Untersuchungen über die Fichtengespinstblattwespen Cephalcia spp. Panz. (Hym., Pamphiliidae) : IV. Massenwechsel von Cephalcia abietis L. im oberschwäbischen Forstrevier Erolzheim (Fbz. Ochsenhausen)

Studies on the web-spinning spruce sawflies of the genus Cephalcia spp. 1 Panz. (Hym., Pamphiliidae)

2000

34 Lit. Ang.

Unselbständiges Werk

2479

200065220

The paper describes the population dynamics of the spruce false webworm Cephalcia abietis L. in an 80-year-old spruce stand in the forest area of Erolzheim near Ulm (Germany) between 1986 and 1996. Emerging sawflies and adult parasitoids were trapped in groundphotoeclectors and the descending host larvae in funnel-traps. About 7000 host larvae and prepupae were dissected to evaluate the degree of parasitism. The emergence pattern of the sawfly was determined as well as the duration of the feeding period. The development of the majority of the sawfly individuals lasted 3 yeras; during the warm summer of 1994 univoltinism was observed in 36% of the population. Temperature was ascertained as the key factor for the variable duration of the dormancy and for the population dynamics of the forest pest. Since 1986, the year of heavy feeding, the density of emerging females, of descending larvae, and of prepupa in the soil decreased finally to a subepidemic level in 1996. 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 were predominating (SI = 0.16). The annual average number of full-grown and developing 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 Cephalcia abietis consists of the egg parasitoid Trichogramma cephalciae, nine primary larval parasitoids (eight ichneumonids, one tachinid), and the hyperparasitoid Mesochorus nov. sp. Within 5 years the average of the total (effective) parasitism of descending larvae amounted to 74%, of prepupae in October to 45%, and of emerging adults to 40%. The possible reasons for the discrepancies are discussed, taking into account the effects of a single parasitoid species.