Zur Biologie und Gradologie des Grauen Lärchenwicklers, Zeiraphera diniana Gn. (Lep., Tortricidae), im schweizerischen Mittelland1

Studies on the bionomy and gradology of the grey larch Tortricid, Zeiraphera diniana Gn. in Middle Suisse. 1: Biotop, phenology and population dynamics. The larch bud moth, Zeiraphera diniana Gn. (Lep.: Tortricidae), exhibits a cyclic fluctuation in numbers, in the subalpine larch-cembran pine forest above 1600 m causing extensive defoliation to the larch at 9-year intervals, but remains more or less stable at low densities in larch plantations within the beech-oak forest on the Swiss Plateau at 500 m. The development of the various stages was established and showed the existence of two independent moth flights; one in June and the second in August. The latter is almost undoubtedly due to immigration from the optimum areas at higher altitudes. Life table studies for 4 generations allow the following statements: 1 Egg development and coincidence of hatching with sprouting of the host plant is well adapted to the climatic conditions of the Swiss Plateau. 2 Limiting factors to population growth are: low fecundity, 80 % predation during the egg stage, 67 % mortality due to abiotic influences and predators during the small larvae period and 51 % abiotic and biotic mortality in the ground during the prepupal and pupal period. Provided that the low coefficient of reproduction of 0.7 is generally valid, the bud moth population on the Swiss Plateau could not survive without repeated immigration from high altitudes.