The pine processionary caterpillar, or the hairy enemy

The ‘caterpillar with a coat’ that ruins walks and how to spot it in time

The pine processionary caterpillar was scientifically described in Europe more than 250 years ago, but its impact on pets’ health has clearly increased in recent decades. The pine processionary caterpillar (Thaumetopoea pityocampa) is, without exaggeration, one of the most underestimated environmental risks to dogs during the warmer months. Every spring it reappears in pine forests, urban parks and private gardens, and every year it causes numerous veterinary emergencies that could have been avoided with adequate information and prevention. 

Its hairy and seemingly harmless appearance deceives even the most attentive owners. However, this “fur” is made up of thousands of microscopic stinging hairs (also called urticating setae), which act like micro-needles and release irritating and allergenic compounds. To put this into perspective with a very tangible fact, scientific literature describes that a mature larva can carry up to around 1 million detachable stinging hairs, about 200 μm long and 6 μm wide, which explains why the risk can exist even without “obvious direct contact” when these setae remain in the environment.

Dogs, explorers by nature, sniff, lick or touch whatever they find during their walk, and even minimal contact is enough to trigger a serious problem 😟. And here is a clinical fact that is worth knowing: in a retrospective study of 41 cases of contact in dogs (2002–2006), the pattern of signs was surprisingly consistent, with oral signs and pain as typical presentations, reinforcing the fact that we are not talking about a “theoretical” risk, but a real and repeated scenario in clinical practice. Therefore, knowing this insect well and learning to recognise it in its environment is not just a curious fact: it is a real measure of protection.

A caterpillar with a history… and very bad intentions

The pine processionary caterpillar is not a recent or isolated phenomenon. It is the larval stage of a nocturnal moth that is perfectly adapted to temperate climates, especially the Mediterranean ecosystem. From an entomological point of view, its life cycle is carefully synchronised with the seasons and the biology of its host tree.

The larvae feed on the needles (leaves) of pine trees and other conifers, gradually weakening the tree. During autumn and winter, they build silky white nests, clearly visible in the treetops, which act as thermal shelters. Inside, the caterpillars remain grouped together, reducing heat loss and increasing their survival during the cold months.

Your vet advises: don’t wait to see the procession

If you detect pockets in the treetops, the risk is already apparent. Changing your route weeks before the caterpillars descend avoids most accidental contact. Anticipation is the best preventive tool. 

As temperatures rise, usually between February and May, they leave the tree and descend to the ground, forming the well-known processions. This behaviour is not anecdotal: moving in a line increases their effectiveness in finding a suitable place to burrow and complete the pupal stage. It is precisely this massive descent to the ground that explains why, year after year, cases in dogs are concentrated at this time of year. Knowing this allows you to anticipate: if you see processions, it is not a good day for walks “at your leisure“.

At the ‘chemical-biological’ level, it is useful to understand a technical term: thaumetopoein. This is a stinging protein described in classic studies; in laboratory terms, it was identified as a ~28 kDa protein (with 13–15 kDa subunits) associated with the hairs, and is related to the stinging effect observed after exposure. In plain language: it is not just ‘mechanical irritation’; there is a real biochemical component.

What’s more, the processionary moth does not contain a single “irritant”, but several. Research on mushroom extracts has described at least seven relevant allergens, which helps to understand why some exposures trigger disproportionate reactions in certain individuals (more sensitive animals or people).

Your vet advises

Instead of relying on “I’ll deal with it when it happens”, prepare a mini walking kit that really works in the real world: nitrile gloves, single-dose saline solution and a squeeze bottle (sports bottle) for continuous irrigation without rubbing. It’s not sophisticated: it’s pragmatic, inexpensive and reduces mistakes due to nerves.

Practical tip: if your dog tends to “walk with its nose to the ground”, during the risk season consider walking it on a short leash in areas with pine trees, and reinforce a simple command such as “leave it” or “come on” to stop compulsive sniffing. It’s not about obedience for aesthetics: it’s about prevention.

Where the processionary caterpillar hides and why it may be closer than you think

Although many people associate the processionary caterpillar only with rural environments, the current reality is very different. Its adaptability and the expansion of green areas with conifers have made it present in everyday places such as urban parks, private gardens, residential areas and peri-urban trails.

It is commonly found in natural pine forests and reforested areas, parks with ornamental pines, communal or private gardens, recreational areas and forest paths.

Climate change has played a key role in this expansion. This is not just a cliché: a widely cited study documented the latitudinal and altitudinal expansion of the processionary moth’s range, and more recent analyses continue to suggest that milder winters favour its advance and persistence. In practical terms for tutors: the risk period may start earlier and last longer, so do not let your guard down even if ‘it is not yet spring‘.

