Between ash dieback and emerald ash borer: two Asian invaders in Russia and the future of ash in Europe.
Four ash species are native to Russia (Fraxinus excelsior, F. angustifolia, F. chinensis, F. mandshurica) while F. pennsylvanica was introduced from North America. Ash forests cover 666 300 ha (0.1% of total forest area of Russia) and constitute a volume of 77.91 mln m3. Ash is widely used in the greening of populated places, around fields and along inter-city roads. We review the current situation with two recent invaders - ash dieback fungus Hymenoscyphus fraxineus (Ascomycota) and emerald ash borer Agrilus planipennis (Coleoptera). Hymenoscyphus fraxineus was likely accidentally introduced from Asia to Western Europe, expanded its range eastward and by 2014 reached Moscow, whereas A. planipennis was accidentally introduced from Asia to Moscow Region, expanded its range in all directions but most noticeably southwards. By 2012, A. planipennis reached Smolensk Region bordering Belarus, and by 2013, Voronezh Region bordering Ukraine. At least between Belarus and Moscow city, the ranges of invaders overlap. Both species are a threat to the native as well as introduced ash in Europe. We list known records of two invaders in Russia (as of 2016) and for A. planipennis also review food plants, seasonal cycle, dispersal, parasitoids and susceptibility of different ash species. We analyze the synergetic effect of two invaders on ash in the area of overlapped ranges and potential losses of biological diversity associated with ash decline and conclude that the future of ash in Europe is precarious. The following directions of actions in Eurasia are proposed: (1) studies of resistance mechanisms to both agents in Asian ash species (first of all, F. chinensis and F. mandshurica) and hybrids between Asian and European or North-American ash species, (2) studies on selection of resistant ash forms and hybrids (to both agents), (3) controlled introduction of resistant Asian ash species, (4) slowing down of expansions of A. planipennis to Western Europe and H. fraxineus within Russia, (5) studies of natural control agents, (6) monitoring of invasions and sanitary condition of ash, and (7) studies on synergetic effect of H. fraxineus and A. planipennis on ash.