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|ŠUMARSKI LIST 5-6/2014 str. 43 <-- 43 --> PDF|
OUTBREAK OF THE SPRUCE WEB-SPINNING SAWFLY Cephalcia arvensis (HYMENOPTERA: PAMPHILIIDAE) IN SLOVENIA
GRADACIJA SMREKINE OSE PREDIVICE Cephalcia arvensis (HYMENOPTERA: PAMPHILIIDAE) U SLOVENIJI
Gregor Meterc, Danijel Borković, Maja Jurc
The first local outbreak of the field web-spinning sawfly (Cephalcia arvensis) appeared in 2009 on one location on 106 ha in the northern part of Slovenia in a stand of 60-90-year-old Picea abies and on 600-800 m a. s. l. In the period from 2009 to 2011 the density of the prepupae, bio-ecology and defoliation of the trees were studied.
The density of the population of prepupae of Cephalcia species was studied on the six plots in the soil samples (25 x 25 x 20 cm). In 2009 it was high (average = 595 individuals/m2 of soil), while in 2010 the average number of prepupae was lower for 68%. The collected data of soil and air temperatures indicate their impact on the beginning of the emergence of Cephalcia species in 2011; we found that it had started in large number when the average daily temperature was 14.7 °C and the average soil temperature was 8.7 °C.
In the autumn of 2009, six circular plots (50 x 50 m each) were defined where the defoliation of the crowns of 88 conifer trees with the breast height over 10 cm was estimated. The defoliation of P. abies has grown over two years; in 2009, the average defoliation was 28%, and in 2010 it was 32%.
KEY WORDS: Cephalcia arvensis, temperature, defoliation, outbreak, Norway spruce, Slovenia.
Cephalcia spp. are Holarctic species, inhabiting Eurasia and North America. In Europe, more than 10 species are known. Cephalcia spp. can cause extensive defoliation of conifers within the family Pinaceae, especially the species of the genera Picea, Pinus, and Larix, and occasionally Cedrus and Abies. The causes underlying the growing populations of the different species vary but include the presence of tree species more susceptible to attack due to weakening by different biotic and abiotic factors, appropriate ecological conditions for the development of Cephalcia spp. (higher temperatures and dry and hot weather), and a lack of natural enemies and pathogens of Cephalcia in the environment. Damage is caused by larvae that eat needles, and the defoliation of needles over multi-year periods weakens the host trees, reducing their growth and making those trees even more susceptible to damage from other biotic and abiotic factors (Jurc 2009). In north east Italy possible predominate causes of the outbreaks of C. arvensis were climatic and soil factors. In the years before outbreaks, the climate was hot