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There are two species in France having an impact on human health: the pine processionary caterpillar (Thaumetopoea pytiocampa) and the oak processionary (T. processionea). The first colonizes conifers of the pine family (Scots pine, black pine, Atlas cedar, etc.) and the second hardwoods of the oak family (sessile oak, pedunculate oak, etc.).
Why this name and how to recognize them?
Processionary caterpillars have the particularity of living in groups and move in single file.
Throughout their development, they weave silken nests in the colonized tree and come out at night to eat the leaves. In its last larval stage, the pine processionary descends along the trunk to bury itself in the ground and turn into a chrysalis – the oak processionary, on the other hand, remains in its tree for these last stages.
Once its metamorphosis is complete, usually a few weeks later (note that some pine larvae can remain buried in the ground for several years), the butterfly emerges and the male and female individuals reproduce. The females will lay their eggs at the top of another tree and thus start a life cycle again.
Pine processionary caterpillars are orange-brown and oak processionary caterpillars silvery gray; they measure up to four centimeters in length at the end of their growth. However, they can be difficult to tell apart outside of their habitat.
Where are they present in France?
Together, these two species have been identified throughout the city. There is no longer any department that escapes one or the other of the species, when both are not present. However, they do not share the same story.
The pine processionary caterpillar first developed in the coniferous forests of the Atlantic and Mediterranean regions, but less and less harsh winters allowed its rise throughout the territory. The mild winter temperatures, both in latitude and in altitude, favor the hatching of eggs and the growth of larvae.
The species has thus extended its implantation towards the northeast since the 1960s and has progressed towards the north and Île-de-France since 2010, due to climate change. The expansion of its range is also one of the biomarkers of climate change retained by theNational Observatory on the Effects of Global Warming (ONERC).
Their geographical proliferation is also the consequence of regular planting of pines in non-forest environments in new regions.
Periodic pullulations of oak processionary caterpillars are known in oak groves mainly in the north-west, Île-de-France and an endemic area in the east. These caterpillars develop, in a discreet way, over a good part of France, except in certain departments of the southern half. Its range has not changed significantly in recent decades, but the species swarms more and more.
When can you be exposed?
The pine and oak processionary caterpillars do not have the same seasonality: exposures to pine processionaries are observed from January to May, with a peak in March, as observed in a study of health effects related to exposure to processionary caterpillars carried out using data from poison control centres. Those of the oak from April to August, with a peak in June. Fall heat waves and mild winters promote early larval development.
The period most at risk is when the pine processionaries migrate along the trunk to bury themselves ten or twenty meters further into the ground. A cord of several hundred individuals (up to 300) stretches in an uninterrupted line, which can be crossed by walkers, residents of forests, woods or gardens, as well as domestic animals.
Very visible on the trunk at breast height, then on the ground, they can be caught by young children and put in their mouths, which represents a major risk of poisoning.
Even out of season, the risk exists: empty nests contain a large concentration of stinging hairs that have detached. Being nearby, when pruning a colonized tree for example, may be enough to be reached. Nests also crumble over time, and the wind spreads their irritating contents.
It is therefore not necessary to directly touch a processionary to be in contact with its stinging hairs. Analysis of data from poison control centers showed that in one out of two cases, those exposed had not seen caterpillars. They were exposed to hair deposited by the wind on their clothes, animal fur, gardening equipment, a terrace, etc.
What are the health risks ?
A processionary caterpillar neither bites nor stings: its “weapons” are some of its hairs, or bristles, which are like so many microscopic needles or harpoons. These hairs detach very easily from the body of the caterpillar when it feels threatened. Carried by the wind, they can reach their target several tens of meters away.
Urticating hairs contain a very irritating and inflammatory toxic protein: thaumetopoein. It penetrates where the “harpoon” is planted: most often in the skin (90% of cases, generally mild according to an ANSES study conducted from January 2012 to July 2019, with more than 1,200 exposures identified), but also in the eye, nose, mouth…
The hair breaks and the toxin released causes a reaction similar to that of nettle stings: red pimples in the form of blisters, itchy red patches, appear on the skin.
Systemic symptoms may occur with significant exposure. The immune system then races, which can lead to a sudden drop in blood pressure, discomfort or loss of consciousness.
In the ANSES study, less than 5% of the cases were of average severity and therefore concerned hairs received in the eyes. Eye exposure can cause pain, tearing, redness, swelling of the eyelids. In more severe cases, the hairs can also cause corneal damage (keratitis) and persistent visual disturbances.
But airborne hair can also reach the respiratory system. If inhaled, respiratory discomfort may occur with coughing, runny nose, sneezing and sometimes asthma attacks.
Finally, caterpillar mouthing can occur in toddlers who exhibit hand-to-mouth behavior while exploring their environment, or in people with cognitive impairment. Hair can cause swelling of the face and throat, difficulty in breathing.
What to do to not be exposed?
The first thing to do is, of course, not to touch a caterpillar and not to approach the processions. Don’t touch where they went either.
Likewise, do not touch the nests but contact professionals who will have the equipment to intervene.
You have to be vigilant for children, who may be tempted to put the caterpillar in their mouths. Avoid walking in parks and gardens where trees are affected.
If infested trees are near waterfront homes, avoid drying clothes outside, be careful while mowing the lawn. Wash fruits and vegetables picked from the area.
In the forest, to avoid airborne hairs, it is preferable to wear long, covering clothes, and to avoid rubbing your eyes during and when returning from the walk.
Is there a risk for animals?
The nose at ground level, animals are exposed to contact with these stinging hairs, especially dogs. They can have it on their coat, even take a caterpillar in their mouth, which can cause very serious inflammatory reactions up to necrosis of the tongue.
If an animal is affected, you must immediately consult a veterinarian who can examine it, or call a veterinary poison control center for advice. Among the signs to be aware of, there is a significant salivation of the dog.
What are the means of struggle?
The classification of processionary caterpillars as a species harmful to health obliges regional and local authorities to put in place treatments and strategies to fight against their proliferation where they risk affecting populations and domestic animals. There are several approaches, varying according to the regions and the extent of the problem.
For pine processionaries, a collar in the form of a gutter can be placed around the trunk, which sends the caterpillars in the process of descending towards a closed bag where they are trapped. The bag is then destroyed.
Their natural predators can also be favoured: bats, tits, etc. Or microbiological control, with spraying on trees infested with a biological insecticide (the bacteria Bacillus thuringiensis kills the larvae that ingest it). Male butterflies can also be attracted and trapped using pheromone boxes (sex hormones) hung in the trees, in order to prevent reproduction and thus limit the pullulation.
Finally, the nests can be destroyed by equipped professionals. The branch of the tree is cut and the whole is burned.
There is no single, definitive solution. It is not possible to eradicate these two species, they must be managed as well as possible. This requires the combination of several means of control and, also, the management of the landscape and biodiversity: ensuring that the host trees of the caterpillars are limited and favoring species that they avoid, such as the silver birch, constitutes a prospect of coming.
What to do in case of exposure?
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If you think you have been affected, it is recommended to take a shower, rinse well to remove the hair and change your clothes. Do not rub the lesions to avoid breaking the invisible hairs present.
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In the event of emergency signs (respiratory distress, loss of consciousness, etc.), you must immediately call 15, or 112 or 114 for the deaf and hard of hearing.
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When you have had hair in your eye, you must quickly consult an ophthalmologist.
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In case of other symptoms (redness, itching): call a poison control center or consult a doctor. If possible, photograph the caterpillar to help identify the species with certainty.
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For pets, consult a veterinarian or call a veterinary poison control center.
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