Urticating insect · Thaumetopoea processionea · Last updated 14 August 2026
Processionary caterpillars
The oak processionary caterpillar (Thaumetopoea processionea) is present in Belgium, especially in Flanders and the north of the country. Its thousands of microscopic hairs release an urticating substance dangerous to humans and animals alike.
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The caterpillars live in colonies on oak trees and move in single file — hence their name. They weave beige silky nests on trunks and branches. Their urticating hairs, released on contact or by the wind, cause intense itching, skin rashes, conjunctivitis and respiratory discomfort. The risk persists even after the caterpillars leave, as the hairs remain active in the nests and on the ground. Intervention involves vacuuming the nests with protective equipment, or applying a biological treatment (Bacillus thuringiensis) in spring on the young caterpillars.
Identify
Signs of infestation
- Caterpillars moving nose-to-tail in procession on oak bark (April to June), forming a chain of 20 to 300 individuals
- Silky white nests, tennis-ball to football sized, on oak branches
- Urticating hairs scattered on the ground beneath infested trees, invisible to the naked eye
- Defoliated oak leaves above the nest
Where it's found
Found exclusively on oak trees (Quercus sp.). In Belgium, the oak processionary moth (Thaumetopoea processionea) is present throughout Wallonia and Brussels, with infestation peaks in Walloon Brabant and forested areas. Cycle: eggs overwinter on branches, caterpillars active from April to June.
Impact
Associated risks
Urticating risk (microscopic hairs)
Microscopic hairs detach and are carried by the wind well beyond the infested tree. Skin contact: intense urticating rash. Eye contact: conjunctivitis that can last for weeks. Inhalation: bronchial reaction. The hairs remain active in the environment for months after the caterpillars are gone.
Regulatory status in Belgium
The oak processionary moth is a regulated pest in Belgium. On roads and public spaces, the municipality is responsible. On private land, intervention must be carried out by equipped professionals (full-body suit, FFP3 mask, industrial extraction). No DIY intervention is legally recommended.
Good practice
Prevention
- 01Never touch or approach a nest: even old nests retain active urticating hairs for months after the caterpillars have left
- 02Report immediately to municipal services if the nest is on a public tree or roadside
- 03On private land: call a fully equipped specialist to extract the nest and the surrounding hairs
Frequently asked questions
What you're asking
Why are processionary caterpillar hairs dangerous?
Each caterpillar carries hundreds of thousands of microscopic hairs containing an urticating protein (thaumetopoein). On contact with skin, eyes or airways, they cause itching, redness, conjunctivitis and sometimes serious allergic reactions. Animals, dogs especially, are highly exposed when sniffing the ground.
When should processionary caterpillars be treated?
The preventive biological treatment (Bacillus thuringiensis) is applied in spring, on the young caterpillars. Once the nests are formed (late spring and summer), the intervention involves vacuuming them mechanically. Never wait: the bigger the colony, the more urticating hairs are dispersed.
Can I remove a nest myself?
No. Handling a nest or caterpillars without full protection exposes you to severe urticating reactions, and burning disperses the hairs into the air. Removal must be carried out by an equipped technician who vacuums the nests, neutralises the hairs safely, and disposes of the waste in a regulated manner.
Insect Control
Related pests
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