North-westward expansion of the invasive range of emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) towards the EU: from Moscow to Saint Petersburg.
Agrilus planipennis is a devastating invasive pest of ash trees in European Russia, Ukraine, and North America. To monitor the north-western limit of its European invasive range, in June 2018 we established 10 study plots along the federal highway M10 (Russia) that runs between Moscow and Saint Petersburg through Tver' City (approx. 180 km from Moscow), and lined with ash trees. On each plot, 2-4 Fraxinus pennsylvanica trees with heights ranging 6.1-17.0 m and diameters ranging 7.0-18.0 cm were girdled, i.e., 50 cm of their bark were removed. The study plots were visited and girdled trees were examined in September and November, 2018, and in October, 2019. Observations revealed that the current continuous north-western limit of A. planipennis range in European Russia coincides with the north-western border of Tver' City and this range limit has not distinctly shifted north-westward during 2015-2019. In spite of the rich food supply (due to abundant F. pennsylvanica and F. excelsior plantings) in Tver' City and along roads going to and from, the population density of A. planipennis in the area is currently low. Recent (September 2020) sudden detection of a spatially isolated A. planipennis outbreak approx. 520 km far north-westward from Tver' (in Saint Petersburg) suggested that A. planipennis most likely had arrived at Saint Petersburg not by gradual stepwise (flying tree-to-tree) expansion of its continuous invasive range in Tver' City, but as a result of its accidental introduction by means of, e.g., "insect-hitchhiked" vehicles, transported plants for planting, and/or other commodities. The proximity of the reported A. planipennis outbreak to the borders of the EU (approx. 130 km to Estonia and Finland) requires urgent measures for its containment and control, and constant monitoring.