Resistance of European ash (Fraxinus excelsior) saplings to larval feeding by the emerald ash borer (Agrilus planipennis)

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European ash (Fraxinus excelsior) has been heavily damaged by ash dieback caused by Hymenoscyphus fraxineus (Ascomycotina).  Another potential threat, emerald ash borer (EAB; Agrilus planipennis, Coleoptera: Buprestidae), has been destructive to North American Fraxinus species and there is concern that ash-dieback resistant F. excelsior trees may be more susceptible to EAB.Fraxinus excelsior was obtained from Norfolk and from collections representative of Great Britain, and replicated by grafting.  The Norfolk trees were selected as apparently more resistant to ash dieback.  Trees were inoculated with EAB eggs and the development of feeding galleries and larvae recorded. The response of F. excelsior to EAB attack was compared to those of a resistant species, F. mandshurica, and the highly susceptible F. nigra.Second and third instar larvae developed much less frequently in F. excelsior and F. mandshurica than in F. nigra, and there was no significant variation between F. excelsior accessions.  Purple discoloration of F. excelsior inner bark was associated with altered rates of larval development.Fraxinus excelsior saplings appear to have partial resistance to EAB larval development, but as EAB can kill even resistant Fraxinus species when they are stressed, it remains a threat to F. excelsior suffering from ash dieback or other stresses.  If responses in saplings reflect those of mature trees, breeding F. excelsior for resistance to ash dieback is unlikely to alter responses to EAB greatly.