Influence of silvicultural practices and population genetics on management of the spruce sawfly, Cephalcia arvensis

Abstract

The web-spinning sawfly Cephalcia arvensis is a defoliator of the Norway spruce ( Picea abies) in Northern Eurasia. Four outbreaks have been recorded in the last 15 years in Czech Republic and in Italy. A comparative analysis of the involved populations revealed noticeable differences in phenology, leading to a strong variation in life cycle and voltinism. A population already known as `spring form' for the early emergence of adults, and morphologically distinguishable at the prepupal stage, has been found to be genetically different from populations emerging later, as far as allozymes and chromosomes are concerned. The high biological and genetic variability observed among populations of C. arvensis allows these sawflies to become adapted to a wide range of local conditions, contributing in this way to the differentiation of local races and species. In spite of this variability, all the outbreaks occurred in Norway spruce plantations characterized by about the same silvicultural parameters (50-to-100-year-old, high density and medium to high quality), located outside or on the border of the natural range of the host plant, and thus undergoing various environmental stresses. As the silvicultural conditions of the stressed stands may determine the risk of outbreak, the integration of cultural methods (selection of planting areas and of regeneration material, selective cutting of mature stands to increase diversity) in pest management seems to be very important. The management of the pest would also benefit from extending monitoring to cover all the phenological variants, because an early detection of the outbreak is essential for a successful application of the biocontrol techniques presently available.