Pine Processionary Moth

Overview

The pine processionary moth is one of the most damaging pests of pine forests and one of the most hazardous Lepidopteran species to human and animal health in Europe. The adult moth is relatively inconspicuous—a grayish-brown insect with a wingspan of 30-40 mm—but its larvae are notorious. The caterpillars are famous for their characteristic nose-to-tail processions as they move between feeding sites and pupation locations. More critically, the larvae are covered in thousands of microscopic urticating hairs that cause severe allergic reactions, rashes, and respiratory problems in humans and animals. The pest primarily attacks various pine species (Pinus spp.) including Aleppo pine, Scots pine, maritime pine, and black pine, though it occasionally infests cedars and other conifers.

Geographic Distribution

Native to the Mediterranean Basin and southern Europe, the pine processionary moth has historically ranged from Portugal and Spain through southern France, Italy, Greece, and into parts of North Africa and the Middle East. However, climate change has driven dramatic northward and altitudinal expansion over recent decades. The pest now occurs in central France, has been detected in the Netherlands, and has spread to higher elevation mountain pine forests previously too cold for survival. This expansion threatens pine forests outside its traditional range and exposes new populations to health hazards. The moth's range is primarily limited by winter temperatures, but milder winters are progressively removing this natural constraint.

Life Cycle & Damage

The pine processionary moth has an unusual life cycle with most larval activity occurring during autumn, winter, and early spring. Adults emerge in summer (July-September) and females lay egg masses of 150-300 eggs on pine needles. Eggs hatch after 30-40 days, and larvae begin feeding immediately.

As they grow through five larval stages, the caterpillars construct increasingly large, distinctive white silken nests, usually on the south-facing side of trees. These nests become most prominent during winter and can reach the size of a football. Larvae emerge at night to feed on needles, returning to their communal nest during the day. In late winter or early spring (February-April), fully developed larvae descend from trees in their characteristic head-to-tail processions to pupate in the soil.

Damage manifests as defoliation, with larvae consuming pine needles and weakening trees. Severe, repeated defoliation can significantly reduce growth, increase susceptibility to other pests and diseases, and in extreme cases, lead to tree mortality. However, the health hazard often overshadows forestry impacts. The urticating hairs can become airborne, causing reactions without direct contact. Dogs are particularly vulnerable—they may investigate or mouth the processions, leading to severe tongue and throat swelling that can be life-threatening.

Detection & Monitoring

The pine processionary moth is relatively easy to detect due to conspicuous nests and larval behavior. From late autumn through winter, look for white silken nests in pine canopies, typically on outer branches of the upper and middle canopy on the sunny side of trees. In late winter and early spring, watch for characteristic processions of caterpillars descending tree trunks or moving across the ground.

Pheromone traps can be deployed in summer to monitor adult populations and predict infestation severity. These traps are useful for area-wide monitoring and timing control interventions. Any evidence of nests warrants immediate attention, particularly in areas frequented by people or animals such as parks, school grounds, or residential properties. Even old nests pose hazards as urticating hairs remain potent for extended periods.

Management & Treatment

Managing pine processionary moth requires a multi-faceted approach prioritizing both pest control and public safety.

Mechanical control involves physically removing nests during winter before larvae descend. This requires extreme caution with full protective equipment including gloves, eye protection, and respiratory masks. Nests should be cut from trees using long poles, carefully bagged, and burned or disposed of safely. This method is practical for individual trees but impractical for large-scale forest infestations.

Chemical control is most effective when applied to young larvae before they develop irritating hairs and build substantial nests. Spraying in autumn targets this vulnerable stage. The biological insecticide Bacillus thuringiensis var. kurstaki (Btk) is widely used as it specifically targets caterpillars while being safe for other organisms. For urban trees, trunk injections of systemic insecticides provide alternatives to spraying.

Biological control includes Btk and various natural parasitoid wasps and flies that attack eggs and larvae, though they rarely provide complete control.

Physical barriers such as collar traps placed around tree trunks can intercept descending larvae in spring. These traps must be monitored regularly and captured larvae destroyed safely. They require a thorough maintenance to adapt the trap to tree’s growth and removing the captured caterpillars

Public awareness is crucial in affected areas. Warning signs should be posted, and the public educated about hazards, particularly to children and pets. Dog owners should be especially vigilant during procession periods and seek immediate veterinary attention if pets contact caterpillars.

For large-scale forest management, integrated approaches combining monitoring, targeted insecticide application to high-value or heavily infested areas, and promotion of forest diversity provide the most sustainable long-term strategy.