Oak Processionary Moth Thaumetopoea processionea


The larvae, or caterpillars, of Oak processionary moth (Thaumetopoea processionea)ᅠare a pest because they pose a serious threat to oak trees and to human and animal health.

OPM caterpillars are easily recognised by their distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which their names derive, and the fact that they almost exclusively live in and feed on oak trees.
They also build distinctive white, silken webbing nests in oak trees and leave white, silken trails on the trunks and branches in early summer.
The adult moth is a brown moth, very similar in appearance to other, harmless species. They are active from mid to late summer and lay their eggs on the smaller twigs and branches in oak trees.


The threat

OPM caterpillars can threaten the health of oak trees because they feed on the leaves. Large populations can strip whole oak trees bare of leaves, leaving them vulnerable to attack by other pests and diseases, and less able to withstand adverse environmental factors such as drought and flood.

The caterpillars have thousands of tiny hairs contain an urticating, or irritating, substance called thaumetopoein, from which the species derives part of its scientific name. Contact with the hairs can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems. This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown about by the wind.

Health precautions

This pest is a dangerᅠto oak trees,ᅠto human health and to animal health. Some simpleᅠactions can help keep the risk of serious harm or disruption very low.


•ᅠtouch or approach nests or caterpillars
•ᅠlet children touch or approach nests or caterpillars
•ᅠlet animals touch or approach nests or caterpillars
•ᅠtry removing nests or caterpillars yourself – call an expert


•ᅠteach children not to touch or approach the nests or caterpillars
•ᅠseek medical advice if you think you or someone you care for has been seriously affected
•ᅠsee a vet if you think your pet or livestock has been seriously affected
•ᅠcall in a pest control expert to remove infestations in your own trees
•ᅠreport sightings of OPM to your Council or Forest Research
•ᅠtrain or restrain your pets from touching or approaching them



Adult moths emerge from the pupae in mid to late summer. They live for an average of three days, during which time they mate and lay their eggs in healthy oak trees, usually high in the canopy.
The eggs spend the winter on the trees, and the larvae/caterpillars emerge the following spring. Larval emergence begins about mid to late April in an average spring. As they grow, the caterpillars descend lower in the trees to feed and build nests, and this is when they are most likely to be seen by the public. It is also when they develop the irritating hairs which pose the health problems. They spend their days in the nests, emerging mostly at dawn and dusk to feed, although this pattern can vary. They can sometimes be seen processing across the ground between oak trees.

In late June and July the caterpillars congregate in the nest to pupate into adult moths, which can take up to four weeks. Spent nests found after adult moth emergence will invariably contain pupae cases.



People are advisedᅠnot to try to remove OPM caterpillars or nest themselves because of the health risks, and because to be most effective the job should be carefully timed and carried out by professionals with appropriate training and equipment.ᅠReport the presence of the pestᅠto your local council orᅠto our Plant Health Service and get a professional pest control operator to remove the infestations. Your local council or our Plant Health Serviceᅠcan provide details of suitable pest control operators working in the area.



Please use the following link :- http://www.forestry.gov.uk/treealert


Or email: plant.health@forestry.gsi.gov.uk


Or inform your local council.


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