Harrow's trees

Oak Processionary Moth (OPM)

Oak Processionary Moth (OPM) gets its name from the distinctive processionary behaviour of the caterpillars moving in nose-to-tail lines.

The caterpillars of these moths feed on oak foliage leading to severe loss of photosynthetic material which can reduce trees longevity and make them more vulnerable to other diseases. However, the main concern with this pest is the threat to human/livestock/pet health because the caterpillars urticating hairs can cause severe allergic reactions.

Oak processionary moth has been spreading rapidly and is now established throughout much of greater London and some surrounding areas as well – including Harrow.

Please note: All sightings of the oak processionary moth should be reported to the Forestry Commission immediately. See below for more information about How to report a sighting. The Forestry Commission does not fund control, this is up to the tree owner and not a legal requirement.

Health Precautions

The oak processionary caterpillars’ tiny urticating 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 at a safe distance.
  • 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.

Advice for oak tree owners: Preventing the spread of Oak Processionary Moth

Taking action will depend in which zone of the three OPM management zones your affected trees are in.

Government action and support for affected owners depends on which one of the three OPM management zones the affected trees are in. Guidance on the zones can be found in The Oak Tree Owners' OPM manual, but these are subject to change as the Forestry Commission review each year.

Report any sightings immediately

Make sure you have correctly identified the species.

Find out how to identify an oak processionary moth caterpillar on the Oak processionary moth (Thaumetopoea processionea)

Reporting your sighting online:

If you are unable to use the form, please use one of the following contact options:

OPM Treatments

Depending on the life-cycle stage of moth development, different treatments are recommended. These are: spraying, nest removal and pheromone traps.

Spray treatments

During April-June whilst the caterpillars are young and out feeding in the canopy, the entire canopy of the Oak tree can be sprayed with an approved / environmentally friendly pesticide specifically designed to target the caterpillars. The tree is then re-sprayed 7-10 days later. 

Nest removal

The most effective time to remove OPM is between June & July when the caterpillars have settled into their nests and are less likely to be feeding in the foliage. Full body PPE is worn and nests are collected by hand,then safely removed from site and disposed of.
Nests observed outside April-June period are likely to be old / abandoned nests but should still be removed. Between April and July is the greatest risk period and when caterpillars are most numerous. Nests found during this time should not be approached. 

Pheromone traps

Pheromone traps are used between July-Sept, the final life-cycle stage, to trap adult male moths. This controls the spread and also provides insights into moth numbers 

Further information

For further information see: