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and dry. This had effect on lower mortality and faster development of the insect and also on the increase of food quality as a consenquence of the water stress suffered by the trees, according to the soil and stand characteristics (Marchisio et al. 1994). In Europe, common outbreaks of the species Cephalcia abietis (Linnaeus, 1758) last several years (Martinek 1980); other species, including Cephalcia alpina (Klug, 1808), Cephalcia annulicornis (Hartig, 1837) and Cephalcia arvensis (Panzer, 1805), rarely occur in outbreaks (Battisti 1993; Martinek 1991, 1992; Battisti et al. 1998). The first record of the outbreak of C. arvensis was recorded on 400 ha on the late 19th century, between the years 1868 and 1874 (Escherich 1942). In the 20th century, outbreaks have been found in Sweden between 1916–1919 (Escherich 1942), in Denmark between 1927-1931 (Escherich 1942), in the Czech Republic between 1981–1986, 1982–1988 and 1996- 1999 (Křístek and Švestka 1986; Liška et al. 2001), in Italy between 1985–1993 (Battisti et al. 2000) and in Poland between 1982–1996 (Jachym 2003). In Slovenia, no records of growing populations of the species within the genus Cephalcia have been recorded until 2009. In 2009, damage appeared in a 106-ha, old stand of Picea abies (L.) Karst. located in the northern part of Slovenia (Jurc et al. 2010) caused by the species C. arvensis (Jurc 2012).
C. arvensis is considered particularly variable from a phenological and morphological point of view (Battisti 1993). The emergence period of this species takes place from the end of April until the end of August. Spring and summer forms can be identified based on the time of emergence. Both forms also have differences in morphological and behavioural features (Křístek and Švestka 1986; Battisti 1993). Females lay eggs on the one-year needles found in the upper and middle parts of the crown. Eggs are laid throughout the entire lifetime of the female, with a maximum number laid in the first week. The larvae hatch after 12-20 days and are isolated inside shelters, which are connected with a silk tube to the spruce needles by several silk threads. The falling period of the larvae takes place from July until the end of August. As the mature larvae reach the soil surface, they immediately begin to penetrate into the soil, where they build oval earth-walled chambers. Prepupae develop into pronymphs or enter into prolonged diapause as eonymphs. Only pronymphs develop into pupae and then into adults the following spring (Battisti 1993; Battisti et al. 2000).
The aim of this study was to identify the dominant species of web-spinning sawfly that entered an outbreak phase in 2009 at one location in the northern part of Slovenia in a Picea abies stand, Also, some bio-ecological characteristics like cardinal temperatures (soil and air temperature during the adult emergence period) were assessed along withthe defoliation intensity and its impact on the tree health status.
MATERIALS AND METHODS
Materijal i metode
Description of the research area – Područje istraživanja
The affected area was located on the slope of Riflov vrh (coordinates: x = 492607, y = 155471), which is located over Prevalje near the Austrian border.
The infested area comprised a 106 ha region of a stand of 60-90-year-old Picea abies that was located on a northern slope with a 20% slope incline, 600-800 m a.s.l. The growing stock varied between 321 m3/ha and 408 m3/ha, and the proportion of Norway spruce in the growing stock was over 70%, with individual admixtures of Fagus sylvatica, Acer pseudoplatanus, Fraxinus excelsior, Pinus sylvestris and Larix decidua. Naturally, these sites are part of the acid beech forest, and the forest community is Omphalodo – Fagetum (Forestry Service of Slovenia 2009).
Sampling of prepupae and adults – Uzorkovanje pretkukuljica i imaga
In 2009, soil samples (25 x 25 x 20 cm) were taken from the plots under the crowns, on six plots located along the slopes covering the entire area. We counted prepupae (eonymph/pronymph) of the different species of Cephalcia that were present and we photographed them, so that they could be identified (NIKON D 200, Tokyo, Japan, objective AF-S NIKKOR 105 mm). In total, 147 specimens were photographed and identified (Zanocco and Battisti 1995; Battisti and Sun 1996).
In 2010 we repeated this method and the number of prepupae in each sample were counted.
In spring 2012 we caught along the entire slope, with the entomological nets, 85 adults and identified them (Beneš 1976; Pschorn-Walcher 1982; Schwenke 1982; Achteberg and Aartsen 1986, Escherich 1942). The identities of the adults that were caught were confirmed at the University of Padova by Prof. Andrea Battisti, Ph.D.
Climatic factors – soil and air temperature – Klimatski čimbenici – temperature tla i zraka
Air and soil temperature data were obtained from the Slovenian Agency for the environment (ARSO) for the Šmartno pri Slovenj Gradcu meteorological station over the period between 2007 and 2010 (average max., min., and year temperature) for each year in this period. Soil temperature was measured at a depth of 20 cm, and air temperature at a height 2 meters above ground.
For each day in the year 2010 we also measured the temperature of the soil at a depth of 20 cm on location Prevalje. Soil temperature was measured with ‘T-button’ thermometers (T-buttons, Dallas Semioconductor, USA; accuracy 0.1°C, 0.1%), which record the soil temperature every 30 minutes.
Monitoring of the emergence of the web-spinning sawfly in year 2011 – Praćenje pojave imaga smrekine ose predivice u 2011. godini
After 10th of April 2011, we began with a detailed inspection of the stand, with the aim to identify the beginning of the emergence of web-spinning sawfly adults. By the scrutiny of the ground vegetation and trees, we determinate the start date of the emergence of Cephalcia. Monitoring was carried out till the last third of May, when Cephalcia adults were no longer observed. For each day in this period the average daily air temperature at a height of 2 meters above the ground, and the temperature of the soil at a depth of 20 cm was determinate. Soil temperature was measured with ‘T-button’