Dynamics of populations of pine processionary moth (Thaumetopoea pityocampa) in the Republic of North Macedonia during the period 2007-2017

This study presents results for the dynamic of the populations of the pine processionary moth (Thaumetopoea pityocampa) during the period 2007-2017 and the impact of reducing factors in the Republic of North Macedonia. The population density was determined by the number of larval nests both per tree and per hectare that varied during the research period. The number of individuals went up between 2007 and 2010, while during the following years it rapidly decreased. Since 2011, an overall prograding trend has been recorded in the several subsequent years. This could be explained by the huge number of individuals from the previous years that were in a diapause. The abundance of pest populations continued to grow in 2016 as no measures were taken. During the winter of 2016/2017, the impact of the extremely low temperatures on pest mortality was monitored in 2016-2017 generation. The density of the populations came to latency due to the very large number of dead larvae of second and third larval stages. We recorded a 100% mortality of larvae in plantations of Pinus nigra in the regions near Prilep, Sveti Nikole, Shtip, Kochani and Negotino Villages.


Introduction
The pine processionary moth, Thaumetopoea pityocampa (Denis, Schiffermüller, 1775) (Lepidoptera: Notodontidae) has been recorded for first time at the end of 1950s in nearly all natural stands of Pinus nigra Arn. in the southern part of the Republic of North Macedonia (Serafimovski, 1959;Kusevska et al., 1978). The pest has been also observed from Gevgelija (535 m a.s.l.) to Visoka Cuka (1200 m a.s.l.) and from the Mariovski Basin, where the main area of pest infestation is, to Ligurasa -Tribot -Karavastina at 1200 m a.s.l. In the central part of the country, T. pityocampa has reached Skopje. It is occasionally found near Krusevo (1300 m a.s.l.) and to the east -along the border with Bulgaria (Maleshevo Mountain, Golak, Plackovica, Ograzden), and in the west direction -in the region of Bitola, Resen and Ohrid.
In the natural stands, the pine processionary moth is not subjected to a particular threat because of the natural regulation of the populations. As a result, damages caused by the pest have greatly decreased. However, since 1970s, the area of pine plantations attacked by T. pityocampa has extended to 160 000 ha and among them 95% of the plantations were P. nigra (Ivanov, Nacheski, 2001, 2005Nacheski, Papazova-Anakieva, 2013). The pest has been registered occasionally also on P. sylvestris L., P. peruke Gris., P. brutia Ten., P. pinaster Ait. and Pseuodotsuga menziesii (Mirb.) Franco (Nacheski, Papazova-Anakieva, 2015).
During the last decade, as a result of global warming, T. pityocampa has enlarged its range and has been registered for the first time in the locality Zmiski Rid near Kumanovo (2009) as the most northern location, with high abundance on the area of 500 ha in 2010 (Nacheski, Papazova-Anakieva, 2015).
The aim of this study was to explore the population dynamics of T. pityocampa and the impact of limiting factors on its survival in the Republic of North Macedonia.

Materials and Methods
The studies were carried out during the period 2007-2017 at 11 localities where T. pityocampa is found. The study area encompassed seven regions (Vinica, Kochani, Kavadarci, Sveti Nikole, Prilep, Bitola and Negotino) of North Macedonia (Table 1).
Standard methods for evaluating the abundance of T. pityocampa, assessment of attack intensity and for degree of damage to trees, were used in selected stationary experimental plots (25 x 25 m) in plantations of P. nigra. The abundance of the pest was determined according to the number of larval nests on trees in the experimental plots and calculated per hectare. The mortality of larvae owing to the extremely low temperatures in the winter of 2017 was established by dissection of 10-15 larval nests from each locality.
The laboratory studies were carried out in the entomological laboratory of the Faculty of Forestry in Skopje.

Results and Discussion
During the period 2007-2017, the pine processionary moth attacked yearly between 3500 and 17280 ha of pine forests in the Republic of North Macedonia (Fig. 1). The population density of T. pityocampa varied during the research period. The number of individuals in different populations from the generations 2007-2010 was growing, and then rapidly decreased. This was a result of the control measures taken in 2010, when air treatment with Rimmon E-10 was carried out.
In 2009, there was another outbreak and in May 2010, significant damages were reported. We have monitored the current situation by estimating numbers of nests and the degree of infestation was critical in newly planted pine forests with some regional fluctuations. In the region of Bitola, the numbers ranged from 400 to 20 400 larval nests per ha. In the region of Kochani, the range was from 460 to 19 000 larval nests per ha. We have permanently followed the intensity of attack and population density of the pine processionary moth in all regions of the Republic of North Macedonia, where its presence has been determined.
After 2010, the abundance of T. pytiocampa was brought to normal limits in all regions. From 2011, a progradation trend has been recorded as a result of the huge number of individuals which were in the pupae stage during the winter diapause during the previous years. The population density was increasing in the several subsequent years. In conjunction with this, the percentage of defoliation increased almost at all sites in North Macedonia from 2011-2015. Based on the results obtained in this period and predominantly due to the very high population density, recommendations were given with measures for gradual regulation of the populations of the pine processionary moth in plantations of P. nigra in 2015 and 2016.  The population density of T. pityocampa came to latency due to the very large number of dead larvae. That was owing to the extremely low temperatures that were measured for a long period in January 2017.
During this research, we identified 100% mortality of larvae of pine processionary moth in the black pine plantations in the studied regions (Fig. 3).
Notable mortality of larvae of T. pityocampa as a result of low winter temperatures was also registered in the winter of 2005-2006 (unpublished). The mortality of larvae of pine processionary moth as a result of the low temperature in the winter of 2005-2006 were: from 17.8% in the location Goceva Gora -Negotino, 42.8% in Belski Rid -Kochani up to 99.9% in the locality Alinci -Prilep.
In conclusion, it should be noted that the low winter temperatures in some years are the main limiting factor of pine processionary moth in the Republic of North Macedonia.