Studies on the significance of pathogenic fungi in the population dynamicsof the Lesser Spruce Sawfly, Pristiphora abietina Christ, (Hym., Tenthredinidae)

The population dynamics of Pristiphora abietina, an important pest in Norwa y spruce in central Europe, is significantly influenced by factors causing nymph mortality during the cocoon stage. In contrast to other findings repo rted in the literature, pathogenic fungi of the genera Beauveria, Paecilomy ces and Metarhizium were found to be important natural enemies. Their epizo otics occur soon after cocoon formation by the larvae in the ground litter, i.e. in June/July, and progress only slightly in autumn and winter. 'Obscu re' mortality. which is ascribed mainly to abiotic factors, is observed mur e in winter. Field and laboratory experiments as well as histological analy ses suggest that the fungal disease in nymphs is due to spore contamination occurring before cocoon formation, and that penetration of fungi from outs ide through the intact cocoon wall appears very unlikely. Thus, the infecti on rate of cocoons depends on the spore density in the ground litter of spr uce stands during the cocoon spinning period of May/June. perspectives for biocontrol arising from these findings are discussed.