Historical distribution of the oak processionary moth Thaumetopoea processionea in Europe suggests recolonization instead of expansion

1 The oak processionary moth Thaumetopoea processionea (Notodontidae) is presently distributed in almost all European countries and in part of the Middle East. In the North, its range limit passes through the Netherlands and Germany, and the southern part of Poland and Ukraine. In the South, the species is present in all the countries located on the northern shore of the Mediterranean Sea, in Anatolia, and in the mountains surrounding the Dead Sea Transform. 2 Using information from museum and personal collections, the available literature and other relevant datasets, we show that the species was already largely distributed throughout Europe before 1920. The available data do not provide any evidence of any long-term latitudinal shift of the species betwen 1750 and 2010. 3 In the northernmost part of its range, the population dynamics of the species is characterized by important fluctuations. We studied their pattern in Belgium, the Netherlands and part of Germany, after the apparent regional disappearance of the species during the first half of the 20th Century. The data suggest a continuous extent of the apparent distribution of the insect between 1970 and 2009, at a rate of approximately 7.5 km per year. 4 To explain the present distribution of the species, we discuss possible improvements of environmental conditions that could have triggered local population increases and favoured dispersal to adjacent areas. In addition, human activity, including the commercial movements of infested nursery trees, was recently suspected to be another source of spread over geographical barriers.