Formation of the Invasive Range of the Emerald Ash Borer Agrilus planipennis in St. Petersburg, Russia: Developmental Characteristic Peculiarities and Associated Nematodes

Emerald ash borer Agrilus planipennis was previously detected in St. Petersburg in 2020, and this paper presents an update of the situation. In 2024, the presence of seven active outbreaks was observed. In Nevsky District an area very remote from the other known finding sites was detected, close to the south-eastern border of the city. Earlier, in Nevsky District, a hotspot of infestation was detected in Park Stroiteley, after which the trees were designated for felling. Three new foci were identified in Petrodvortsovy District. In previous years, two more infested areas were detected—in the ash alley plantings on Suvorovskaya Street, buildings 5 and 7, as well as in the railway station square in Lomonosov. These findings were fixed in 2022—in the eastern part of Saint Petersburg and Petrodvortsovy District at a distance of 0.5–3 km from the southern coast of the Gulf of Finland; the western and south-western part of the Nevsky District close to the Neva River (approximately 750–1000 m). Complete removal of infested ash trees in this district was not carried out. Nematodes, both bacteriophage (Panagrolaimus sp.), mycophytotrophs and plant parasites (Laimaphelenchus sp.), were found in the emerald ash borer larval galleries, but not in the imagoes and larvae themselves. Presumably, the nematodes do not use the emerald ash borer as a vector, but were transmitted by other insects or entered the ash trunks by another means. Both species of ash trees, Fraxinus excelsior and Fraxinus pennsylvanica, were observed to be infested. The density of settlements of the infested trees was at a low level, and a significant increase in larval mortality was noted compared findings in 2019–2020. Further factors restraining the population density may be an increase in the number of parasitoids, low heat availability, and timely removal of infested trees. The increase in the number of ash borer outbreaks suggests that the spread of this pest is continuing and can lead to massive death of ash trees in St. Petersburg.