Interaction of European Chalcidoid Parasitoids with the Invasive Chestnut Gall Wasp, Dryocosmus kuriphilus
Insect herbivores and their parasitoids are estimated to comprise between one and two thirds of all multicellular life on earth. Insect herbivores are key primary consumers, and occupy economically important roles as agricultural pollinators and pests. Natural insect parasitoid enemies of insect herbivores can inflict very high mortality, and provide economically important biological control of many insect pests. While the processes involved in some specific host-parasitoid interactions have been studied in detail, the recruitment of parasitoids to herbivore hosts in nature remains poorly understood. In this thesis I consider the recruitment of native European parasitoids to an invading herbivore – the Asian chestnut gall wasp, Dryocosmus kuriphilus. Originating from China, D. kuriphilus has rapidly become an economically important pest of sweet chestnut, Castanea sativa, in Europe, where it is now found from Portugal to Turkey and from southern Italy to the U.K. Since its arrival in ca. 1996, it has become locally super-abundant and has recruited over 30 native chalcid parasitoids as opportunistic enemies. Most are known to attack native oak gall wasp hosts. This thesis seeks to understand the processes underlying recruitment of native enemies to the novel host. Specifically, I seek to understand whether recruitment of native parasitoids is a rare and localised process, or a frequent and widespread one. I address this question using widespread geographic sampling of oak and chestnut gall wasps, rearing their parasitoids and using multi-species analyses of community structure and composition. Because some currently recognised parasitoid morpho-species have been shown to include cryptic molecular taxa, I explore the consequences of a DNA barcoding approach for analyses of the parasitoid communities attacking D. kuriphilus and native gall wasp hosts. At my study sites I found D. kuriphilus to be attacked by 29 parasitoid morpho-species, extended by DNA barcoding to a total of 39 molecular and morphological taxa. The majority of native cynipid galls in Europe are associated with oak and most of the parasitoid species found to attack D. kuriphilus are also known to attack gall wasp hosts on oak. My data provide new records of parasitoid species recruited to D. kuriphilus on chestnut. This includes parasitoids known to attack non-oak cynipid galls suggesting that other sources for the recruitment of parasitoids need to be considered. Multi-species community analyses suggest that parasitoid host shifts to chestnut have happened repeatedly in multiple locations. My study thus suggests that, while gall wasps are highly specific to particular tree taxa, their chalcidoid natural enemies are not so constrained.