Processionary Caterpillars: Why Their Urticating Hairs Are a Hidden Risk

A line of caterpillars moving head to tail may look too orderly to pose any danger. With processionary caterpillars, however, the greatest concern is barely visible: thousands of microscopic urticating hairs can detach from their bodies and remain in the surroundings, causing reactions even when nobody touches the animal directly.

Why do processionary caterpillars move in a procession?

The name processionary comes from their distinctive single-file movement. The caterpillars leave their nests to find food or travel between parts of a tree, staying in contact with one another and forming a route that resembles a small living train. This behaviour is particularly associated with the oak processionary caterpillar and pine processionary caterpillar.

Although the line makes the group easy to identify, it should not encourage people to get closer. European forestry authorities advise keeping away from both caterpillars and their nests. Children and pets should also be kept at a distance, as curiosity and direct contact make exposure to the hairs more likely.

Where are the hairs that cause irritation?

At later stages of development, processionary caterpillars produce extremely small urticating hairs, which are different from the longer hairs that can be seen with the naked eye. These hairs contain substances that can irritate the skin and mucous membranes, leading to itching, redness and eye discomfort. More sensitive people may develop more significant respiratory symptoms.

The issue is that these hairs come away very easily. They can remain caught in old nests, bark, leaves, clothing and other nearby materials. As a result, the hazard does not simply disappear once the caterpillar has moved on from an area or because nobody has touched its body.

Can you react without touching a caterpillar?

Yes. Microscopic hairs may be carried by the wind and reach the skin, eyes or airways. This is why official guidance stresses that abandoned nests must not be handled and colonies should not be removed without suitable equipment and training. Indirect exposure is a major part of the risk.

In the United Kingdom, management of the oak processionary caterpillar involves tree monitoring and restrictions in infested areas. The species has become a public-health concern precisely because its hairs can remain in the environment beyond any immediate contact with a caterpillar.

• Do not touch caterpillars, even if their line seems slow-moving and easy to watch from close by.
• Avoid old nests, as urticating hairs may still be trapped in the material.
• Keep children and pets away from trees marked as infested.
• Seek medical help if symptoms are severe, especially if breathing becomes difficult.

Watch the video shared by the Germano Woehl Junior YouTube channel, showing the caterpillars moving in procession.

What should you do after exposure?

If hairs reach the skin, avoid rubbing the affected area, as this can spread them and worsen the irritation. Public-health guidance commonly recommends removing contaminated clothing, washing the area and seeking advice from a doctor or pharmacist according to the severity of symptoms. The eyes and airways require particular care.

Breathing difficulty, significant swelling, dizziness or a widespread reaction should not be dismissed as ordinary itching. In these circumstances, prompt medical attention is needed. With mild exposure, symptoms are usually short-lived, but reactions differ between individuals and may be worse for people with allergies or asthma.

Why does a beautiful procession deserve distance?

Their collective behaviour makes processionary caterpillars easy to photograph, yet their defence works on an almost invisible scale. The caterpillar does not need to bite or chase anyone: it is enough for hairs to break off and land on a nearby surface for the surrounding area to become a possible source of irritation.

The safest way to observe them is from a distance. The contrast between the orderly line and the microscopic defence helps explain why these animals attract so much attention. If you find a concentration in a tree, park or garden, do not attempt to remove it yourself, and alert the person responsible for the site where there is a risk of public contact.