Taxonomic information	Current valid scientific name: Thaumetopoea processionea Synonyms: Cnethocampa processionea Name used in the EU legislation: Thaumetopoea processionea Order: Lepidoptera Family: Notodontidae Common name: Oak processionary moth (OPM), oak processionary caterpillar Name used in the Dossier: Thaumetopoea processionea
Group	Insects
EPPO code	THAUPR
Regulated status	Thaumetopoea processionea is listed in the Annex III of Regulation (EU) 2019/2072 as protected zone quarantine pest for Ireland. It is protected zone quarantine pest in the UK and included in A1 lists for Argentina and Türkiye (EPPO, 2024a). The Panel noted that the species is present in Türkiye (Groenen & Meurisse, 2012).
Pest status in the UK	T. processionea is present in the UK with restricted distribution. It is a species under official control, currently found in the London area and in the Southeast of England (EPPO, 2024b; Forestry Commission, 2024a). According to Suprunenko et al. (2022) the eradication of T. processionea from the UK territory is ‘no longer considered a feasible option’. In 2006 it was found breeding at three separate sites in southwest London (Townsend, 2006). There were other previous records of the moth in the UK (south coast from Cornwall to Essex, islands Jersey and Guernsey), however, these records refer to immigrant moths caught in traps (Foster, 1983; Riley, 1985, 1987; Townsend, 2006).
Pest status in the EU	T. processionea is a native European species reported to be present in 22 EU member states; it is absent from Estonia, Finland, Latvia, Lithuania and Malta (EPPO, 2024c; GBIF, 2024). In Ireland it was introduced in 2020 and eradicated in 2021. In June 2023 the NPPO of Ireland has newly detected the pest in the municipality of Castleknock and eradication measures have been immediately applied. The current pest status for Ireland declared by NPPO is ‘under determination’ whereas the current pest situation evaluated by EPPO is transient (EPPO, 2024d). According to Groenen and Meurisse (2012) the discontinuous occurrence of T. processionea in central‐northern Europe in the last two centuries, and its recent massive reappearance in north‐western Europe, are due to long‐term population fluctuations rather than range expansion.
Host status on Betula pendula and B. pubescens	No information was found on whether B. pendula and B. pubescens are hosts for T. processionea. Stigter et al. (1997) reports Betula as an occasionalhost of T. processionea in the Netherlands. Moreover, according to Evans (2008) and Baker (2009) Betula is a host or occasional host to T. processionea.
PRA information	Available Pest Risk Assessment: –Oak processionary moth Pest Risk Analysis (Evans, 2008); –Evaluation of a pest risk analysis on Thaumetopoea processionea L., the oak processionary moth, prepared by the UK and extension of its scope to the EU territory (Baker et al., 2009); –UK Risk Register Details for Thaumetopoea processionea (DEFRA, 2024).
Other relevant information for the assessment
Biology	T. processionea is native to southern and central Europe, where it is more abundant and widespread in warm and sunny sites; in central and western Europe its presence is mainly dependent on population fluctuations which can be determined by aridity and climate change (Csóka et al., 2018; Groenen & Meurisse, 2012). The moth is also present in Türkiye and in the Middle East (Syria, Lebanon, Jordan, Israel) (Battisti et al., 2015; Basso et al., 2017; CABI, 2024; Groenen & Meurisse, 2012). T. processionea has four life stages: egg, larva (six instars), pupa and adult; it is a univoltine species, overwintering as 1st instar larva, but at egg stage too (CABI, 2024; Forestry Commission, 2024b; Zielonka, 2020). Adults, 25–35 mm wingspan, fly from July to September and can survive 4–10 days. Females lay 30–200 eggs, occasionally up to 300 (CABI, 2024), which are 2 mm long. The eggs are laid in batches on small branches of oaks (3.5–10 mm diameter), more rarely on other hosts (Battisti et al., 2015). In autumn 1st instar larvae are found within the eggs; eggs and larvae are known to withstand up to −30°C, and a 90% rate of survival of overwintering eggs is observed after severe winters (Baker et al., 2009; Battisti et al., 2015). Egg hatching in April–May is usually well synchronised with oak bud flushing. The larval stage can last 60–70 days. Larvae feed on foliage gregariously from April to July and build a silky nest for each of the instars (CABI, 2024); however, a large bag‐shaped nest incorporating hairs, frass and silk, is built only at 5th–6th larval stage in the medium‐lower part of the trunk. The 35–40 mm mature caterpillars rest in the nest during the day and move in nose‐to‐tail processions during the night in search of food. Larvae from 3rd instar onwards develop urticating hairs on the dorsal part of abdomen (CABI, 2024; EPPO, 2024e; Zielonka, 2020). In the UK, mature larvae pupate inside the nests from June to early September and adult flight can be normally observed from end of July to late September (Forestry Commission, 2024b). Natural dispersal of T. processionea