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

The web-spinning sawfly Cephalcia arvensis is a defoliator of the Norway sp ruce (Picea abies) in Northern Eurasia. Four outbreaks have been recorded i n the last 15 years in Czech Republic and in Italy. A comparative analysis of the involved populations revealed noticeable differences in phenology, l eading to a strong variation in life cycle and voltinism. A population alre ady known as 'spring form' for the early emergence of adults, and morpholog ically distinguishable at the prepupal stage, has been found to be genetica lly different from populations emerging later, as far as allozymes and chro mosomes are concerned. The high biological and genetic variability observed among populations of C. arvensis allows these sawflies to become adapted t o a wide range of local conditions, contributing in this way to the differe ntiation 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 o utbreak, the integration of cultural methods (selection of planting areas a nd of regeneration material, selective cutting of mature stands to increase diversity) in pest management seems to be very important. The management o f the pest would also benefit from extending monitoring to cover all the ph enological variants, because an early detection of the outbreak is essentia l for a successful application of the biocontrol techniques presently avail able. (C) 2000 Elsevier Science B.V. All rights reserved.