Modern invasions within the Western Palearctic: Cynipid gallwasps as study models for the development of parasitoid communities
Creators: name: McCormack, Koorosh
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: Dryocosmus kuriphilus
invasion ecology
Plagiotrochus
gallwasp
community ecology
Ecological filtering
cryptic speciation
Torymus flavipes

description: Invasion ecology is a topic of great interest, in large part because of its immediate value in conservation and biocontrol efforts which serve to curb or prevent the negative impacts of invasive species. Invasion ecology is also of great interest because invaders are excellent model systems for the study of community assembly under different ecological paradigms, from ideas stemming from island biogeography, neutral theory, niche theory and coevolutionary processes. Each time an invader arrives into a novel environment, we are provided with a naturalised experiment in which to study these community assembly processes. To that end, invading gallwasps within the tribe Cynipini (Hymenoptera; Cynipidae) provide an excellent study system; the base of the community consists of the host plant, usually oak (Quercus spp.), and a parasitic gallwasp whose larvae modify the development of plant tissue of particular host plant organs, upregulating the supply of nutrient rich material in the process and also often providing them with a degree of protection. The complexity of this interaction enforces strict limitations in the breadth of suitable host plant species that can be exploited by the gallwasps themselves. Centred around the gallwasps and their associated gall tissues are a suite of additional associates: the inquilines and parasitoids. Cynipid inquilines live inside and feed upon gall tissues either peripheral to the main inducer’s chamber, or in the main chamber after having killed the inducer. Non-cynipid inquilines including moths and weevils feed on gall tissue and gall-inhabitants. Parasitoids are a form of specialised predator whose larvae prey directly upon the immature stages of gall inhabitants, including gallwasps, inquilines, and sometimes other parasitoids and gall tissue. Cynipid inquilines, while incapable of gall induction, induce the development of their own nutritive gall tissues in ways similar to the inducing gallwasp, and hence tend to have strict limitations in host breadth, broadly echoing the strict relationship between gall-inducers and their host plants. Almost all parasitoids associated with gallwasps also tend to be specific to gallwasps and their inquilines, although strict limitations in host breadth are less common. The level of specialism at each trophic level of the gallwasp community serves as an ecological filter for the set of community members likely to interact with invaders, and in this sense, gallwasp communities are thought of as being ‘ecologically closed’ compared to other invading species where ecological filtering processes are more diffuse. Our level of understanding for a vast proportion of gallwasp invaders is however limited or unknown, and the unique set of ecological trait combinations that novel invaders provide has implications for the set of parasitoids attacking them. This thesis centres around the distribution and community developments of four contemporary invaders of the UK and Europe, with a particular focus on the chestnut gallwasp (Dryocosmus kuriphilus) and Plagiotrochus quercusilicis. Globally, D. kuriphilus is the most significant gallwasp pest due to the damage it causes to its sweet chestnut (Castanea spp.) hosts and hence is a species of great interest. Past studies of gallwasp invasions in Europe have all concerned range expansions of species within the Western Palearctic bearing little economic significance, utilising host oak trees primarily of Quercus section Quercus, or host-alternating species whose sexual generations utilise section Cerris hosts in spring and asexual generations on section Quercus in autumn. The phylogenetic placement of the gallwasps’ host tree often has important implications for their set of parasitoid associates, and indeed we often observe that parasitoid communities are more alike between distantly related gallwasp species on the same host tree section, than to different generations of the same gallwasp species on different host tree sections. The set of invaders I cover in this thesis are unique within a UK invasion context due to the host plants that they exploit. D. kuriphilus is doubly unique within a European context in being the only gallwasp to attack sweet chestnut (Castanea spp.), and being the only widespread invading gallwasp of non-Western Palearctic origin (originally from China). The other three (Plagiotrochus australis, Plagiotrochus coriaceus and Plagiotrochus quercusilicis) are the only invaders utilising Quercus section Ilex hosts, for which the parasitoid community is generally less known than for galls on other oak sections, and are completely unexplored within an invasion context.