The “worms” you’re seeing on or around your pine trees are almost certainly caterpillars, most commonly pine processionary moth larvae, bagworms, or pine sawfly larvae. Pine processionary caterpillars are the most widespread and problematic of the group. They’re easy to identify by their distinctive behavior: they travel in long, nose-to-tail chains that can include hundreds of individuals, and they build conspicuous white silk nests in the branches of pine trees. Beyond damaging your trees, these caterpillars pose real health risks to people and serious dangers to pets.
How to Identify Pine Processionary Caterpillars
Pine processionary caterpillars are covered in fine, grayish-white hairs and travel in unmistakable single-file lines along branches and down trunks. Their silk nests look like dense white or grayish pouches, typically anchored to the sunny side of the tree’s canopy. A single egg mass can contain up to 300 eggs, so nests often house large colonies. If you see a procession of caterpillars marching across the ground near a pine tree, especially in late winter or early spring, you’re almost certainly looking at this species.
Pine sawfly larvae are sometimes confused with processionary caterpillars but behave differently. Sawfly larvae look more like smooth, pale green or yellowish caterpillars and tend to feed in clusters along individual branches rather than forming processions. Bagworms, another common pine pest, are recognizable by the small spindle-shaped bags they construct from silk and bits of foliage, which dangle from branches like tiny ornaments.
Their Life Cycle and When They Appear
Pine processionary moths are unusual insects because they’re active during the coldest months of the year. Adult moths emerge from the soil in August, mate, and lay eggs on pine needles. The caterpillars hatch about four weeks later and remain active throughout the entire winter, feeding on pine needles even on subfreezing nights. Colonies build and enlarge their silk nests as temperatures drop, using them as insulated shelters between feeding bouts.
By March, the caterpillars reach their final growth stage and leave the tree in their famous ground-level processions, sometimes stretching dozens of feet long with up to 300 individuals in a single chain. They’re heading underground to pupate, where they spend the warm summer months before emerging as moths in August to restart the cycle. This March procession is the most visible and dangerous phase, because the caterpillars are at their largest, carry the most irritating hairs, and are moving across ground where people and pets encounter them.
Damage to Pine Trees
Processionary caterpillars feed on pine needles throughout winter, and a large colony can strip significant portions of a tree’s canopy. You’ll notice browning or thinning of needles, particularly on branches near the silk nests. A single season of heavy defoliation rarely kills an otherwise healthy pine, but repeated infestations over several years weaken the tree, making it vulnerable to drought, disease, and secondary pest attacks. Young or already stressed trees are at the greatest risk.
Bagworms and sawfly larvae cause similar defoliation. Sawflies tend to eat older needles first, leaving new growth intact, which gives the tree a better chance of recovery. Bagworms are less selective and can cause patchy, irregular needle loss across the canopy.
Health Risks to People
Pine processionary caterpillars are covered in thousands of microscopic hairs that contain an irritating protein. These hairs detach easily and can become airborne, meaning you don’t need to touch the caterpillars directly to be affected. Contact with the hairs causes painful skin rashes, eye irritation, and throat inflammation. In rare cases, people experience allergic reactions that require medical attention.
The hairs remain irritating long after the caterpillars are gone. Old nests, shed skins, and even soil where caterpillars have traveled can still carry loose hairs capable of triggering reactions. This is why you should never handle nests, dead caterpillars, or debris from infested trees with bare hands.
Serious Dangers for Dogs
Dogs are especially vulnerable because they tend to sniff or lick processionary caterpillars on the ground. When the irritating hairs contact a dog’s mouth or tongue, the reaction can escalate from a medical problem to a life-threatening emergency within hours.
The first signs are intense drooling and swelling of the tongue and mouth. As inflammation worsens, the tongue can develop tissue death caused by tiny blood vessel blockages triggered by the local inflammatory reaction. In severe cases, portions of the tongue darken and die, potentially requiring surgical removal. Dogs may also develop breathing difficulty from throat swelling, vomiting, fever, and signs of systemic infection. One documented case involved a dog that progressed from initial mouth swelling to full tongue necrosis, dangerously low blood pressure, and sepsis, requiring intensive care including a medically induced coma and feeding tube.
If your dog shows sudden excessive drooling, pawing at its mouth, or a swollen tongue after being near pine trees during caterpillar season, treat it as an emergency. Time matters significantly with these exposures.
How to Safely Remove Nests
Never remove processionary caterpillar nests without proper protection. Full body coverage is essential, including protection for your head, face, neck, and a respirator or high-quality mask to avoid inhaling airborne hairs. Exposed skin anywhere on your body can develop a painful rash.
The safest manual method is to cover each nest with a plastic bag, carefully pull it off the branch while keeping it inside the bag, then immediately seal the bag and place it inside a second bag. Spraying the nest with hair lacquer before bagging helps prevent hairs from drifting loose during removal. If you have access to a vacuum with a HEPA filter, that’s the most efficient option for collecting nests, loose caterpillars, and stray hairs.
Removed nest material should be burned on site if your local regulations allow it. If you need to transport it for disposal, be aware that processionary moth material may be classified as hazardous waste in some regions, with specific rules about transport documentation. Check with your local council or waste authority before moving it off your property.
Treating an Infestation
For active infestations, a biological insecticide containing a naturally occurring soil bacterium (sold under the name Btk) is one of the most effective and environmentally targeted options. It works only when eaten by caterpillars and has no effect on other animals, birds, or beneficial insects like bees. The key limitation is timing: it works best on young, actively feeding larvae. If the caterpillars are already large and mature, the treatment is unlikely to be effective.
Good spray coverage is critical. Every needle surface, including undersides, needs to be coated so feeding caterpillars ingest enough of the bacterium. Sunlight breaks the product down quickly, so reapplication is often necessary, especially after rain or several days of strong sun. For tall pine trees, professional application with specialized equipment may be needed to reach nests high in the canopy.
For smaller infestations or individual trees, physical nest removal during winter (before the March procession) is often sufficient. Catching the problem early, when nests are small and caterpillars are young, makes both biological treatment and manual removal far more effective. Once caterpillars leave the tree and disperse across the ground in spring, control becomes much harder and the risk to people and pets increases significantly.
Preventing Reinfestation
Because adult moths emerge from the soil in August and seek out pine trees to lay eggs, monitoring your trees from late summer through early autumn helps you catch new infestations before colonies establish. Look for cylindrical egg masses wrapped around individual pine needles, typically on the outer branches of the canopy. Removing egg masses in autumn eliminates colonies before caterpillars ever hatch.
Trunk barrier bands, which are physical collars wrapped around the tree trunk, can intercept caterpillars during their March descent and funnel them into a collection bag. These are particularly useful for protecting high-traffic areas where children or pets play near infested trees. In regions where processionary moths are established, annual monitoring and early intervention each season is more realistic than expecting permanent elimination.