Thaumetopoea Pityocampa) Larval Survival in Bulgaria
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Silva Balcanica, 17(1)/2016 Impact OF low temperatures ON pine processionary moth (THAUMETOPOEA PITYOCAMPA) larval survival IN Bulgaria Plamen Mirchev1, Georgy Georgiev1, Margarita Georgieva1, Lilia Bocheva2 1 Forest Research Institute – Sofia, Bulgarian Academy of Sciences 2 National Institute of Meteorology and Hydrology – Sofia, Bulgarian Academy of Sciences Abstract Thaumetopoea pityocampa winter nests were collected in January 2011 and 2015 from six localities situated in different 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. The ‘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: Thaumetopoea pityocampa, instar structure, larval survival, distribution, Bulgaria INTRODUCTION Pine processionary moth, Thaumetopoea pityocampa (Denis & Schiffermüller, 1775) (Lepidoptera: Notodontidae, Thaumetopoeinae) 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 specific ecological form T. pityocampa bulgarica occurs as more adaptive to extreme climatic conditions than the typical Mediterranean form (Tsankov et al., 1996). The 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. The 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. 51 The aim of this work was to investigate the main biological characteristics of T. pityocampa (larval instar structure, survival, adaptability expressed by the level of building winter nests, the time of nest leaving and moving of the larvae into the soil) in different regions of its range during the cold period of year (November-February) and relationship between these characteristics and temperature conditions. MATERIALS AND METHODS The biological material was collected from six localities over the pine processionary moth range in Bulgaria (Table 1). In one of them, Pesnopoy (Plovdiv district), T. pityocampa bulgarica occurs and in the rest ones, the typical form is distributed. The locations Marikostinovo and Ivaylovgrad are situated in regions with Mediterranean climate influence where T. pityocampa occurs permanently. The climate of Pchelarovo is less influenced by Mediterranean climate than Ivaylovgrad in the Eastern Rhodopes. Staro selo is situated in a region with continental climate where T. pityocampa appears in some years. Muhovo occupies an intermediate position between Sofia and Plovdiv, with mild and warm microclimate determined by the influence of close Topolnitsa dam and south mountain slopes of Sredna gora. Climatic data (average daily and minimal monthly temperatures) for November and December (2010 and 2014), and January and February (2011 and 2015) for four of the investigated locations were obtained by the meteorological stations in Sandanski, Kardzhali, Plovdiv and Sofia (Table 1). In 2011, T. pityocampa nests were collected from Austrian black pine (Pinus nigra Arn.) plantations in six sites to study the larval instar structure during the cold period (Mirchev et al., 2011a). In January 2015, the same places were visited again in order to collect biological material for new analyses (Table 1). In each site, ten winter nests were randomly selected and all available caterpillars in them were taken to determine the larval instar structure and their survival. The dead caterpillars were separated for a further microbiological analysis, and the living ones were placed in 75% ethanol. In the entomological laboratory of Forest Research Institute – Sofia, the larval developmental stage was determined by the size of head capsule (Zankov, 1960). The nests were assigned by nest silk index according to a three-stage scale (Battisti et al., 2005): 1 – loose nest wall, larvae visible through the wall; 2 – thick nest, larvae not visible, external silk layer sparse; 3 – very thick nest, larvae not visible, external silk layer dense. At recent study, the intermediate degrees of 1.5 and 2.5 were used. The data were analysed using descriptive statistic of MS Excel 2013. The relationship between the various variables was determined by the Pearson correlation coefficient (r). RESULTS T. pityocampa winter nests characteristics In 2015, with 60 analyzed nests, eight of them were empty because the caterpillars had already moved into the soil. Out