DIGITALNA ARHIVA ŠUMARSKOG LISTA
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|ŠUMARSKI LIST 5-6/2014 str. 47 <-- 47 --> PDF|
During the 2009 – 2010 assessment period, the number of trees, classified as belonging to the first (0-10%) and second (10-20%) level of defoliation, decreased; in all of the other defoliation classes defoliation increased, with a maximum increase in the trees belonging to the fourth (30–40%) and fifth (40–50%) level of defoliation, which indicates, that tree health declined.
At the Prevalje location, where, according to the data available in Slovenia, visible damage of the Norway spruce stand first appeared, the species C. arvensis was dominant. C. arvensis is considered the most variable species from both a morphological and biological standpoint (Battisti 1993). Comparisons of specimens caught in Slovenia and Italy revealed a difference in the colour of the specimens; the population in Slovenia had darker patterns compared to the population from Italy. The large number of C. arvensis males in 2012 was most likely due to the time they were caught (the end of April). Battisti (1993) showed that males are always the first to emerge, preceding the females by approximately one week. It is known that, of all the European species of the genus Cephalcia, this species is the first to emerge and does so at the end of April, while the other species emerge later (Battisti 1993). Past identification of the prepupae found in 2009 showed a 71% occurrence of C. arvensis (Klepec 2013), and, according to the damage and characteristics of older larvae and prepupae, we can assume that C. abietis was also present. Cescati and Battisti (1992), in their research on the distribution of species of the genus Cephalcia in non-outbreak areas of Trentino in northern Italy, noted that C. arvensis is a dominant species of the genus Cephalcia. Jensen (1988) also reported similar findings in his research of the distribution of species of the genus Cephalcia in Danish spruce forests. In the research of the outbreak of the species C. arvensis in the Czech Republic between 1997 and 1999, the presence of three other species, C. abietis, C. annulicornis and C. alpina, was recorded but they comprised less than 2% of the population (Liška et al. 2001).
In soil samples taken from the Prevalje location, we found in 2009, 994 eonymphs/pronymphs per m2 of soil. As a result, we could very reliably expect outbreaks of Cephalcia spp. in the coming years (Jurc and Mlinšek 2009). Battisti and Rodeghiero (1988) reported that harmful defoliation starts with an abundance of 20 pronymphs/m2 of soil or 14.26 adults/yellow sticky trap. The number of prepupae found in the soil in 2010 was lower, but still large; in some parts of the studied area, we found up to 320 specimens/m2 of soil. For the outbreak of C. arvensis in the Czech Republic between 1997 and 1999, the average number of prepupae in the soil was between 200 and 300 specimens/m2; the highest recorded density was in 1998 and was 1.300 specimens/m2 of soil (Liška et al. 2001). The number of prepupae at the Prevalje location even in 2010 was ten times higher, compared to the number of prepupae in the soil in the year when the harmful defoliation started.
Temperature is a natural factor that significantly affects the development, time of emergence and abundance of the species of the genus Cephalcia. In boreal spruce forests and forest boundary areas of the Alps, where the temperatures are lower, development proceeds more slowly, and C. arvensis does not pass into outbreak. This species can be harmful in warmer areas, such as in the Italian pre-Alps and at lower altitudes in central Europe (Martinek 1991; Battisti 1993).
The temperature of the air is particularly important for the beginning of emergence and the existence of imago in spring and at the same time also presents the temperature threshold at which larvae from the crowns of the trees fall on the ground and penetrates into the soil (Battisti and Cescatti 1994).
Studies in Italy have shown that the emergence of C. arvensis takes place from May until June, when the air temperature is approximately 12 °C. In Slovenia in 2011, the emergence of a larger number of Cephalcia spp. was observed on the 23rd of April, when the average daily temperature was 14.7 °C. Individual imagos were observed on the 20th of April, when the average daily air temperature was over 12 °C, and the emergence ended after 30 days. The slightly earlier emergence of this species (before May) was also noted by Martinek (1991) when researching the outbreak of C. arvensis between 1982 – 1988 in the Czech Republic, where this species was emerging during the second half of April.
The air temperature should be the most important factor affecting the temperature of the soil (Spurr and Barnes 1980 cit. after Battisti 1994 ). The temperature of the soil is important in understanding the ecology of species of the genus Cephalcia because they spend most of their lifetimes in the soil. Battisti (1994) has shown that temperature conditions are important for larvae, both when they are entering into the soil and when they live in the soil as eonymphs/pronymphs. Soil temperature has an effect on the timing of the development of the different developmental forms. If the soil temperature is above 12 °C when the larvae are migrating into the soil, the larvae immediately change into pronymphs, and adults will occur the next spring. If the temperature of the soil is below 12 °C, the larvae will change into eonymphs and then into pronymps the following summer (Battisti 1994). The development can last for two years, in which case diapause is extended.
Consequently, soil temperature has an effect on the dynamics of the population. Long-lasting soil temperatures below 0 °C may result in higher mortality of the prepupae found