Measured climate data: its expansion has been quantified in kilometres (and in altitude)

This is not just an impression: a classic study documented a shift in the distribution area of the processionary caterpillar of ~87 km to the north (between 1972 and 2004) and, in alpine areas of northern Italy, an altitudinal rise of ~110–230 m (between 1975 and 2004). The main explanation: greater winter survival linked to milder winters. ([reference study on expansion due to warmer winters])

A simple but effective tip is to observe the trees you usually walk past during autumn and winter. Identifying the presence of pockets in the treetops allows you to anticipate the risk weeks or even months before the caterpillars descend to the ground. Many guardians avoid problems simply by changing their route for a few weeks, choosing streets without conifers or prioritising walks along the beach/in the city when the pine forests are ‘active’.

A practical “pocket” method: make a mini-rule at home for the season: if there are pine trees and there are several mild days in a row, walk with more caution. It is surprisingly effective, because the real risk skyrockets with mild temperatures.

Learning to recognise it: the first step to avoiding disaster

Recognising the processionary caterpillar in the environment is a fundamental preventive skill. On trees, white or greyish, cotton-like pockets are the most obvious sign. They are usually located in the sunniest parts of the crown, as the caterpillars seek to maximise heat.

On the ground, during the critical months, the most characteristic sign is the presence of brown caterpillars with orange bands, moving in single file, touching each other. This striking behaviour should not arouse curiosity, but caution 🚨. When faced with a visible procession, the most prudent thing to do is to move away without allowing your dog to approach, even if the animal shows only visual interest.

Practical tip: cases repeat themselves every year following the same pattern

Veterinary articles agree on one key point: most exposures occur during routine walks in familiar areas. The problem is not the rarity of the event, but its predictability. 

It is important to remember that the danger is not always obvious at first glance. Stinging caterpillars develop in the later larval stages, can detach themselves and become airborne, so the risk is not limited to ‘seeing a caterpillar’ on the path. Therefore, in areas with a known presence of processionary caterpillars, it is advisable to prevent your dog from sniffing the ground insistently, keep it under constant visual control and, in the case of particularly curious dogs, opt for a short leash during the risk period.

A very useful tip: if you are walking in an area with pine trees and you spot pockets in the treetops, don’t wait to see the procession. Change your route immediately. In prevention, being “one step ahead” is what avoids emergencies.

Sources and recommended reading

Fabre, J., et al. (2011) – Contact with pine processionary caterpillar (Thaumetopoea pityocampa): a retrospective study of 41 cases in dogs – Veterinary Dermatology – Retrospective clinical study analysing 41 canine cases of exposure to processionary caterpillars and describing repetitive and well-characterised clinical patterns in dogs – https://pubmed.ncbi.nlm.nih.gov/21824369/

Battisti, A., et al. (2005) – Expansion of geographic range in the pine processionary moth caused by increased winter temperatures – Ecological Applications (Ecological Society of America) – Reference work demonstrating the latitudinal and altitudinal expansion of the pine processionary moth associated with increased winter temperatures – https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/04-1903

Godefroid, M., et al. (2025) – Climate-driven changes in the distribution and phenology of the pine processionary moth – Science of the Total Environment – Recent study confirming how mild winters prolong the risk period and bring forward larval activity – https://www.sciencedirect.com/science/article/pii/S0048969725011076

Lamy, M., Novak, F. (1985) – Isolation and characterisation of thaumetopoein, the urticating protein of the pine processionary caterpillar – Biochimica et Biophysica Acta – Biochemical identification of thaumetopoein, the urticating protein responsible for the toxic effect of the hairs of the processionary caterpillar – https://pubmed.ncbi.nlm.nih.gov/3933773/

Lamy, M., et al. (1986) – Thaumetopoein: a protein toxin from the pine processionary caterpillar – Toxicon – Toxicological study describing the protein structure and irritant mechanism of thaumetopoein – https://www.sciencedirect.com/science/article/pii/0041010186901947

Rodríguez-Mahillo, A. I., et al. (2012) – Allergenic components of the pine processionary caterpillar: clinical and molecular characterisation – Clinical & Experimental Allergy – Research identifying at least seven relevant allergens in the stinging hairs, with sensitisation demonstrated in more than 60% of the patients studied – https://pubmed.ncbi.nlm.nih.gov/22708991/

Moneo, I., et al. (2023) – Health effects of airborne setae from the pine processionary caterpillar – International Journal of Environmental Research and Public Health – Comprehensive review detailing the presence of up to ~1 million stinging setae per larva and their ability to disperse into the environment – https://pmc.ncbi.nlm.nih.gov/articles/PMC10049294/

Sánchez-Morillas, L., et al. (2011) – Skin reactions after exposure to pine processionary caterpillar hairs – Actas Dermo-Sifiliográficas – Clinical article describing skin reactions caused by airborne fungi, even without direct contact with the caterpillar – https://www.actasdermo.org/en-skin-reactions-on-exposure-pine-articulo-resumen-S1578219011000928