of the rest 52 nests with caterpillars present, the 52 Table 1. Main characteristics of the studied areas and meteorological stations Meteorological Date of collection station Geographical Altitude, Altitude, region Site Coordinates 2011 2015 Name m m Petrich- Marikosti- 41º26’24.5”N, 130 January 17 January 30 Sandanski 270 Sandanski novo 23º10’39.3”E kettle Arda river Ivaylov- 41º31’40.0”N, valley, the 250 January 20 January 20 Kardzhali 290 grad 26º07’07.3”E Eastern Rhodopes Central part 42º20’37.7”N, Pesnopoy 300 January 18 January 14 Plovdiv 130 of Thracian 24º40’15.1”E plain 42º28’49.4”N, Golo bardo Staro selo 830 January 21 January 30 Sofia 600 23º06’53.3”E Mt. Topolnitsa 42º25’10.0”N, river valley, Muhovo 400 January 18 January 14 - - 23º59’57.1”E Sredna gora Mt. Uplands of 41º46’47.9”N, Pchelarovo 480 January 20 January 20 - - the Eastern 25º21’27.1”E Rhodopes nest silk index was assessed at 2.0 or 3.0 degree in predominate part of them (28.9% and 26.9%, respectively), followed by the nest silk index degree of 2.5 (19.2%), 1.5 (15.4%) and 1.0 (9.6%). The number of caterpillars in different nests varied over a rather wide margin: from 8 in Ivaylovgrad to 568 in Pchelarovo. During both periods of study, the lowest average number of caterpillars in one nest was registered in the region of Pesnopoy where T. p. bulgarica occurs. The highest number was assessed in Pchelarovo and Staro selo (Table 2). No correlation (r = -0.05) was detected between the number of T. pityocampa caterpillars and the silk thickness of winter nests. Although in Ivaylovgrad and Staro selo it occupies habitats in two different climatic zones (Table 2), the nest silk index was similar in both studied periods. In contrast to them, the nest silk index was different in Ivaylovgrad (2.36-2.60) and Marikostinovo (1.85-2.15), situated in regions with expressed influence of Mediterranean climate and permanent presence of pine processionary moth in them. The data of study showed no correlation between the thickness of nests and caterpillars mortality (r = -0.12). Relationship between T. pityocampa caterpillars survival and climatic parameters over the winter period Feeding of T. pityocampa caterpillars mostly occurs at night. As was mentioned above, wintertime feeding is possible when night temperature is above 0ºC (potential 53 Table 2. Main characteristics of T. pityocampa studied nests Insect Nest silk index Total number No. larvae/colony ± SD Locality generation ± SD of caterpillars (Range) 2010-2011 2.36±0.50 1115 111.50±54.91 (42-215) Marikostinovo 2014-2015 2.60±0.66 952 105.78±27.5 (17-450) 2010-2011 1.85±0.47 879 87.90±64.75 (8-229) Ivaylovgrad 2014-2015 2.15±0.53 1109 110.90±61.74 (19-238) 2010-2011 2.40±0.61 313 62.60±55.43 (10-135) Pesnopoy 2014-2015 2.0±0.58 355 71.00±34.50 (26-111) 2010-2011 1.68±0.26 927 115.87±46.87 (37-204) Staro selo 2014-2015 2.20±0.48 1288 128.80±57.28 (26-228) 2010-2011 2.40±0.57 908 90.80±76.27 (23-295) Muhovo 2014-2015 2.30±0.79 898 99.78±36.34 (50-147) 2010-2011 2.05±0.50 760 108.57±60.67 (20-188) Pchelarovo 2014-2015 2.00±0.85 1334 148.22±161.85 (29-568) feeding temperature) provided that the colony temperature during the day before has reached 9ºC (activation temperature). In respect to the number of favourable days for pine processionary moth caterpillars’ development, it was established that the most favourable conditions occurred during 2010-2011 in the region of all four meteorological stations ranging between 24 for Staro selo and 46 for Marikostinovo (Table 3). In 2014-2015, the number of favourable days for all stations was lower. In Sofia region it was less with 33.3% as compared to 2010-2011 and in for the rest stations the results were similar, respectively: Plovdiv – 61.1%, Kardzhali – 63.4% and Sandanski – 78.3%. As concerning to the average daily temperatures sum, these differences during the two studied periods were not noticed for all stations (Table 3). Critical for the caterpillars’ survival temperature values (below -16ºC) were not registered. Beside availability of pest host plant, the number of the favourable days in cold period is also a necessary condition one biotope to be appropriate for the pine processionary moth caterpillars survival and development.