Oak processionary moth

Oak processionary moth (Thaumetopoea processionea) is a serious regulated pest of oak trees. Gregarious caterpillars feed on oak foliage, form nose-to-tail processions and build silken nests on trunks and branches. Heavy infestations can defoliate trees, while mature larvae and old nests contain irritating hairs that may cause skin, eye, throat and respiratory reactions in people and animals.

The damaging stage is the caterpillar, not the inconspicuous adult moth. Larvae commonly occur in groups, feed mainly at night and rest during the day in silk-lined shelters. Their characteristic movement is a nose-to-tail procession along oak branches, trunks or the ground. Feeding may range from localized window-paning and skeletonization of leaves to extensive canopy defoliation. Repeated or severe defoliation can reduce growth and increase tree vulnerability to drought, secondary pests and diseases.

Identification should be confirmed by an appropriately trained diagnostician because several hairy moth caterpillars can occur on oak. Do not approach, brush against or collect caterpillars or nests for casual examination.

Typical tree symptoms and signs include:

• Irregular chewing, window-paning or skeletonization of oak leaves.
• Progressive thinning of the crown or patches of severe defoliation.
• Groups of gray-bodied, black-headed caterpillars moving in conspicuous processions, especially from late spring into early summer.
• Silken, bag-shaped nests on trunks, at the bases of branches or in lower portions of the crown; nests may contain frass, shed skins and many irritating hairs.
• Rows or plaques of eggs covered with protective scales on small oak twigs.

Human or animal exposure can cause itchy rash, eye irritation, sore throat and breathing difficulty; severe allergic reactions are uncommon but possible. These health effects are not plant symptoms, but they are important for safe diagnosis and management.

The pest spreads locally when adult moths fly between nearby oaks, usually remaining relatively close to the source tree. Caterpillars may also move collectively across the ground when foliage becomes scarce. Longer-distance spread is strongly associated with movement of live oak planting stock carrying egg plaques, larvae or nests, and potentially with contaminated equipment or materials.

In the United States, do not move suspected infested oak trees, branches, nests, caterpillars, soil or equipment. Photograph the find from a safe distance, record the location and contact the state department of agriculture or USDA APHIS for reporting and identification guidance.

This is generally a univoltine moth with one generation per year. Adults typically fly from mid- or late summer into early fall and lay batches of eggs in rows on one- to two-year-old oak twigs. First-instar larvae develop within the eggs and overwinter there. Hatching commonly occurs in April or May, often close to oak bud break.

Larvae pass through six instars and feed communally from approximately April through July, although exact timing varies with climate and latitude. Urticating hairs begin developing from about the third instar onward and become more abundant and hazardous in later instars. From the fifth or sixth instar, larvae construct larger silk nests on trunks or near branch bases. Pupation occurs in the nest, followed by adult emergence later in summer.

First isolate the area and keep people and animals away. Do not touch caterpillars, nests, fallen branches or apparently empty nests, because old silk and shed skins may retain irritating hairs.

For a suspected occurrence in the United States, photograph the pest without disturbing it and report it to the state plant regulatory agency or USDA APHIS. Confirmation and any regulatory response should precede movement or disposal of the material.

On confirmed infestations, a qualified arborist or licensed applicator may combine targeted treatment of young larvae, physical removal of nests and caterpillars, monitoring and restrictions on tree movement. Timing is critical: control is generally safer and more effective when larvae are small and before large nests and mature urticating hairs accumulate. Heavy defoliation does not automatically justify removal of a healthy oak; assess tree condition, structural risk and the official management plan after the infestation is addressed.

Organic and low-toxicity management should be based on exclusion, early detection and professional removal rather than home handling:

• Keep people, pets, horses and livestock away from suspected caterpillars, nests and the ground beneath infested trees.
• Arrange professional removal or vacuum extraction of nests and caterpillar groups by trained personnel using appropriate protective equipment and regulated disposal procedures.
• Where an approved biological product is legally labeled for the crop, site and target pest, an applicator may consider a treatment directed at young larvae, before large nests and abundant urticating hairs develop. Do not assume that a product labeled for another processionary moth or another country is lawful or effective in the United States.
• Support general oak-tree vigor through appropriate watering during drought, mulch kept away from the trunk and avoidance of unnecessary root or soil disturbance. These measures do not eradicate the pest but may reduce stress after defoliation.
• Do not release predators, parasitoids or pathogens into the environment. Conservation of naturally occurring birds, bats and arthropod predators is preferable, but biological control is not a substitute for official response to a suspected regulated detection.

Prevention is primarily a plant-health and biosecurity task:

• Purchase oak trees from reputable, inspected nurseries and retain plant movement records.
• Inspect twigs, branch unions, trunks and lower crowns for egg plaques, silk, frass, caterpillar groups and old nests before planting or moving oaks.
• Do not transport suspected infested oak material. Follow instructions from state and federal plant-health officials for confirmation, quarantine or disposal.
• Survey valuable oaks during spring and early summer, when larvae and processions are most visible, while maintaining a safe distance.
• Use trained arborists for work in or beneath suspect trees; avoid mowing, pruning or other activity that could disturb nests or spread hairs.
• After confirmed exposure, follow official decontamination guidance for clothing, tools, vehicles and equipment.

• The common name refers to the caterpillars’ distinctive nose-to-tail processions.
• The long white hairs visible to the eye are not the main source of irritation; much smaller barbed urticating hairs contain the allergenic protein thaumetopoein.
• Old nests and shed skins can remain hazardous after the caterpillars and adult moths have gone.
• Quercus is the principal host genus. Other broad-leaved trees, including beech, birch, hornbeam, hazel and sweet chestnut, may be occupied or fed upon when oak foliage is scarce, but successful completion of the life cycle has not been reliably demonstrated on those non-oak hosts.
• Severe outbreaks are particularly concerning in urban trees, parks, forest edges and other places where people, pets and livestock may contact nests or fallen hairs.