Insect outbreaks are among the most important natural disturbances in temperate forest ecosystems.  Development of historical records of outbreaks of forest insects over multi-decadal and centennial time scales is crucial to assess the temporal dynamics and the potential effects of climate variation on the occurrence of outbreaks. In the Patagonian Andes, Nothofagus pumilio, one of the most widespread southern beech species in South America, experiences extensive one-season duration defoliations caused by Ormiscodes species (Lepidoptera). Nevertheless, there is no systematic information about past frequency and severity of outbreaks of Ormiscodes spp. It is unknown if these outbreaks are relatively common phenomena in these forests or if their dynamics have been recently altered. We used dendroecological techniques to reconstruct and evaluate the temporal dynamics of Ormiscodes outbreaks in N. pumilio forests. Our study area comprises two regions on the eastern slope of the Argentinean Andes in Patagonia. One region is located at mid-latitudes (40-41° S; 71° W) and the other at high-latitudes (49° S; 72° W). In each region we collected tree-core samples from 20 stands (c. 350 trees) with at least one defoliation event on a known date.

Results/Conclusions

Ormiscodes defoliation years were relatively infrequent in both study areas over the last c. 180 years. (i.e. only 10 defoliation years per region). In the mid-latitudes region outbreak events occurred mostly during the 1860-1900 (50 % of the events) and the 1960-2005 (40 %) periods; while in the high-latitudes region most of the outbreaks (70 %) occurred during the 1960-2005 period. In the context of the past c. 180 years, our results suggest that outbreak frequency has recently (i.e. past 40 years) increased in the high-latitude region. Further research is needed to assess the potential influences of the recent climate warming trend documented in Patagonia on the observed recent increase in outbreak frequency.