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An overview on the biological control of the invasive Dryocosmus kuriphilus (Hym: Cynipidae)

Dryocosmus kuriphilus is a globally invasive insect pest, spreading very quickly in new habitats and making seriously damage to sweet chestnut forests in many European countries. Indigenous parasitoid species trophically associated with oak gallwasps have adapted to this new host but cannot effectively regulate its population density. Classical biological control using parasitoid Torymus sinensis (Chalcidoidea: Torymidae) has been proven to be the only effective method of controlling the populations of D. kuriphilus and has been successfully applied in Japan, South Korea, USA in the mid 1970th. It was introduced to Italy in 2005 and was released in several sites covering most of the infested area. In 2011 it was introduced and released in several sites in France. In 2013 Torymus sinensis was also introduced to Croatia and Hungary and was released into deciduous mixed forests with infested Castanea sp. The aim of this presentation is to provide overview and up-to date knowledge about biological control of D. kurphilus and to describe first steps of introduction of T. sinensis to sweet chestnut forests in Croatia and Hungary. The history and results of introductions of T. sinensis to Japan, South Korea, USA and Italy are shown as well as the adapted biology and behavioural traits of T. sinensis to its host D. kuriphilus are discussed. First results of T. sinensis release in France indicate that even though few biocontrol agents were released, the rate of establishment of T. sinensis is high (c. 80%). Although the parasitoid abundance was still low, the T. sinensis populations display a positive inter-annual growth rate. Possible negative effects of T. sinensis on native parasitoid fauna and risks that could influence the successful establishment of T. sinensis are discussed. Previous experiences have shown that T. sinensis can successfully control the population density of D. kuriphilus, slowing down the spread and mitigating negative impact of this invasive chestnut pest and keeping the damage of D. kuriphilus at acceptable level. High specificity of T. sinensis suggests that it has limited potential of exploiting native hosts but further detailed monitoring of native parasitoid and possible interactions with introduced T. sinensis is strongly suggested. The introduction of T. sinensis has raised some important questions to be further studied and answered: (i) the general risk that T. sinensis could shift to native gall wasps related to D. kuriphilus and (ii) could hybridize with native Torymus species. No other host than D. kuriphilus was ever mentioned in the literature to be parasitized by introduced T. sinensis. Till now, the only reported case of T. sinensis hybridization is with the native T. beneficus in Japan: T. sinensis and T. beneficus were successfully crossed in the laboratory to produce fertile hybrid females. The probability of hybridization with native European Torymus species was also tested, no mating was recorded yet in the laboratory experiments.

Torymus sinensis; Dryocosmus kuriphilus; classical biological control; parasitoid; sweet chestnut; host specificity; gallwasp; release

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