WHY THE LARCH BUD-MOTH CYCLE COLLAPSED IN THE SUB-ALPINE LARCH-CEMBRAN PINE FORESTS IN THE YEAR 1990 FOR THE 1ST TIME SINCE 1850

The larch bud-moth cycle has been observed in the sub-alpine larch-cem bran pine forests 16 times since 1850. Infestation is easily recognize d by the characteristic red-brown discoloration of the larch crowns du e to the wasteful feeding of the bud moth larvae. The heaviest defolia tion recurs at intervals of 8.47 +/- 0.27 (SE) years, and the larval d ensity per kilogram of larch branches varies more than 10000-fold over four or five generations. The basic regulatory mechanism for this cyc le is the induced change in food quality for the two or more subsequen t larval generations. Defoliation functions as a negative feedback mec hanism acting on larval density. In 1989 local discoloration in the Up per Engadine valley was observed in the usual first focus. In 1990 and 1991, however, instead of the expected widespread defoliation damage, larval densities decreased drastically. Based on extensive field data collected from 1961 to 1991 on the development and the survival of th e bud moth (Zeiraphera diniana Gn.) and the phenology of the host, Lar ix decidua L., this paper shows the effect of weather on survival in t he egg stage and on the coincidence of larval hatching with the sprout ing of the larch. It is shown that the winter and spring weather condi tions in 1989 1991 were conducive to unusually high egg mortality. Sin ce these conditions occurred in three successive generations, populati on growth was effectively reduced and the cycle collapsed prematurely. Thus the rather persistent cyclicity of the larch-larch bud-moth syst em was disturbed by weather conditions with a very low probability of occurrence, but due to the inherent high resilience of the system, the next population peak with visible defoliation is expected to occur 19 96/1997, provided that the weather conditions return to the climatic s tandard.