Black sawfly (Tomostethus nigritus), the foliage browsing pest of common ash (Fraxinus excelsior) after several years of depression was registered in forest stands, forest shelter belts and urban ornamental stands of the East Ukraine. The aim of this research was to reveal the peculiarities of ash defoliation by black sawfly during three years of outbreak and to recognize the changes in health condition of defoliated trees. Research was carried out for 2013–2015 in Molodezhny park of Kharkiv (50°00' N; 36°25' E) (Ukraine, Forest-steppe natural zone) in two groups of common ash trees (forest belt and compact stand). For each labeled tree, defoliation of the whole crown and separately of upper, middle and lower layers, outer and inner parts of crown was assessed in June of 2013, 2014 and 2015, and additionally in July 2015. Health condition of each labeled tree was assessed in June 2013 and July 2015. Defoliation of common ash trees did not significantly differ in 2013 and 2014. It almost twice increased in 2015 and in average exceeded 90 %. Dependence of defoliation from crown layer, crown part (outer or inner) or tree diameter was not proved statistically, but correlation analysis shows the better health condition of larger trees. The fast recovery of ash crowns after severe damage by black sawfly in 2015 can be explained by favorable weather conditions. Analysis of ash trees distribution by categories of health condition show the improvement of their health after three years of foliage damage by ash sawfly, but forest stand continues to be weakened. Considerable parts of the trees, which belonged to the 2nd, 3rd and 4th cate-gories of health condition in 2013, changed it by one-three grades. Totally 50 % of trees did not change the category of health condition, 35.3 % of trees improved it and 14.7% worsened it.

Keywords: black sawfly (Tomostethus nigritus), common ash (Fraxinus excelsior); defoliation, crown condition.