The oak processionary moth (Thaumetopoea processionea) and its caterpillars originated in southern Europe but have now become established in Greater London. Oak processionary moths and OPM caterpillars are pests of oak trees (Quercus) and are hazardous to humans and animals.
In this article, we will provide you with some vital information about OPM, including how to identify it. However, please don’t get close to the moths, caterpillars or their nests because of the potential health risks involved. If you suspect OPM, we recommend contacting a qualified tree company such as North London Tree Surgeons.
This greyish-brown moth, with a 25-35mm wingspan, was first found in southwest London in 2005 after an accidental introduction. The species has grown in numbers through natural population growth and further unintentional introductions. OPM is now subject to a government-led survey and control programme to minimise its population, spread and impacts.
You can find more information about the government-led OPM management program by visiting the GOV.UK website.
The adult Oak processionary moth’s tan, brown, and white pattern helps it blend naturally with oak bark. These moths spread their wings during July and August and lay eggs that develop through winter until March-May the following year.
The moth’s name comes from its caterpillars’ unique moving behaviour in nose-to-tail processions. The larvae build shared nests using white silk on tree trunks and branches and come out at night to feed. These nests can stretch from 25 millimetres to several metres long, and they often become harder to spot as they darken over time.
The caterpillars go through six growth stages and develop long white hairs that stand out against shorter, barely visible, irritating hairs. These irritating hairs contain thaumetopoein, a protein that gives the moth its scientific name. While oak leaves are the larvae’s preferred food, they eat other broadleaved species like sweet chestnut, hazel, beech, birch, and hornbeam when oak leaves become scarce.
What areas have been affected by Oak Processionary Moth?
The Oak processionary moth was originally only found in London, but it has now spread to the surrounding counties, including Surrey, Kent, Hertfordshire, Essex, Hampshire, and Berkshire. Despite government-led control programs, scientists estimate that the Oak processionary moth is spreading at a rate of over 3.7 miles per annum. The Forestry Commission have an interactive Oak processionary moth UK map showing the locations of trees infested with OPM.
How to identify Oak Processionary Moths and Oak Processionary Moth Caterpillars
Oak processionary moths and OPM caterpillars have several distinct features you can use to help identify them. Oak processionary moth caterpillars have grey bodies, dark heads, and long white hairs that stick out from reddish-brown warts. As they grow, these caterpillars develop a dark stripe down their back and whitish lines on each side. The easiest way to identify the presence of OPM is by spotting their white, silken nests. Oak processionary moth nests can be as small as a golf ball or as large as a rugby ball, and they show up primarily on oak tree trunks and branches. The nests look like domes or teardrops that turn muddy brown as time passes.
Here are the main identification markers:
- White silken trails on trunks and branches
- Nose-to-tail processions, often 2-3 caterpillars thick
- Clusters of caterpillars, especially visible at dawn or dusk
- Nests positioned on trunks or underneath branches, never among leaves
Spring brings tiny 2mm caterpillars that grow to 2cm when fully developed. These insects stay active at night and hide in their nests during the day.
Adult moths aren’t as easy to spot. They have mottled grey wings with a comma-shaped marking in the centre of each forewing. Their wingspan reaches 30-40mm. Identifying adult moths is challenging because they look so much like other species.
What are the dangers of Oak Processionary Moths to trees?
Oak processionary moth caterpillars pose a major threat to trees through severe defoliation. These pests devour oak leaves, and large groups can leave trees completely bare. The extensive leaf loss limits a tree’s ability to photosynthesise and affects its growth and health.
Defoliation’s effects change with intensity, and studies reveal that severe cases reduce tree growth by 10-60% in the outbreak year. The damage persists, as affected trees show a 10-30% growth reduction even the following year.
The damage goes beyond just losing leaves. Trees that weaken from OPM caterpillar attacks become easy targets for:
- Secondary pest invasions
- Disease infections
- Environmental stresses like drought and floods
Which tree species do Oak Processionary Moths affect?
Oak processionary moths target trees in the Quercus genus. Britain’s native species – pedunculate (Quercus robur or English oak) and sessile oak (Quercus petraea)- face significant risk. The oak tree caterpillars feed on other oak varieties, including Turkey oak (Quercus cerris), chestnut-leaved oak(Quercus castaneifolia), and holm oak (Quercus ilex).
The caterpillars might turn to other broad-leaved species when oak leaves become scarce. Alternative hosts include sweet chestnut, hazel, beech, birch, and hornbeam. Yet, these non-oak species rarely support the caterpillars’ full development cycle.
What are the dangers of Oak Processionary Moth to people and animals?
The microscopic hairs on oak processionary moth caterpillars contain thaumetopoein, a protein that creates health risks for humans and animals. Wind easily spreads these hairs, which stay active for several years after shedding. Human contact with these hairs causes many symptoms. Studies show that health complaints affect 5-7% of people in these areas. The most common reactions include:
- Skin irritation and itchy rashes
- Eye irritation and conjunctivitis
- Sore throat and breathing difficulties
- Fever and general malaise
Oak Processionary Moth Rash Picture
Outdoor workers such as arborists, landscapers, and forestry workers face a higher risk from these caterpillars because they’re more likely to encounter infected trees. Children are also at high risk due to their inquisitive nature and because they often like to explore their surroundings without thinking of possible dangers.
The toxic hairs can severely affect pets, especially dogs. Dogs that sniff or make direct contact with infested oak trees often face exposure. Pets might experience tongue swelling, excessive drooling, and breathing problems. Some cases can lead to fatal allergic reactions.
Pets need immediate veterinary care for symptoms like gagging, vomiting, or mouth inflammation. A study with 41 dogs showed that affected animals suffered from ptyalism, dysphagia, and pain. Some dogs needed up to 15 days to recover.
Important Warning: Never attempt to remove nests or caterpillars yourself. Medical attention becomes necessary after exposure. Remember to tell healthcare providers about possible contact with oak processionary moth.
What should I do if I suspect or spot Oak Processionary Moth?
When you spot signs of oak processionary moth, report it right away.
Don’t try to remove it yourself. Use the Forestry Commission’s TreeAlert online system to report sightings.
You can also email them at opm@forestrycommission.gov.uk or call 0300 067 4442.
Make sure you check these vital points before filing a report:
- The location has oak trees nearby
- You have clear, well-lit photos taken from a safe distance
- You know the exact location
- You’ve noted any visible OPM symptoms or damage
TreeAlert needs photos to verify the situation, so take pictures from a safe spot.
Health issues need proper medical attention based on your symptoms. A pharmacist can treat mild skin or eye irritations. Serious allergic reactions need attention from a GP or NHS111. Pet owners should take their animals to a vet immediately if affected.
Oak Processionary Moth Treatment
The most effective treatment for oak processionary moth infestation is physically removing the nests and caterpillars. Only trained professionals should remove OPM nests or Oak processionary moth caterpillars. These OPM removal specialists use proper protective gear, including full body protection, head coverings, and breathing apparatus. Oak processionary moth waste is also hazardous and requires safe and special disposal.
We provide safe and efficient oak processionary moth treatment in North London, so if you suspect you have OPM in North London, contact us today.
