51 Silva Balcanica, 17(1)/2016 IMPACT OF LOW TEMPERATURES ON PINE PROCESSIONARY MOTH (THAUMETOPOEA PITYOCAMPA) LARVAL SURVIVAL IN BULGARIA Plamen Mirchev 1 , Georgy Georgiev 1 , Margarita Georgieva 1 , Lilia Bocheva 2 1 Forest Research Institute – Sofa, Bulgarian Academy of Sciences 2 National Institute of Meteorology and Hydrology – Sofa, Bulgarian Academy of Sciences Abstract Taumetopoea pityocampa winter nests were collected in January 2011 and 2015 from six localities situated in diferent regions over pest’s range in Bulgaria. In each site, ten winter nests were randomly selected and the larval instar structure and their survival were determined. Te ‘silk index’ of winter nests and the level of their building were assessed. Using data about average daily and minimal monthly temperatures from November (2010, 2014) to February (2011, 2015), the number of favourable days for T. pityocampa larval development was evaluated. A relationship between the climatic characteristics over the cold period of year and the larval instar structure, survival, adaptability expressed by the level of building winter nests and the time of nest leaving and moving of the larvae into the soil was found out. Key words: Taumetopoea pityocampa, instar structure, larval survival, distribution, Bulgaria INTRODUCTION Pine processionary moth, Taumetopoea pityocampa (Denis & Schifermüller, 1775) (Lepidoptera: Notodontidae, Taumetopoeinae) is considered to be one of the most frequent and dangerous insect pests in the Mediterranean region. A part of the northern border of pest distribution passes through Central Bulgaria. In this area, the specifc ecological form T. pityocampa bulgarica occurs as more adaptive to extreme climatic conditions than the typical Mediterranean form (Tsankov et al., 1996). Te geographic range of T. pityocampa in Bulgaria covers pine forests in the Rhodopes Mt. and the mountainous areas of South Bulgaria, predominantly along Struma and Mesta river valleys. Te border of its northern distribution reaches to Golo Bardo Mt., Sredna gora Mt. and to the south slopes of Central Balkan Range (Mirchev et al., 2011b). During the cold period, the survival of pine processionary moth caterpillars depends on the value of air temperature, which should not fall below -16ºС (Démolin, 1969b). In winter period, the most suitable for larval development are daily temperatures of over 9ºC followed by night temperatures above 0ºC allowing caterpillars to leave their nests and feed on the needles of the pine trees (Battisti et al., 2005). Milan (1990) reported that well-constructed nests protect the caterpillars against unfavourable climatic conditions.