Sick Series: The hairy caterpillars that are as harmful to us as our plants

3rd August 2018

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The Oak Processionary Moth (OPM) is a relatively recent scourge of the UK oak tree population. They live and feed almost exclusively on oak leaves and can strip a tree bare, leaving it weakened and more vulnerable to pests, diseases, and stress. But this pest does not stop at compromising the health of our oak trees – people and pets need to watch out as well.

Oak Processionary Moth caterpillars were given their name because you’ll often see them moving along oak tree branches or between trees on the ground in a nose to tail procession. They have a distinctive black stripe and are covered in long white hairs, which gives the impression that they are furry and this is part of the problem. Small children think they’re cute and want to pick them up to play with them, or pets get curious and think they’re food. But these fine hairs contain a defensive toxin, designed by nature to prevent predators eating them. Unfortunately, this toxin will affect anyone or anything that touches them or their nest and can cause itchy skin, rashes, and on occasion, a sore throat, breathing problems and eye issues.

Which is why the Forestry Commission recommends that you…


  • touch or approach nests or caterpillars;
  • let children touch or approach nests or caterpillars;
  • let animals touch or approach nests or caterpillars; or
  • try removing nests or caterpillars yourself.

But that you

  • teach children not to touch or approach the nests or caterpillars;
  • see a pharmacist for relief from skin or eye irritations after suspected OPM contact;
  • call NHS 111 or see a doctor if you think you or someone in your care has had a serious allergic reaction; and
  • call in a pest control expert to remove infestations in your own trees.

OPM are normally found on the continent, specifically southern Europe, but milder winters have enabled them to move further north. In 2005 they were presumed to be accidentally introduced into Britain as eggs on oak plants, and due to a lack of their natural predators here in the UK they have flourished and may well expand their infestation territory into most of England and Wales given time.

Their life cycle goes something like this:

  • Late spring to early summer. This is when you are most likely to spot the caterpillars as they feed and, of course, this is when children are running around gardens after a long winter stuck indoors.
  • Early summer. This time of year they build their nests, which are initially white and silken, in the branches of oak trees, those this will discolour over time and become less visible.
  • July to September. Caterpillars pupate into adult moths in these nests.

If you suspect that you have Oak Procession Moth caterpillars in your garden, do not go near them, tell children to stay away and try to keep cats, dogs and other pets from going near them. Do contact a friendly local tree surgeon or arborist and they can assess any damage they have done to your trees and advise whether the forestry authorities need to be informed.

Don’t panic. There are many hairy caterpillars out there and not all are harmful to humans and trees, but it does make sense to take precautions and ask an expert.

To book a survey to determine whether the furry little critters you’ve found in your garden are anything to worry about, call us on 0208 292 8992. We’re always happy to pop round or to help with a little advice.

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