Why oak processionary moth matters in British woods

Oak processionary moth (OPM) is a native of southern Europe that has become established in parts of the UK, particularly London and the South East. Its caterpillars feed on oak leaves, often stripping branches bare in early summer. Repeated defoliation weakens trees, making them more vulnerable to drought, disease and other pests. For woodland managers and conservationists, it is a balancing act: we want healthy oaks and rich biodiversity, but we also need to protect people, pets and the trees themselves. Understanding the moth’s life cycle and knowing how to respond is now part of responsible woodland care.

Know the signs: caterpillars, nests and silken trails

From May to July, OPM caterpillars are active. They are greyish with distinctive white hairs and a dark stripe, and they move in nose-to-tail processions – hence the name – often on the ground or along branches. Look for white, silken webbing nests on oak trunks and branches, sometimes low down or at the base of trees. These nests can remain visible in winter after the leaves have fallen, and old nests may still contain irritating hairs.

• Caterpillars: present May to July, often in lines on bark, grass or paths.
• Nests: white, candyfloss-like webbing on trunks, branches or under bark crevices.
• Silken trails: fine threads on bark, and shed hairs can be blown by wind.

Do not touch caterpillars or nests. Even brief contact can transfer hairs to skin, clothing and tools. If you find a nest, mark the location from a safe distance and keep others away.

The health risk: hairs, skin and breathing

The caterpillars’ hairs contain thaumetopoein, a protein that causes irritation. Contact with skin can produce an itchy, red rash that may worsen over hours. Hairs can become airborne, so inhalation may lead to a sore throat, coughing or breathing difficulties. Eye contact causes redness and pain. Dogs are especially at risk: they sniff or lick the ground near oak trunks, and hairs can cause swelling of the mouth, tongue and throat, sometimes severe enough to need urgent veterinary care.

If you think you have been exposed, wash the area with cool water and soap, do not rub, and change your clothes. A cool shower can help. Antihistamine cream may ease itching, but seek medical advice if you have breathing problems or severe swelling. For dogs, contact a vet immediately. Never handle a caterpillar or nest without professional protective equipment.

Practical steps for woodland visitors and workers

If you are walking in oak woods between May and August, plan ahead. Check local alerts for OPM, and avoid dense oak areas if you have asthma or sensitive skin. Wear long sleeves, long trousers and closed shoes, and tuck trousers into socks where practical. Keep to paths, and do not lean on or hug oak trunks. Keep dogs on a short lead, and stop them from nosing around the base of trees or eating grass near infested trunks.

For woodland workers, a risk assessment is essential. Use binoculars to inspect oaks rather than getting close. Avoid working directly under infested trees during caterpillar season, and never use leaf blowers or strimmers near nests, as these can launch hairs into the air. Wear disposable coveralls, gloves and eye protection, and wash tools and clothing after work. Prevention is far easier than treating a reaction.

Managing woods: what conservationists and landowners can do

Good woodland management reduces risk while supporting biodiversity. Monitor oaks regularly from May to August, recording any nests or caterpillar activity. Prevent spread by not moving oak material, leaf litter or timber from infested areas, and clean boots, tools and vehicle tyres before leaving a site. Nest removal should be done by trained professionals using appropriate PPE, often in early morning or evening when caterpillars are clustered. Pheromone traps can help track moth numbers, and biological controls are being researched, but none replace careful observation.

Conservation balance matters. OPM is a natural part of oak ecosystems in continental Europe, and blanket removal of oaks would harm wildlife. Instead, promote mixed woodland with a range of tree ages and species, thin oaks to reduce stress, and retain deadwood for invertebrates. Natural predators such as great tits, cuckoos and parasitic wasps may help, though the caterpillars’ hairs deter many. Report sightings promptly and work with neighbouring landowners to avoid gaps in control.

Reporting sightings and keeping records

If you see caterpillars or nests, do not disturb them. Note the location, date and what you observed, and take photographs from a safe distance. Report the sighting through the official tree health reporting service or to your local council’s environmental health team. In the UK, Tree Alert is the recognised channel, but any official plant health authority will pass details on. Early reporting helps contain outbreaks and protects others.

For woodland owners, keep OPM records in your management plan. Mark nests on maps, note dates and monitor annually. Review your approach with a forestry adviser, and share findings with local recording groups. By staying alert, keeping dogs close and never touching caterpillars or nests, we can all help protect British woods – and each other – from this irritating and sometimes serious pest.