The influence of climatic factors and the leaf-miner Cameraria ohridella on growth and condition of European horse-chestnut Aesculus hippocastanum

• Tree growth and defoliation by Cameraria ohridella monitored for 21 years. • Growth increments positively related to the number of wet days in spring and autumn. • Maximum defoliation occurred after a dry autumn and a warm, dry spring. • Defoliation had a significant impact on tree growth only in drought years. • No evidence that Cameraria ohridella has a long-term effect on tree health. The intensity of damage caused by Cameraria ohridella and radial and basal area increments of 74 horse-chestnut trees, Aesculus hippocastanum (and 53 sycamore Acer pseudoplatanus , included as an undamaged comparison) were estimated each year from 2003 to 2023. Growth increments were compared with the percentage leaf area damaged by C. ohridella and different combinations of monthly mean temperatures, precipitation and the numbers of wet days per month using Generalised Linear Models. Radial and basal area increments in A. hippocastanum responded positively to a greater number of wet days in April–August of the current year and in October of the previous year, conditions that at the same time led to a decrease in the abundance of C. ohridella . Compared with climatic factors, the effect of C. ohridella on tree growth was marginal and the only significant impact attributable to the leaf miner occurred in the hottest and driest year, when tree growth was already greatly reduced. The general condition of A. hippocastanum changed little over the course of the study, and there was no evidence for C. ohridella having a major long-term impact on tree health.