Processionary caterpillar numbers on rise in northern France: what to do if you spot them

The record-breaking heatwave at the end of May has led to an increase in processionary caterpillar numbers across northern France.

It comes after a spike in caterpillar numbers was observed in and around Paris at the end of last week. 

Departments issuing warnings included Seine-Maritime, Yvelines, Eure, and Oise, and the Île-de-France Regional Health Authority also issued its own alert over the caterpillars.

In Yvelines, certain forest areas were closed to walkers to reduce the risk of coming into contact with the caterpillars.

These caterpillars are known for their tiny urticating hairs, which can irritate humans and animals they come in contact with.

Symptoms include irritation of skin and eyes, allergic reactions, and in some cases breathing problems. Dogs are particularly at risk, as we explain in our article here. 

In Seine-Maritime, up to a third of all people visiting pharmacies in the Bray area were seeking medical advice after being irritated by processionary caterpillars last week, according to media outlet Actu.

Irritating ‘hairs’ travel in wind

Oak processionary caterpillars (chenille processionnaire du chêne) are found across northern France, with Pine variants more common in the south of the country.

The northern variants are usually spotted between April and July. This year, they are expected to be a major nuisance until mid-June. 

While they are an annual occurrence and a domestic species (despite being designated as harmful), milder winters mean an increasing number of caterpillar larvae make it to the spring.

As their name suggests they are most commonly seen in oak trees.

A combination of higher temperatures leading to increasing caterpillar numbers, and winds over the weekend led to public health concerns. 

The caterpillars are particularly dangerous because their urticating hairs remain dangerous long after becoming detached from the caterpillars themselves. In some cases, they can still cause reactions for three years.

These hairs – more than 60,000 from a single caterpillar – detach from caterpillars when they are stressed or threatened, pushing out in all directions. They can also fall off naturally as caterpillars march in their processions.

The hairs fall on the ground, get stuck to leaves or trees, and even remain in still water. Either at the moment they detach, or after settling, they can be swept up by wind and travel.

In Chartres (Eure-et-Loir), at least 230 people across the department reported symptoms in recent weeks, leaving health authorities to deal with a spike in issues.

Local healthcare practitioners believe the issues have been caused by hairs carried in the wind, as pine caterpillars (more common in the area) have already burrowed. 

Some people reported symptoms without going anywhere near oak trees or even forested areas, further cementing the idea the hairs travelled in the wind.

What you should do if you see processionary caterpillars

If you see caterpillars when out in public, you should not attempt to deal with them yourselves as you can be affected.

Instead, report them to local authorities who can decide on the appropriate next steps. 

In some cases, they may opt to install traps to catch caterpillars descending from tree tops. 

In some communes such as Saint-Paul (Oise), residents who spot the caterpillars in their garden must have infested trees treated by a specialist firm. 

You can check with your mairie if there are such requirements in your area.