Taxonomic information	Current valid scientific name: Thaumetopoea processionea Synonyms: Cnethocampa processionea, Traumatocampa 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, online_a). The Panel noted that the species is native to Türkiye (Groenen and Meurisse, 2012).
Pest status in the UK	Thaumetopoea processionea is established in the UK since 2006. It is a species under official control, currently found in the London area and in the Southeast of England (EPPO, online_b). According to the Dossier Section 5.2 T. processionea is present in Great Britain, except in specified pest‐free areas. In North Ireland the pest is under eradication after a recent outbreak in 2023. In 2022, the T. processionea was found in Jersey (Channel Islands) where it is currently under eradication (EPPO, online_c). According to Suprunenko et al. (2022) the eradication of T. processionea from the UK territory is ‘no longer considered a feasible option’.
Pest status in the EU	Thaumetopoea 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, online_b; GBIF, online; de Jong et al., online). It was introduced in Ireland 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, online_b). 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 Corylus avellana	No information was found on whether Corylus avellana is a host for T. processionea. Only ‘Corylus’ is reported as host by Evans (2008) and CABI (online); however, no references are given to support this information (Baker, 2009; Mirchev et al., 2011). Stigter et al. (1997) do not mention Corylus in the list of secondary hosts (Acacia, Betula, Crataegus, Fagus and Sorbus) of T. processionea in the Netherlands.
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); –Scientific opinion on commodity risk assessment of Fagus sylvatica plants from the UK (EFSA PLH Panel, 2023a); –Scientific Opinion on the commodity risk assessment of Quercus petraea plants from the UK (EFSA PLH Panel, 2023b); –Scientific Opinion on the commodity risk assessment of Quercus robur plants from the UK (EFSA PLH Panel, 2023c); –UK Risk Register Details for Thaumetopoea processionea (DEFRA, online).
Other relevant information for the assessment
Biology	Thaumetopoea 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 (Csoka et al., 2018; Groenen and 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, online; Groenen and Meurisse, 2012). Thaumetopoea 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, online; Forestry Commission, online; 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 Compendium, online), 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., 2014). 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., 2014). 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, online); however, a large bag‐shaped nest weaved with 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, online; EPPO, online_e; Zielonka, 2020). In the UK, the mature larvae pupate inside the nests from June to early September and adult flight can be normally observed from end July to late September (Forestry Commission, online).
	Natural dispersal of T. processionea is through larval processions and adult flight. Larvae can move in processions only to very 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, online_d). Males are known to be able to fly over the Channel from France to southern England; this is considered unlikely for females, which are too heavy (Battisti et al., 2014; Evans, 2008; EPPO, online_e). 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, online_e; Evans, 2008).
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 Corylus avellana 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	Thaumetopoea 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	Thaumetopoea 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, online; EPPO, online_f; EUROPHYT, online). Secondary, occasional hosts, only attacked during outbreaks are Acacia, Betula, Carpinus, Castanea, Corylus, Crataegus, Juglans, Fagus, Pistacia, Pinus, Robinia and Sorbus (Stigter et al., 1997; Evans, 2008; Baker et al., 2009; CABI, online; EPPO, online_f). However, beside Quercus, the development of larvae to adults is known only for Fagus (Stigter et al., 1997; EPPO online_e, f).
Reported evidence of impact	Thaumetopoea 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., 2014). 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, online). No information was found about the impact of T. processionea on C. avellana. 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 (lepidopterism). A good synthesis on health effects of T. processionea is provided by Rhalenbeck 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).
Evidence that the commodity is a pathway	Although there are no reports of C. avellana infested by T. processionea, Corylus is reported as a secondary, occasional host, attacked 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	Thaumetopoea processionea is a quarantine pest under official control in the UK. As part of an annual survey at ornamental retail and production sites (frequency of visits determined by a decision matrix), T. processionea is inspected for on Quercus. No specific surveillance of T. processionea is implemented in the UK for Corylus avellana. An additional inspection, during the growing period, is carried out at plant passport production sites. Nursery staff is aware of T. processionea and check all Quercus products for signs, even where the pest is not present in the area. Movement restrictions for growing sites are enforced in the infested area and buffer zone. There is an eradication policy for the buffer zone and pest‐free area (Dossier Section 5.1).