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

Abstract

a b s t r a c tSustainable 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°C. The mean percentage of parasitism was low 10.1% (±4.9) for MaxTw below 10°C, increasing to 70.9% (±3.8) above 11.5°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°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.

Increasing numbers and intercontinental spread of invasive insects on eucalypts

“…This is in part due to the considerable time and financial investment required to implement biological control programmes (including costly and uncertain scouting trips to identify natural enemies in the native range), and the introduction of stricter regulations for the import of biological control agents (Hajek et al 2016;this issue). In addition, for several species there is as yet no biological control available and for others the organisms introduced do no provide sufficient control, as with Gonipterus platensis by Anaphes nitens in the Iberian Peninsula (Reis et al 2012). These challenges have led to increased and more formalized global collaboration between eucalypt growers to share knowledge and biological control agents ).…”

Effects of forest heterogeneity on the efficiency of caterpillar control service provided by birds in temperate oak forests

“…Their leaf consumption may decrease the growth of trees, negatively influencing their health condition, fecundity, and inhibiting regeneration (Rieske and Dillaway, 2008). The economic value of the wood losses due to reduced growth increases exponentially with the extent of defoliation (Reis et al, 2012), and may reach 310 $/ha/year (Lyytikainen-Saarenmaa and Tomppo, 2002). On top of these significant economic losses, regular insect outbreaks may trigger decline chains in the forests resulting in mass tree mortality (McManus and Csóka, 2007;Jepsen et al, 2013).…”

Pest categorisation of the Gonipterus scutellatus species complex

“…Mapondera et al (2012), the species present in Portugal is Gonipterus platensis. It is widespread in mainland Portugal Reis et al, 2012;Valente et al, 2017) Two specimens of the species complex were found in the Azores, one in a Malaise trap, the other on Erica azorica in a native forest area (Orom ı et al, Borges et al, 2013) …”

“…Severe defoliation implies loss of stem growth and important reduction of the volume of merchantable wood (Reis et al, 2012). E. globulus, E. viminalis and E. camaldulensis are indicated as the most vulnerable species (Cerasoli et al, 2016).…”