The Pine processionary moth (Thaumetopoea pityocampa) is a very common moth in many parts of southern Europe (including Spain and Portugal), North Africa and central Asia.

It has caterpillars that have some fascinating behavioral habits, but which can also be serious pests. The caterpillars have hairs that cover their bodies which can cause irritation to humans and other mammals.

a bunch of hairy processionary caterpillars

These hairs come out of the caterpillars when they are stressed, and cause inflammation of human skin if they come into contact with it. They should not be touched because of this.

Some people will develop an allergy to them as well, ranging from extremely itchy blistered skin to anaphylactic shock. In almost all cases, medical treatment will be required.

The itch is especially itchy and painful and lasts typically for three weeks. If these hairs get into your eyes it can cause blindness, especially in a child or animal.

It is not unknown for dogs to have their tongues amputated after getting stung elsewhere on their body, then licking the affected area.

If the dog survived the attack at all, that is…

I did hear of one man, a friend of a friend who strangely had not heard of these creatures, who was driving up a mountain road one day when he spotted a procession crossing in front of him.

Being a kind-hearted fellow, he didn’t want to run them over and so stopped and manually assisted them to the other side of the road. I believe he was nearly a month in hospital!

Me, I’d have stopped only so I could reverse over them again!

Pine processionary larvae cause a lot of damage to pine trees and cedars because they feed on the needles. They will also feed on larch.

These caterpillars normally leave their nests in pine trees to forage for food at night before returning home to sleep during the day, literally stripping their host tree of leaves (needles) and sap. Coming out only at night deprives the birds and wasps of a good feed.

They are called processionary because they form a procession, with the head of one attaching itself to the rear of the one in front to form a long line, or procession.

It is in January/February that you see their nests hanging on pine trees like big cotton wool balls and it is up to the local authority to clear these nests, by removing and burning them while the workers wear protective clothing.

It is not advisable to take them down and burn them yourself, because of the danger of airborne hairs which can be fatal if inhaled.

Pine Processionary Moth Description

The Pine processionary moth has cream forewings with brown markings and white hind wings. It can be seen on the wing between May and July and the females are bigger than the males.

three pine processionary caterpillars marching forward

Pine Processionary Moth Caterpillar Nests and Processions

Pine processionary caterpillars spin quite large tent-like nests is the trees they infest and feed on, and they spend the winter months in these structures. Often it is possible to see several of these nests in the same trees.

The nests or tents can be as large as a pineapple or even bigger, and are messy looking structures because they accumulate a lot of caterpillar frass at the base. The nests are spun of silk and the caterpillars force their way in and out of the structures which they shelter in.

When smaller the caterpillars, which are very social creatures, spin flimsier nests around the pine needles, but in their later stages they make stronger permanent homes out of the nests they build.

They leave them at night to go and feed and return at dawn to spend the days undercover. The caterpillars are active through the cold nights of winter and continue feeding after dark.

They leave these tents for good, usually in March, and descend to the ground in order to find a place to pupate. The caterpillars move in conspicuous nose-to-tail processions, which as mentioned before, give the moth its name.

They do this because they respond to the stimulus of hairs on the tail of the caterpillar in front, and are also thought to follow a scent trail of pheromones.

Entomologist Jean Henri Fabre studied the Pine processionary caterpillars and published the results in The Life of the Caterpillar in 1916.

the life of the caterpillar Jean Henri Fabre

He found out that their instinct to follow was so strong, that the caterpillars placed in a circle going nose-to-tail would continue going round and round despite there being food just outside the circle.

They pupate singly just under the ground in silken cocoons. They like soft and sandy soil to make their cocoons in.

The pupae are at first a yellowish-brown but become a darker red-brown. In warm springs, many caterpillars pupate successfully and a large number of moths will hatch in the summer.

Is There a Long-Term Solution For The Pine Processionary Problem?

I personally would start by getting rid of all the pine trees; it’s not as if they are useful for anything!

The authorities tend to spray the affected areas with insecticides from helicopters once a year, but this has the undesired effect of killing many other insects that are necessary for a balanced ecology, as well as small birds which feed off the insects.

This approach could best explain why every year here there seems to be an imbalance of nature’s creatures. Some years there are thousands of earwigs.

Other years it’s wasps. Yet other years it’s snails or the little wriggly grey things that smell when you stand on them (I don’t know what they are).

Left to nature, there should be no sudden rise in the populations of any insects or small creatures.

There have been some attempts to control the processionary caterpillars by laying pheromone traps to disrupt the mating season, but at the same time, there has been a policy to plant yet more pine trees!

And so the cycle continues…