Efficiency of biological control of Gonipterus platensis (Coleoptera: Curculionidae) by Anaphes nitens (Hymenoptera: Mymaridae) in cold areas of the Iberian Peninsula: Implications for defoliation and wood production in Eucalyptus globulus

Sustainable management of forest plantations and cost-effective control strategies depend on previous estimations of the economic level of damage caused by the pests. The eucalyptus weevil, a key pest of Eucalyptus plantations worldwide, is mainly controlled using classical biological control, using the mymarid egg-parasitoid Anaphes nitens (Girault). Nevertheless, in several temperate regions, the parasitoid fails to reduce the weevil populations to economically sustainable levels. This study attempts to (i) relate the efficiency of the parasitoid with climate variables, (ii) relate the level of damage caused by the weevil with the rate of parasitism, (iii) estimate the implications of weevil damage on wood production. Weevil density, damage caused by defoliation on the upper crown and parasitism rates were monitored in 2007, in 34 Eucalyptus globulus stands. Elevation, temperature and precipitation were assessed by using the Worldclim database. Using historic inventory data, wood production was projected to an age of 10 years, prior to the arrival of the weevil, and compared with current data for the same stands. Parasitism rate by A. nitens was a key element explaining weevil density and tree defoliation, r(2) = 0.37 and 0.41 (p < 0.001), respectively. Significant relationships between parasitism rates and maximum temperature of the winter months (MaxTw), r(2) = 0.55 and elevation r(2) = 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 degrees C. The mean percentage of parasitism was low 10.1% (+/- 4.9) for MaxTw below 10 degrees C, increasing to 70.9% (+/- 3.8) above 11.5 degrees 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 degrees 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. (C) 2012 Elsevier B.V. All rights reserved.