Pine processionary moth

Pine processionary moth, Thaumetopoea pityocampa, is a quarantine pest of concern to U.S. plant health because its gregarious caterpillars feed on pine needles and can cause severe defoliation. The larvae build conspicuous white silk nests and later travel in nose-to-tail processions. Their detachable urticating hairs can irritate skin, eyes and airways in people, pets and livestock, so suspected infestations should be handled cautiously and reported through plant-health authorities.

The pine processionary moth is a Eurasian and North African species associated mainly with Pinus and, less consistently, Cedrus and Larix. Larvae feed in groups and construct silken nests in the crown. Heavy infestations remove current and older needles, reducing growth and increasing stress, although healthy mature trees may recover from a single episode of defoliation.

In the United States, do not assume that a hairy processionary caterpillar is this species. Similar-looking caterpillars occur in North America, and official identification is important. Avoid touching larvae, nests, shed skins or contaminated vegetation.

• White to gray, oblong silk nests among branch tips and foliage, often high in the pine crown.
• Partly or completely stripped shoots, with consumed needles turning yellow to brown and remaining attached.
• Communal orange-brown larvae with darker markings, often feeding at night or resting inside the nest.
• Single-file, nose-to-tail processions on branches or across the soil, commonly during late winter and spring in much of the native range.
• Repeated or severe defoliation may cause reduced shoot growth, chronic decline and greater vulnerability to drought or secondary pests.

Because the caterpillar hairs can cause dermatitis, conjunctival irritation, respiratory symptoms and allergic reactions, human or animal exposure is a safety issue as well as a plant-health issue.

Damage is caused by the larval stages feeding on pine needles. Females generally lay several hundred eggs in a helically arranged batch around needles or young twigs; the batch is camouflaged with abdominal scales. Larvae remain together, build progressively larger silk shelters and spread locally as mature caterpillars descend to pupate.

Long-distance movement is associated mainly with infested plants for planting and other regulated plant material. Adult males can disperse several miles, while warmer conditions may enable range expansion and alter seasonal development. In the United States, suspected finds should be reported to the state plant-health director or USDA APHIS rather than moved, released or distributed.

The species is usually univoltine. Adults commonly emerge from soil in summer, mate soon afterward and females lay egg batches on nearby pines. Eggs hatch after roughly 30–45 days. Larvae pass through five instars, feeding and enlarging communal silk tents. From about the third instar they develop urticating setae containing the allergenic protein thaumetopoein.

In the typical cycle, mature fifth-instar larvae leave the tree in processions during late winter through spring, burrow approximately 2–8 inches into soil and pupate. Adult emergence follows in summer, but prolonged diapause can delay some adults for more than one year. Timing varies with latitude, elevation, host and weather.

Management is most effective when directed at young larvae before large winter nests and urticating hairs become a major hazard. Have a qualified arborist or licensed pest professional inspect the tree and confirm the identification. Small, accessible nests may require carefully controlled removal and secure disposal by trained personnel; do not cut or handle nests casually.

For large trees, public areas or suspected U.S. detections, use professional containment and follow state or federal instructions. Protect people and animals from the work zone, prevent larvae and nest fragments from becoming airborne, and clean equipment after work. Trees with substantial needle loss should receive appropriate water management and monitoring for secondary pests, but avoid injuring already stressed roots or bark.

Use an integrated approach rather than relying on one organic input. Remove accessible egg batches or young nests only when this can be done safely and legally by trained personnel. Microbial products based on Bacillus thuringiensis can be useful against early caterpillar stages when a product is authorized for the crop, site and target pest; performance declines once larvae are larger and protected inside dense tents.

Conserve native predators, parasitoids and insectivorous birds by avoiding broad-spectrum sprays and preserving habitat. Trunk barriers or collection systems may intercept descending larvae in some settings, but they must be designed and serviced by professionals because trapped caterpillars remain hazardous. Biological control and pheromone-based monitoring are best incorporated into a documented IPM program.

• Buy conifers from reputable nurseries and inspect branches, twigs and crowns for camouflaged egg batches or silk tents before planting.
• Do not transport suspect trees, branches, nests, soil or caterpillars. In the United States, contact your state plant-health director or USDA APHIS for reporting and confirmation guidance.
• Maintain tree vigor with appropriate irrigation during drought, adequate planting space and mulch kept away from the trunk; avoid unnecessary nitrogen fertilization.
• Monitor susceptible pines from summer through spring, using trained inspection and, where appropriate, pheromone traps for adult activity.
• Keep children, pets, livestock and workers away from nests, processions and contaminated ground. Do not sweep or use leaf blowers around suspect material.

• The caterpillars' distinctive procession gives the pest both its common name and much of its diagnostic value.
• Egg batches are covered with scales and can resemble a small piece of dried plant material.
• Urticating hairs are actively released when larvae are disturbed and can remain irritating after the caterpillar or nest is no longer alive.
• The moth itself is not the damaging stage; the adult is short-lived and mainly responsible for mating and egg-laying.
• Host susceptibility differs among pine species and can vary by geographic region, so a host record does not mean every tree of that species is equally vulnerable.