Parasitism rate by A. nitens was a key element explaining weevil density and tree defoliation, r2 = 0.37 and 0.41 (p < 0.001), respectively. Significant relationships between parasitism rates and maximum temperature of the winter months (MaxTw), r2 = 0.55 and elevation r2 = 0.59 (p < 0.001) were found. Other climatic variables, such as temperatures of the warmest months and precipitation, were not significantly related to parasitism rates. An upper threshold limit for the efficiency of the parasitoid appears for MaxTw of 10-11 掳C. The mean percentage of parasitism was low 10.1%(卤4.9) for MaxTw below 10 掳C, increasing to 70.9%(卤3.8) above 11.5 掳C. A reduction of the efficiency of A. nitens due to differences in the climatic niches of both the host and the parasitoid is hypothesised. The lower temperature threshold in particular, is of paramount importance for this host-parasitoid system. In consequence, in colder areas MaxTw < 10 掳C, a defoliation of 74.1%was attained. Wood volume (projected to the age of 10 years) was estimated to decrease to 51%in the affected areas in 2004-2006, compared to the previous period of 1995-1998. Estimated loss in wood volume increased exponentially reaching 43%and 86%, for 75%and 100%of tree defoliation, respectively. Therefore, considering the increase in the economic costs calculated for these regions, due to the high defoliation caused by the weevil, research into alternative control strategies is urgently needed.