Processionary Caterpillars: Why Their Ordered Line Requires Distance

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

Why do processionary caterpillars move in a line?

The name processionary comes from their distinctive habit of travelling in a single file. Caterpillars leave their nests to find food or travel between parts of a tree while staying in contact with one another, forming a trail resembling a tiny living train. This behaviour is particularly associated with oak processionary and pine processionary caterpillars.

Although the line makes the group easy to spot, it is not an invitation to get closer. European forestry authorities advise people to keep away from both caterpillars and nests. Children and pets should also be kept clear, as curiosity and direct contact increase the likelihood of exposure to the hairs.

Where are the hairs that cause irritation?

At their more advanced stages, processionary caterpillars develop extremely tiny urticating hairs, which differ from the longer hairs visible to 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 experience more significant respiratory symptoms.

The issue is that the hairs detach readily. They may remain caught in old nests, bark, leaves, clothing and other nearby materials. As a result, the risk does not simply disappear after the caterpillar has left an area or because nobody has touched its body.

Can you react without touching a processionary caterpillar?

Yes. Microscopic hairs can 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 an important part of the risk.

In the United Kingdom, management of the oak processionary caterpillar includes 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 immediate contact with the caterpillar.

• Do not touch the caterpillars, even if the line appears slow-moving and easy to observe closely.
• Avoid old nests, as urticating hairs may still be trapped in the material.
• Keep children and pets away from trees marked as infested.
• If severe symptoms occur, especially breathing difficulties, seek medical help.

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

What should you do after exposure?

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

Breathing difficulties, substantial swelling, dizziness or a widespread reaction should not be dismissed as ordinary itching. In these situations, prompt medical care is needed. Mild exposures are usually limited, but reactions differ from person to person and may be worse for those with allergies or asthma.

Why should you keep your distance from the striking line?

Their collective movement makes processionary caterpillars easy to photograph, yet their defence operates on an almost invisible scale. The caterpillar does not need to bite or chase anyone: it is enough for hairs to come loose and reach a nearby surface for the surrounding area to become a potential source of irritation.

The safest way to watch is from a distance. The contrast between the orderly line and the microscopic defence explains 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; notify the person responsible for the area where there is a risk of public contact.