A cost-benefit analysis for the management of Gonipterus platensis by comparing chemical and augmentative biological control

Abstract

Around 1.5 million ha of the Iberian Peninsula is currently occupied by Eucalyptus globulus, the preferred species for commercial plantations due to its fast growth and wood technological properties. However, these economically important plantations are being heavily damaged by Gonipterus platensis (Col: Curculionidae), an invasive species native to Australia which was established in the Peninsula, in the nineties. A classical biological control with the release of the egg parasitoid Anaphes nitens (Hym: Mymaridae) was launched in the Peninsula. Yet, in the colder regions, in the North of the Peninsula, the parasitoid is not efficient enough to control the weevil population. The resulting damage overcomes more than 200 million euros every year.Current control strategies rely on the application of insecticide (acetamiprid) or augmentative biological control (ABC) with yearly releases of A. nitens in spring. However, uncertainties about the best option persist. In this study, chemical and ABC treatments were submitted to a cost-benefit analysis with the aim of determining their viability against the weevil, based on a treatment efficacy and stand productivity trade-off analysis.A simulation tool (3-PG.d) was used to simulate the growth of E. globulus stands, under four defoliation intensity scenarios mimicking the removal of 25, 50, 75 and 100% of the spring new leaves biomass. Each defoliation scenario was combined with three treatments: (i) chemical (acetamiprid), (ii) ABC+ (optimistic approach), and (iii) ABC- (pessimistic approach). For each treatment, several plans of yearly treatments were tested. The Net Present Value (NPV) and the harvested volume were used to compare all treatment alternatives.The best treatment decision, based on a cost-benefit analysis comparing economic results and harvested volume, varied with the intensity of defoliation. Treating was always better than doing nothing, except for the 25% defoliation scenario. ABC+ offered the best economic results, relative to other treatments, for 25, 50 and 75% defoliation scenarios. ABC- was only as good as the chemical treatment for the 25 and 50% defoliations. Under the heaviest weevil attack (100%), applying insecticide was the best alternative, overcoming both ABC treatments. These results show that augmentative biological control was a strategy to consider for this study, and thus, may be a strategy to be considered in the future.The 3-PG.d simulator showed that it can be an asset for forest managers aiming at integrating chemical and/or biological treatments against forest pests in stand management decisions.