Following a request from the European Commission, the Panel on Plant Health was asked to deliver a scientific opinion on a pest risk analysis for Thaumetopoea processionea L. prepared by the UK. The Panel was also asked to consider in its opinion the plant health risk of T. processionea to the whole EU territory. The oak processionary moth, Thaumetopoea processionea, is established in Europe and feeds primarily on deciduous oak (Quercus) species. The insect has one generation per year and overwinters as eggs laid on branches of oak trees. After emergence, the larvae feed gregariously and from the 5th instar form a communal silken nest on the tree from which they typically migrate in procession to feed. Feeding may result in partial or complete tree defoliation. From the third instar, the larvae produce urticating hairs which may cause allergic reactions in humans and animals. The Panel examined in detail the UK document to determine whether the evidence presented in the document supports the conclusions reached for the assessment area of the UK, including Northern Ireland and the Channel Islands. To consider the risks posed to the whole area of the European Community, the Panel analysed additional information obtained from a review of the literature and from consultation with experts in the EU. The Panel conducted a preliminary climatic analysis to explore the establishment in the whole EU area, based on temperature accumulation. Based on the above, the Panel reaches the following conclusions: 1.With regard to the evaluation of the UK document: •the Panel agrees that the probability for entry on the plants for planting pathway is moderate to high. It further agrees that the probability of entry of T. processionea on the oak roundwood pathway is low; •the Panel agrees that the probability of establishment of T. processionea in the southern area of the UK is high given the presence of breeding populations in London and Jersey, the widespread distribution of oak trees and the favourable climatic conditions in the southern part of the assessment area; •in the UK document evidence is not presented to support the statement that the impact of the oak processionary moth on oak trees is major. Pest management measures applied in areas where the pest is established relate primarily to human health effects and thus are not considered to be a reliable indicator of the magnitude of plant health impact of T. processionea; •the Panel concludes from an evaluation of the risk assessment provided for the assessment area of the UK and review of additional information, that T. processionea may enter, establish and spread in the UK and has the potential to cause negative effects on the health of Quercus spp. although there is a high level of uncertainty relating to the magnitude of the effects on wood yield and quality which are directly attributed to T. processionea; •for the plants for planting pathway, visual inspection, pest surveillance and the establishment of pest free areas or places of production are proposed as risk management options in the UK document. The Panel agrees that uncertainties relating to adult dispersal and the absence of tested surveillance methods may influence the effectiveness and practical implementation of the management options proposed. Further analysis on natural dispersal by flight would assist in evaluation of entry pathways and risk management measures, including those taken in the area of the UK where the pest is present and under official control; •the Panel agrees that the management options proposed for the roundwood pathway can reduce the risk of introduction i.e. removal of bark, restriction of time of year of felling and export of roundwood. There are uncertainties regarding the effectiveness of visual inspection for the presence of nests and the practicality of bark removal for oaks. 2.With regard to the risk assessment conducted for the whole EU territory: •T. processionea occurs in many Member States of the EU territory, but the Panel found no reports to suggest that the pest is established in Denmark, Estonia, Finland, Ireland, Latvia, Lithuania, Malta or Sweden; •the presence of the pest in many areas of the EU territory provides opportunity for natural dispersal of T. processionea into adjacent areas where it is not currently established. Natural dispersal of adults may be restricted by geographic barriers (e.g. sea, mountains); •the results from the exploratory analysis conducted by the Panel indicate that spring and summer temperatures are suitable for larval and pupal development in parts of all EU member states where the pest is currently absent. Overwintering survival is not considered to be a key factor in defining the northern limits to the distribution. Limited availability of host plants (Quercus spp.) and low summer temperatures are likely to restrict the potential area of establishment to southern areas of the most northern EU member states; •reports of the plant health impact of T. processionea range from low to high. Due to the high level of variation in the level of plant health impact in the pest’s current area of distribution, more detailed analysis is required to assess the consequences of further spread in areas where it is not currently established in the EU; •the Panel considers that the degree of uncertainty is high. The main uncertainties relate to: ■differences in the magnitude of the pest effects reported from different areas of the EU where the pest is established, ■the plant health impact directly attributed to T. processionea alone and in combination with other stress factors contributing to tree mortality, ■factors affecting the health status and susceptibility of Quercus spp. to defoliation by T. processionea, ■the current distribution of T. processionea in the EU territory and lack of biological data needed to estimate the potential expansion of the range of T. processionea, ■natural dispersal capabilities of T. processionea females and effectiveness of surveillance methods. Phytosanitary measures are unlikely to prevent natural dispersal of the pest. Phytosanitary measures aimed at plants for planting could, however, reduce the probability of introduction of the pest into areas of the EU territory where the pest is currently absent, or present but under official control. Therefore, the Panel concludes that T. processionea may be considered as a harmful organism and hence is potentially eligible for addition to the list of harmful organisms in Council Directive 2000/29/EC