If you go down to the woods today, watch out for oak processionary moths, advises Sutton Council

Sutton’s Liberal Democrat Council has advised residents visiting the borough’s parks and woods to be on the lookout for a moth caterpillar which can cause an unpleasant allergic reaction.

The oak processionary moth (Thaumetopoein processionea) is a pest that was recently identified in the Worcester Park area in the west of the borough. The moth was introduced to England from mainland Europe and first identified in London in 2005.

The oak processionary moth caterpillars emerge around May, coinciding with bud burst when leaves emerge from trees at the start of the growing season. In their early stages of growth, the newly-hatched caterpillars feed exclusively on oak leaves and it is possible for large populations to strip whole trees of their leaves. On a healthy oak tree, this generally will not cause any permanent damage, but it can leave trees vulnerable to other pests and diseases, and less able to withstand events such as drought and flood.

As the caterpillars develop they produce thousands of tiny barbed hairs containing a substance called thaumetopoein that can cause itching skin rashes, eye irritations and sore throats in people and animals that come into contact with them. In rare cases the barbed hairs can cause breathing difficulties and allergic reactions. However, symptoms are not usually serious and can be treated by a pharmacy.

The hairs can be shed by the caterpillars as a defence mechanism, be blown off by the wind, and left in the silken webbing nests the caterpillars build on the trunks and branches of oak trees, sometimes at or close to ground level. These nests can fall to the ground, and hairs can stick to the trunks and branches of oak trees.

The oak processionary moth caterpillars have a distinctive habit of moving about in or under oak trees in nose-to-tail processions, which gives them their name. The silken webbing nests are white when new, and often have silken trails leading to them. They quickly become discoloured and harder to see against the dark colour of oak-tree bark.

Sutton Council’s Parks Service is monitoring the situation closely and has a term contractor to deal with both spraying and nest removal. 

Cllr Jill Whitehead, Chair of the Environment and Neighbourhoods Committee at Sutton Council, said: “If you see any oak processionary moth nests or caterpillars, do not approach or touch them. Report them immediately to the council or the Forestry Commission, which is leading efforts to control its population, spread and impact.”

The main risk period is between now and July, when the caterpillars are active. However, borough residents are advised to avoid nests, even “spent” nests, at any time, because the hairs in them can remain irritating for many months. 

  • Sutton residents who spot oak processionary moths on borough oak trees should contact Sutton Council’s Parks Service tree officers on 020 8770 4789.
  • The preferred way to report sightings is with the Forestry Commission’s Tree Alert online pest reporting form, which can be accessed via its oak processionary moth website page at forestry.gov.uk/opm. You will have to add a photograph to your report, but do not risk contact to get a photograph. If you cannot use Tree Alert or get a photograph, you may report the moths by emailing the Forestry Commission at opm@forestry.gsi.gov.uk or telephone 0300 067 4442.

Note that oak processionary moths:

  • Do NOT live on walls, fences or other structures; and
  • Do NOT build nests among oak leaves, so there is no need to report caterpillars or nests in these places.
  • They also only attack other trees if they run short of oak leaves to feed on, so please do NOT report caterpillars or nests in other trees unless there are stripped oak trees nearby.

What to do if you are affected by oak processionary moths

  • It is important to avoid contact with the hairs, to teach your children to avoid them, and to protect your pets and livestock from them. Curious pets might need to be restrained from approaching nests and caterpillars.
  • However, if you are affected, the symptoms, although unpleasant, are not usually medically serious and will pass in a few days. You can ask a pharmacist for something to relieve the symptoms.
  • If you do have a serious allergic reaction, call NHS111 or see a doctor. Similarly, consult a vet for badly affected animals.

Control programme

Sutton Council is working with the Forestry Commission and major oak tree owners on a programme funded by the Department for Environment, Food & Rural Affairs (Defra) to survey for and treat affected oak trees and remove nests to minimise the population, spread and impacts of the pest. You can help us by reporting sightings as above. Details of the programme are available on the Forestry Commission website.

If you own oak trees

  • If you have oak processionary moths in your own oak trees, you will be issued with a Statutory Plant Health Notice by the Forestry Commission, requiring you to have the pest removed. This does not mean you are in any trouble – the notices are a management tool.
  • Sutton lies in the oak processionary moth Control or Buffer Zone where the Forestry Commission is working with us to prevent or limit spread out of the Core Zone. This means that if you are a private tree owner the Forestry Commission will have the infestation removed at no cost to you. Public-sector tree owners must have the work done at their expense.
  • The Forestry Commission has published an ‘oak processionary moth manual’ with advice for people and organisations who own oak trees in the affected areas. See forestry.gov.uk/opmmanual.
  • If you are having work done on oak trees, such as pruning or felling, please check that your tree surgeon is familiar with the health and safety precautions they should follow, and with the regulations for disposing of oak material safely to avoid accidentally spreading the pest to new areas. These are available on the Forestry Commission website.
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