A Multi-Year Study on the Presence and Infestation Levels of Cameraria ohridella and Guignardia aesculi on Horse Chestnut in North-Eastern Italy

The alien leaf miner Cameraria ohridella (Lepidoptera, Gracillariidae) is damaging horse chestnuts in European countries. Since native natural enemies cannot control the moth, integrated control strategies must be implemented. In north-eastern Italy, from 1997 to 2020, sampling was performed on 55 sites to record the presence of and estimate the leaf damage caused by C. ohridella and the fungus Guignardia aesculi. The level of leaf removal from the ground in autumn was estimated, and information on altitude (199–1294 m a.s.l.), average annual temperature (4.4–11.9 °C) and rainfall (954–1394 mm), and the occurrence of trunk injection with abamectin was collected. Damage caused by the leaf miner and the fungus declined with decreasing temperatures, with negligible damage at sites with average annual temperatures below 7 °C (above 1000 m a.s.l. in the study area). Since, in the study sites, complete leaf removal provided comparable efficacy to trunk injections with abamectin, we suggest adopting this environmentally friendly practice to maintain C. ohridella below damaging levels. Interspecific competition occurred between C. ohridella and G. aesculi because leaf damage from one competitor decreased as damage from the other increased.