Oak Processionary Moth

Oak Processionary Moth

Thaumetopoea processionea commonly known as the Oak Processionary Moth can be threatening to some of the Oak (Quercus ) species of tree(s). The caterpillars of this moth will defoliate the tree, leaving crowns of trees sparse or completely bare affecting the tree’s ability to photosynthesize and ultimately declining the health of a tree. Areas unaffected are known as ‘protected zone’ since the introduction of European Legislation in 2014.

Oak processionary moth caterpillars © Forestry Commission

Tiny hairs cover the caterpillars in the thousands. Each hair is home to a substance known as thaumetopoein which can cause on contact with the skin, rashes, itching. Less common side effects can include eye issues, sore throats or breathing difficulties.

Things to look out for are clusters of caterpillars on branches when feeding, Tear shaped or domed shaped nests of silk on undersides of branches and stems of trees. Traveling tail to nose when moving about in Oak trees or on the ground in a procession was the observation which gives way to their common name.

Health precautions

The oak processionary caterpillars’ tiny hairs contain a toxin which can lead to itching skin lesions and, less commonly, sore throats, breathing difficulties or eye problems. This can happen if people touch the caterpillars or nests, or if the hairs are blown into contact by the wind.

For this reason, avoid contact with the nests and caterpillars – and keep pets and livestock away from them too

The nest may persist for several years. Please treat these with the same caution as a live nest.

Adults and children must see a doctor immediately if they have come into contact with OPM and begin to suffer symptoms

If pets or livestock come into contact, they should be removed from the area and a vet should be contacted.


Further information can be found on the following websites:





Author: Mark Kirk ND Horticulture FdSc Arborculture
Contract Manager – Beechwood Trees and Landscapes Ltd