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Recovering the function of sweet chestnut: classical biological control of invasive Dryocosmus kuriphilus with introduced parasitoid Torymus sinensis

Sweet chestnut (Castanea sativa) is a valuable forest species which is endangered by two highly invasive organisms: chestnut blight (Cryphonectria parasitica) and Asian chestnut gall wasp (Dryocosmus kuriphilus). These harmful organisms have significantly influenced health and function of European sweet chestnut forests. D. kuriphilus is the most recent introduction and in the last 10 years it has spread quickly in Europe making significant damages in sweet chestnut forestst. The only effective option was application of classical biological control and use of Torymus sinensis, a classical biocontrol agent of chestnut gall wasp Dryocosmus kuriphilus. The parasitoid was released in Croatia, Slovenia and Hungary in 2015 and 2016. Following the introduction, the research of parasitism rates and population genetic indices on 40 different sites was performed in order to monitor and evaluate the success of establishment. T. sinensis established viable and genetically diverse populations and also successfully spread naturally from Italy across Slovenia to Croatia and Hungary. Population genetic indices showed that populations of T. sinensis did not suffer from bottleneck-induced founder effect phenomenon. This rapid dispersal of T. sinensis was aided by high concentration of the host, as well as the lack of competition of native parasitoids. This successful application of classical biological control in forestry, first of its kind in Croatia, Slovenia and Hungary has enabled recovery and return of functions of sweet chestnut trees and lowering the population level and damages of D. kuriphilus.

Classical biological control, parasitoid, haplotypes, dispersal, Dryocosmus kuriphilus

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