Studies on the web-spinning spruce sawflies of the genus Cephalcia spp. 1 Panz. (Hym., Pamphiliidae). IV. Population dynamics of Cephalcia abietis L. in Upper Suabia.

The paper describes the population dynamics of the spruce false webworm Cep halcia abietis L. in an 80-year-old spruce stand in the forest area of Erol zheim near Ulm (Germany) between 1986 and 1996. Emerging sawflies and adult parasitoids were trapped in groundphotoeclectors and the descending host l arvae in funnel-traps. About 7000 host larvae and prepupae were dissected t o evaluate the degree of parasitism. The emergence pattern of the sawfly wa s determined as well as the duration of the feeding period. The development of the majority of the sawfly individuals lasted 3 years; during the warm summer of 1994 univoltinism was observed in 36% of the population. Temperat ure was ascertained as the key factor for the variable duration of the dorm ancy and for the population dynamics of the forest pest. Since 1986, the ye ar of heavy feeding, the density of emerging females, of descending larvae, and of prepupa in the soil decreased finally to a subepidemic level in 199 6. The sex ratio (SI) of different host stages averaged from 0.37 to 0.54. In the yellow prepupae - amounting to 11% of the total population males wer e predominating (SI = 0.16). The annual average number of full-grown and de veloping eggs per female varied between 21 and 53, the total number of all oocytes between 46 and 112 (average 79). The complex of parasitoids of Ceph alcia abietis consists of the egg parasitoid Trichogramma cephalciae, nine primary larval parasitoids (eight ichneumonids, one tachinid), and the hype rparasitoid Mesochorus nov. sp. Within 5 years the average of the total (ef fective) parasitism of descending larvae amounted to 74%, of prepupae in Oc tober to 45%, and of emerging adults to 30%. The possible reasons for the d iscrepancies are discussed, taking into account the effects of a single par asitoid species.