is through larval processions and adult flight. Larvae can move in processions only to short distances, but adults are good flyers (50–100 km/year for males and 5–20 km/year for females); windborne spread of adults is also likely (Baker et al., 2009; EPPO, 2024e). Males are known to be able to fly over the Channel from France to southern England; this is considered unlikely for females, which are heavier (Battisti et al., 2015; EPPO, 2024e; Evans, 2008). In the UK, T. processionea has recently increased its expansion rate, passing from 1.66 km/year in 2006–2014 to 6.17 km/year in 2015–2019 (Suprunenko et al., 2022). The spread of T. processionea can also be human supported, mostly via trading of plants for planting carrying eggs, larvae and pupae. Cut branches and round wood with bark are considered pathways of lesser importance (Baker et al., 2009; EPPO, 2024e; Evans, 2008). T. processionea is both an important defoliating insect for oak species and a threat to human and domestic animal health. Marzano et al. (2020) provide a useful summary of how the multi‐face OPM problem is currently felt by people and managers in the UK. The impact of T. processionea on forest health is variable: it is considered a minor pest for oak forests in Ukraine, Romania, Hungary, Slovenia; severe damage was instead reported from Germany, Italy, France, Belgium and Spain (Baker et al., 2009). In western Europe (Belgium, the Netherlands) and in the UK, the pest is mainly harmful to urban and road trees, as well as to amenity oak trees in parks, forest edges and countryside hedgerows (Battisti et al., 2015). Both in canopied stands and open forests, oaks weakened after severe defoliation by the T. processionea become more susceptible to secondary pests as buprestid beetles, bark and ambrosia beetles or root rot fungi. T. processionea may be hence considered a contributing factor in the oak decline, also resulting in loss of biodiversity (Baker et al., 2009; CABI, 2024). No information was found about the impact of T. processionea on Betula. Impact on human health may be relevant mostly in urban areas, due to the severe pseudo‐allergenic reactions caused by the contact of urticating hairs released by the larvae with skin, eyes and respiratory system. A good synthesis on health effects of T. processionea is provided by Rahlenbeck and Utikal (2015). Urticating hairs released by larvae spread by air currents also from nests, exuviae, pupal cases and may remain active in the soil or in the litter for several years lengthening the social impact of the species (Baker et al., 2009).
Symptoms	Main type of symptoms	Main symptoms caused by larvae of T. processionea on oaks are skeletonisation of leaves and defoliation; presence of silken nests mainly on the lower branches and the lower part of the trunk; processions of caterpillars on the branches and trunks; egg batches in rows covered by scales, mostly on 1–2 years‐old twigs. No specific symptoms on Betula are known. Symptoms on humans and animals due to urticating hairs are skin rash, eye irritation, sore throat and breathing difficulty.
	Presence of asymptomatic plants	No information on the presence of asymptomatic plants was found.
	Confusion with other pests	T. processionea is one of 15 species belonging to the genus Thaumetopoea worldwide, recently revised by Basso et al. (2017). The species is easily identified by both morphological features of adults, and features and host plants of larvae (it is the sole Thaumetopoea feeding on Quercus sp.) so that no confusion with other similar species is possible.
Host plant range	T. processionea is a specialist herbivore feeding on oaks in Europe (Damestoy, 2019). Quercus species known to be hosts of T. processionea are Quercus boissieri, Q. calliprinos, Q. cerris, Q. frainetto, Q. infectoria, Q. ilex, Q. palustris, Q. petraea, Q. pubescens, Q. pyrenaica, Q. robur, Q. × turneri (Baker et al., 2009; DEFRA, 2024; EPPO, 2024f; EUROPHYT, 2024). Occasional hosts during outbreaks on which are Acacia, Betula, Carpinus, Castanea, Corylus, Crataegus, Juglans, Fagus, Pistacia, Robinia and Sorbus (Baker et al., 2009; CABI, 2024; EPPO, 2024f; Evans, 2008; Stigter et al., 1997). On these trees larvae were found to feed but without complete development of the life cycle. Only on Fagus they can reach the pupal stage (EPPO, 2024e, 2024f; Stigter et al., 1997).
Reported evidence of impact	T. processionea is an EU protected zone quarantine pest.
Evidence that the commodity is a pathway	Although there are no reports of Betula pendula or B. pubescens infested by T. processionea, Betula is reported bearing dispersed feeding larvae during outbreaks on major hosts. Major hosts of T. processionea (Quercus spp.) are present both in the nurseries and in the surroundings of the nurseries. Therefore, a spillover of larvae may occur making the association with the commodity possible particularly if plants are exported with leaves.
Surveillance information	T. processionea is quarantine pest for which Great Britain is a pest‐free area (excluding the local authority areas in infested zone) (Dossier Section 5.2).