Spot OPM Symptoms

Learn how to identify Oak Processionary Moth (OPM) symptoms in trees across the South East.

A silent threat is returning to South East oak trees: the Oak Processionary Moth (OPM). Homeowners, land managers, and outdoor enthusiasts must know how to spot OPM symptoms and take steps to protect people, trees, and the local environment.

With public health warnings already being issued for 2025, now is the time to stay informed and take action.

The Oak Processionary Moth (Thaumetopoea processionea) is an invasive pest that feeds exclusively on oak trees. Originally native to Southern Europe, it has spread across parts of England over the last two decades. The larvae, or caterpillars, pose the greatest risk—both to tree health and human safety. The infestation typically begins in late April, peaks during May to July, and can remain active well into early August, depending on temperatures. In 2025, the warm spring has accelerated activity, making early detection important.

Why Is OPM a Problem in 2025?

OPM is a dual threat—damaging trees and risking public health.

Caterpillars strip oak trees of their leaves, weakening them and making them vulnerable to disease and pests. A single infestation can defoliate an entire canopy, and repeated attacks may eventually kill the tree.

The caterpillars are covered in tiny toxic hairs containing a substance called thaumetopoein, which can cause serious allergic reactions. These hairs are easily detached and can linger in the environment long after the caterpillars are gone. Many infestations are located in parks, public gardens, schools, and residential areas—especially in the South East. With warmer conditions and increased outdoor activity during summer, 2025 presents a high-risk period.

How to Spot OPM Symptoms in Trees

OPM is now firmly established in Surrey, Reigate, Dorking, Redhill, Leatherhead, and surrounding parts of the South East. Sightings continue to rise each year, especially in areas rich in mature oak trees.

The Forestry Commission has mapped “buffer zones” where infestations are known or suspected. Arborists like Arboright are regularly called out to monitor and treat affected trees in these locations.

Detecting the presence of OPM early can prevent wider spread and reduce health risks.The species gets its name from the way the caterpillars move—in long head-to-tail processions, often in single file, across trunks, branches, or the ground.
Look for dense, white, silky nests on trunks or major branches—not in the canopy. These resemble spun cotton and may appear dry and flaky. Older nests turn brown with age. A thinning or bald-looking oak canopy in late spring or early summer could indicate OPM feeding. Trees may look scorched or unevenly leafed.
OPM caterpillars are black with long white hairs and orange markings. They're often seen clustered tightly together. If you suspect OPM symptoms in trees on your property, do not attempt to remove or touch the nest. Professional treatment is essential.

What Are the Health Symptoms of OPM Exposure?

In humans and animals, contact with OPM hairs can cause rashes, itching, eye irritation, sore throat or breathing issues, and dizziness or fever in severe cases. Pets may experience swelling of the mouth or tongue if they sniff or lick contaminated areas. Symptoms usually appear within hours and may last for several days.

Whether you’re walking in a woodland or maintaining a garden in Dorking or Reigate, you should avoid touching any nests or caterpillars. Even fallen nests can retain dangerous hairs. Wear long sleeves and trousers if walking through oak-heavy areas between May and July.

Keep pets on leads and children away from tree trunks. Report sightings via the Forestry Commission’s Tree Alert tool or contact a professional tree surgeon like Arboright. Never attempt DIY removal. Only trained and licensed operatives should handle OPM infestations, using protective equipment and safe disposal methods.

What to Do If You Spot OPM

Step by step guide

If you suspect an oak on your land has OPM, take a photo from a distance. Mark the location but avoid contact. Then call a qualified tree surgeon immediately.

At Arboright, we provide prompt OPM inspections across Surrey and the South East. Our team can assess the risk, treat the affected tree, and ensure your site is safe for the public, pets, and wildlife.

Dealing with OPM isn’t a one-off task. Annual surveys, treatment cycles, and tree care plans are essential to prevent repeat infestations, keep your site compliant with health and safety regulations, and protect surrounding woodland.

With higher-than-average spring temperatures and increased public reporting, 2025 is already a high-risk year. The earlier emergence of caterpillars has expanded the treatment window, making early detection more important than ever.

CONTACT US TODAY

learn how to spot OPM symptoms

The Oak Processionary Moth is more than just a nuisance—it’s a threat to trees, wildlife, and people alike. By learning how to spot OPM symptoms in trees and acting quickly, we can reduce the impact on our environment and stay safe through summer.

If you’re concerned about oak trees on your land or want professional advice, contact Arboright today to schedule our services. Trust our experience for prompt, safe, and reliable solutions that protect both your property and the local environment.

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