Recognising the oak processionary moth and assessing the risk
By Wespenprofis.ch · Reviewed by:Fachbewilligung Schädlingsbekämpfung VFB-S · Updated: 3 July 2026
At a glance
The oak processionary moth lives on oak trees and builds dense, grey silk nests on the trunk or in branch forks. From the third larval stage onwards the caterpillars carry microscopic urticating hairs containing the toxin thaumetopoein, which cause skin rashes and respiratory irritation on contact. Do not approach the nest or the caterpillars, and report the infestation to a licensed professional.
What defines the oak processionary moth
The oak processionary moth (Thaumetopoea processionea) is a butterfly whose caterpillars feed exclusively on oak trees. Their behaviour gives them their name: at night the animals leave their silk nest and move in a closed line, the so-called procession or “single file”, to the feeding sites in the crown and back again. In the process, dense, grey-woven silk nests form on the trunk or in branch forks; these grow noticeably over the season and often only become properly conspicuous in summer. A single heavily infested tree can carry several such nests at once, with smaller webs merging into one larger main nest as the season progresses.
Why the caterpillars are dangerous
From the third larval stage onwards, the caterpillars develop microscopic hollow urticating hairs that contain the toxin thaumetopoein. These hairs break off at the slightest touch and can be carried long distances by the wind. If they penetrate the skin or mucous membranes, they trigger caterpillar dermatitis with severe itching and weals, and the eyes and airways can also be irritated. The urticating hairs barely lose their potency: even in abandoned, old nests they remain toxic for up to twelve years. So even trees on which no living caterpillars were visible the previous year can still pose a risk if nest remnants have been left behind on the trunk.
What to do if you find one
- Keep your distance and keep children and pets away from the affected tree.
- Do not touch the nest or the caterpillars, and do not remove them yourself.
- Do not mow, sweep or use a leaf blower in the area, as this stirs up the urticating hairs.
- Note the location of the find and the approximate height of the nest.
- Call in a licensed professional (/prozessionsspinner-bekaempfung), who will mechanically vacuum the nest away in full protective equipment.
Good to know
In built-up areas, mechanically vacuuming the nests away in full protective equipment is the standard method, as it safely binds the urticating hairs instead of spreading them further. There is a risk of confusion with the pine processionary moth, which occurs on pines rather than oaks, and with the brown-tail moth, whose webs stand out on deciduous and fruit trees. If a skin rash has already appeared, it is worth taking a look at our guide on caterpillar dermatitis.
Frequently asked questions
How do I recognise the oak processionary moth?
The give-aways are the dense, grey-woven silk nests on the trunks and branch forks of oak trees, along with the caterpillars' nocturnal migration in long lines, the so-called procession.
Why are oak trees in particular affected?
The caterpillars of the oak processionary moth are specialised in oak foliage and feed almost exclusively on this tree species.
How long do the urticating hairs remain dangerous?
The fine urticating hairs stay toxic in abandoned nests and around the tree for up to twelve years, even once no living caterpillars are visible any more.