The effect of chronic foliage damage by Tomostethus nigritus (Fabricius, 1804) larvae on the radial growth of Fraxinus excelsior

The research aimed to evaluate the response of the European ash Fraxinus excelsior L. radial increment to the foliage damage by Tomostethus nigritus (Fabricius, 1804). In Kharkiv's Molodezhny Park, defoliation of European ash by black ash sawfly increased from 2013, reached a maximum (94.1±3.4%) in 2015, and then decreased. The mean defoliation of individual trees in individual years ranged from 31.7±18.1% to 73.3±13.5%. Only three trees were completely defoliated over two consecutive years. No dependence of tree defoliation on their diameter was found. The mean health condition index HCl started to increase in 2017 and reached 2.4 in 2018. Its dependence on the defoliation level was high and significant (r=0.83; p<0.001). The radial growth of trees decreased in years of increasing foliage damage. In 2013–2018 compared to 2007–2012, the average annual, early, and late wood in the total sample decreased by 12.4%, 7.2%, and 14.9%, respectively. However, the changes were statistically significant only for the late wood. The late growth changed almost synchronously with the annual growth. The proportion of the late wood in the annual growth was only 56.1 % in the year of maximum defoliation and exceeded 70 % the following year. In the group of trees with low defoliation (on average < 25%), the annual wood width in 2013-2018 (the years of black ash sawfly outbreak) was higher than in 2007-2012 by 0.2 mm (8.5%). In the group of trees with moderate defoliation (26-50%), the annual wood width decreased by 0.2 mm (8.5%), and in the group with severe defoliation (> 50%) by 0.7 mm (20.9%). However, the change in the annual wood growth was significant only for the group of trees with severe defoliation. In the group of healthy trees (HCI=1), late wood width in 2013-2018 was higher than in 2007-2012 by 0.04 mm (3.7%), in the group of weakened trees (HCI=2) was lower than in 2007-2012 by 0.2 mm (16.6%), and in a group of severely weakened trees (HCI=3) was lower by 0.15 mm (12.4%). However, these differences were not statistically significant. Most Spearman's rank correlations between ash wood growth and weather parameters are low and insignificant. Negative correlations between ash wood growth and air temperature of the growing season, May and September, are the most significant. An increase in the Hydrothermal Index in 2016, compared to the previous year by 0.2 units (by 25%), could have a positive impact on growth recovery. The research indicates that European ash foliage damage by the black ash sawfly does not pose a noticeable threat to the plantings. At the same time, weakened trees became susceptible to infestation by the emerald ash (Agrilus planipennis Fairmaire, 1888 (Coleoptera: Buprestidae) in subsequent years