The worst outbreak in 25 years, arriving in winter
Each winter's end in Spain has long carried the quiet procession of pine caterpillars descending to earth — a natural rhythm now distorted by warming temperatures into something more urgent and more dangerous. The processionary caterpillar, armed with nearly invisible urticating hairs capable of triggering severe allergic reactions, is emerging earlier and in greater numbers than recorded memory allows, turning parks, forests, and family walks into spaces requiring careful navigation. The Balearic Islands face their worst outbreak in a quarter century, and medical professionals across the country are documenting a rise in anaphylactic emergencies, while veterinarians warn that dogs — drawn by instinct toward these creatures — face potentially fatal consequences. What was once a seasonal inconvenience has become, under the pressure of climate change, a genuine and expanding public health concern.
- Milder winters have broken the caterpillars' seasonal rhythm, pushing their descent weeks earlier and swelling their populations to levels not seen in a generation.
- The threat is nearly invisible — urticating hairs too fine to see drift on the wind, embed in skin and eyes, and can trigger anaphylactic shock in sensitive individuals before they realize what has touched them.
- Dogs face the gravest danger, as their natural curiosity leads them to sniff nests and caterpillars, with mouth and throat inflammation capable of escalating to fatal respiratory failure within hours.
- Authorities in the Balearic Islands and southern Spain are cutting and burning infested branches and installing trunk barriers, while emergency rooms and veterinary clinics prepare for a peak season expected to intensify through May and June.
- Climate projections offer no relief — warming conditions continue to favor the species, meaning each successive year risks arriving harder and earlier than the last.
The processionary caterpillar has always been part of Spain's pine forest ecology, descending each year in its distinctive single-file lines to burrow into the soil and complete its transformation into a short-lived moth. But milder winters driven by climate change have accelerated and amplified this ancient cycle, pushing the caterpillars down from the branches earlier in the season and in far greater numbers than before. The Balearic Islands are already bracing for what officials describe as the worst infestation in 25 years, and the peak of encounters — typically falling between May and June — has not yet arrived.
The caterpillar's danger is concentrated in the urticating hairs that cover its body, particularly in its final larval stage when it turns dark with orange and white banding. These hairs are nearly invisible to the naked eye, yet they penetrate skin and mucous membranes with ease, causing irritative dermatitis, eye inflammation, and respiratory complications. They also become airborne, carried on wind currents far from the caterpillar itself. Spain's dermatologists have documented a growing number of anaphylactic reactions in recent years, with some cases severe enough to require emergency intervention.
Children are especially vulnerable — curiosity draws them close, and the hairs adhere to hands and clothing, worsening with any scratching. The recommended response is to wash the area gently with water, apply aloe vera and cold compresses, and seek medical attention if symptoms escalate. Eye contact requires saline flushing without touching; respiratory symptoms demand immediate emergency care.
For dogs, the risk is potentially fatal. Their instinct to sniff caterpillars and fallen nests exposes them to rapid and severe inflammation of the mouth, tongue, and throat — angioedema that can close the airway and kill if veterinary care is not sought immediately. Municipalities are responding with nest removal and trunk barriers, but as temperatures continue to rise, the window of danger grows longer and the outbreaks grow harder to contain.
The processionary caterpillars are on the move again, descending from Spain's pine trees in their characteristic single-file processions, and they're arriving earlier and in larger numbers than ever before. These insects, native to the Iberian Peninsula, are marching down from the branches to bury themselves in the soil and complete their transformation into moths—a legitimate part of their life cycle, but one that has become an increasingly serious problem for dogs, children, and adults who encounter them.
Milder winters across recent years have created ideal conditions for these caterpillars to thrive. Climate warming is not just allowing them to survive; it's accelerating their emergence and swelling their populations. In southern Spain and the Balearic Islands, authorities have already sounded alarms. We are still technically in winter, yet the caterpillars are already descending—a sign of how dramatically the seasonal calendar has shifted. The encounters will intensify as spring approaches, reaching their peak between May and June, when the caterpillars will finally emerge from the soil as adult moths to breed.
The danger lies in what covers their bodies: nearly invisible urticating hairs that can penetrate skin and mucous membranes with ease. Once embedded, these hairs trigger irritative dermatitis on exposed skin, eye problems, and respiratory complications. Airam Jenny Dávalos, coordinator of the Dermatology Group at Spain's General Practitioners Society, explains that in recent years, several anaphylactic allergic reactions to the caterpillar have been documented. The reactions are immediate. For people with heightened sensitivity, the consequences can be severe enough to require emergency room intervention to prevent serious respiratory symptoms. The toxic hairs are concentrated on the caterpillar's surface and especially dense in their nests, but they also become airborne, drifting through the environment on wind currents.
