’ll often spot greyish-white webs on oak trunks that look like cotton wool balls and can be as big as a football.

Keep your hands off. The nests should neither be touched nor removed. Even webs that appear to have been abandoned may still contain millions of poisonous hairs.

Under no circumstances should you touch the caterpillars if you want to help them along – it’s like reaching into glass wool.

What should you do after coming into contact with someone?

Anyone who has driven through an infested area and notices symptoms should change their clothes as soon as possible and wash them. They should then take a shower and wash their hair thoroughly. Do not rub your eyes and, if possible, avoid scratching the affected areas of skin.

If you experience breathing difficulties, dizziness or severe allergic reactions, you should call the emergency services immediately.

The most important rule: the oak processionary moth is no reason to leave your bike at home. If you take the warnings seriously and avoid infested areas, you’ll significantly reduce the risk. Or to put it another way: riding technique helps against roots, rocks and stony ground. The only defence against a cloud of stinging hairs is to keep your distance.