Watering and Leaf Removal as Strategies to Mitigate Cameraria ohridella Damage in Urban Horse Chestnuts

Background Removal of infested leaf litter reduces overwintering Cameraria ohridella populations. Whether supplemental irrigation combined with removal can alleviate drought stress and improve the condition of infested urban horse chestnuts remains insufficiently evaluated. Methods This quasi-experimental study compared treatments applied at 7 sites: untreated control (no leaf removal or irrigation); removal alone; removal plus rainfall-adjusted irrigation of 2 m 3 to 4 m 3 tree −1 every 14 days (regular); removal plus automated irrigation of 70 mm month −1 during April to September (enhanced); and two foliar sprays of the contact insecticide lambda-cyhalothrin at one site. Sampling occurred each September in 2020 and 2021 but only in 2020 at the insecticide site. Leaf-miner damage, total leaf area, dry mass, and estimated specific leaf area were analyzed as tree level means. Results Median tree level damage was approximately 91% at the control and 85% after leaf removal alone (not significantly different) compared with 48% to 69% at regular-irrigation sites and 11% to 44% at enhanced-irrigation sites. Site 6 (L6), comprising solitary trees under leaf removal and enhanced irrigation, had the lowest damage, with leaf area and dry mass among the highest. At the insecticide site, median damaged leaf area was approximately 42% for lower-canopy samples and 75% for upper-canopy samples. Estimated specific leaf area did not differ among sites. Conclusions Combined leaf removal and irrigation were associated with lower damage and better leaf condition, particularly at L6. These site-level associations support the combined strategy as a host-condition intervention without isolating irrigation’s contribution or demonstrating direct pest suppression. L6 had numerically less damage than both insecticide-treated canopy positions, but this limited comparison cannot establish superiority.