The caterpillar only bears these urticating hairs during one stage of its life. As it molts through its larval phase, it changes color—first green, then pink and reddish, and finally black with an orange band of hair along its back and white bands on the sides. It is in this final, darkest form, typically visible by late autumn when the nests have grown large to protect against cold, that the caterpillar becomes most recognizable and most dangerous. Once it transforms into a moth, the hairs disappear entirely. The adult moths live for barely a day, consuming no food at all, existing only to mate and lay eggs before dying.
If you encounter these caterpillars, the advice is simple: do not touch them, and keep children away. The hairs adhere to skin and clothing, and scratching only drives them deeper into the skin and eyes, worsening inflammation. Lesions appear most frequently on exposed areas—the neck, wrists, forearms, and ankles. Children, driven by curiosity, often suffer hand and finger irritation. The immediate response should be to wash the affected area with water without scratching or rubbing, followed by aloe vera and cold compresses to reduce swelling. A doctor may recommend corticoid creams or antihistamines. For eye contact, saline solution and artificial tears can help flush the substance without hand contact. Respiratory effects are uncommon but serious when they occur, manifesting as difficult breathing and air hunger, often associated with anaphylactic reactions affecting multiple organ systems—these require immediate emergency care.
Dogs suffer the worst of the processionary season. Their curiosity drives them to sniff the caterpillars and fallen nests. When symptoms begin, dogs show anxiety and compulsive scratching, intense salivation, and if they swallow the hairs, vomiting. The inflammation of the mouth, tongue, and larynx can develop rapidly and severely, causing angioedema that prevents the mouth from closing, respiratory difficulty, and potentially death. A veterinarian must see an affected dog immediately.
The caterpillar itself does not kill the pine tree directly; instead, it consumes the tree's needles, weakening it over time. The real damage emerges when infestations persist year after year, leaving trees so compromised that they become vulnerable to pathogens, drought, and injury. The density of nests and abundance of caterpillars fluctuate annually based on environmental conditions, but climate warming is making conditions increasingly favorable for the species. This means earlier, more severe outbreaks in pine forests. Municipalities respond with removal measures—cutting and burning branches with nests, or placing rings around tree trunks to funnel caterpillars into bags where they die.
The Balearic Islands have already warned of processionary presence in parks and forests and are bracing for what they describe as an exceptionally difficult year. Luis Núñez, head of the Balearic Islands' Forest Health Service, acknowledged that the infestation levels now appearing are the worst his region has faced in 25 years. As temperatures continue to climb, the caterpillars will keep arriving earlier and in greater numbers, stretching the season when Spain's residents and their pets must remain vigilant.
Notable Quotes
The hairs are nearly invisible and can penetrate skin and mucous membranes, causing irritative dermatitis, eye problems, and respiratory issues; anaphylactic reactions have been documented in recent years.— Airam Jenny Dávalos, Dermatology Group coordinator, Spanish General Practitioners Society
The infestation levels we're seeing now are the worst we've had in 25 years.— Luis Núñez, head of Forest Health Service, Balearic Islands
The Hearth Conversation Another angle on the story
Why are these caterpillars suddenly everywhere if they're native to Spain?
They've always been here, but the winters used to kill many of them off. Now the cold isn't harsh enough. Milder seasons mean more survive, and they emerge earlier in the year.
The hairs sound almost weaponized—invisible, airborne, embedded in skin. Is this a new development?
No, the hairs have always been there. What's new is the scale and timing. We're seeing more caterpillars, more nests, more people and pets encountering them during seasons when we're less prepared.
You mentioned dogs are particularly vulnerable. Why are they at such higher risk than humans?
Dogs investigate with their noses and mouths. They don't understand the danger. If they swallow the hairs, the inflammation in their throat can close their airway. A human might get a rash and know to seek help. A dog might die before the owner realizes what's happening.
The article mentions anaphylactic reactions in humans. How common is that?
Rare enough that it's not the typical case, but common enough now that doctors are documenting it regularly. That's the shift—we're seeing reactions we didn't see before, or at least not with this frequency.
What happens to the pine forests themselves?
The caterpillars eat the needles, which weakens the trees. Over years of repeated infestations, the trees become so fragile they can't survive a drought or disease. You end up with dead forests that are vulnerable to fire.
And there's no way to stop this without cutting down trees?
Municipalities try—they burn branches with nests, they trap the caterpillars. But you can't eliminate them entirely. They're part of the ecosystem. The real problem is that climate change has tipped the balance so far in their favor that our old management approaches don't work anymore.