Aug 07 2018

Oak Processionary Moth – The dangers

Oak processionary moth (Thaumetopoea processionea) was first accidentally introduced to Britain in 2005 and it is possible that it could survive and breed in much of England and Wales. The larvae or caterpillars of OPM are a pest because they can affect the health of oak trees, people and animals.


OPM caterpillars are most easily recognized by their distinctive habit of moving about in late spring and early summer in nose-to-tail processions from which they derive their name, and the fact that they live and feed almost exclusively on oak trees.


They can sometimes be seen processing across the ground between oak trees and clustering together as they feed on oak leaves.


The caterpillars have thousands of tiny hairs which contain an “urticating” or irritating substance. 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 into contact by the wind. The caterpillars can also shed the hairs as a defense mechanism and lots of hairs are left in the nests which is why the nests must not be touched.


The nest may persist for years after the caterpillars have left them and must always be approached with caution.


Woodland Trust staff & volunteers are at risk of exposure when working on or around infested oak trees in areas where the creature is present. (See the map at Any Trust worker or volunteer within these affected areas must be vigilant for caterpillars or nests and check trees carefully before working around them.


Some people can become sensitized by repeated exposure to the hairs meaning that the symptoms become worse.


Health precautions

People in the affected areas can take these simple precautions to help minimise the health risks:


  • Touch or approach nests or caterpillars
  • Let children touch or approach nests or caterpillars
  • Let animals touch or approach nests or caterpillars
  • Try removing nests or caterpillars yourself

If you find nests or caterpillars while working suspend operations immediately and move away from the area – volunteers must report any suspected sighting to the Trust Site manager.


  • Keep any children on site away from OPM areas – teach them not to touch the nests or caterpillars
  • Keep pets away from OPM areas
  • If the Trust has cattle or other livestock in affected areas remove them as soon as possible.
  • Contact your GP immediately if you have been in contact with OPM and begin to suffer symptoms – especially if you have had contact in the eye area.
  • Contact a vet if you believe pets or livestock may have been affected.

Any sighting of OPM must be reported to the Forestry Commission – you can do so here:

1 Comment
  • MaggieMcCallum

    I may have written this before – it has a familiar ring to it, but here goes anyway.

    I had a brush with these caterpillars in 1986 at Lake Annecy. I had to visit the pharmacy with my substantial crop of red itchy ’boutons’ only to be told it was likely to have been caterpillars. We later had a look at an area where we’d been sitting on the grass and then we saw a notice about risks from contact with caterpillars…. it was and is a serious business.

    October 1, 2018 at 5:04 pm

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