A recent ash dieback infection neither affects emerald ash borer performance nor triggers a substantial systemic phytochemical defense response in European ash

The number of non-native invasive species has increased markedly in recent decades, raising the likelihood of co-occurrence and interactions among invaders. Such interactions may exacerbate impacts on native ecosystems, making it essential to understand co-invasion dynamics for effective pest management and conservation. European ash ( Fraxinus excelsior ) is simultaneously threatened by two invasive species: The fungus Hymenoscyphus fraxineus , which causes ash dieback (ADB), and the emerald ash borer (EAB), Agrilus planipennis , a phloem-feeding beetle. Although their invaded ranges overlap in parts of eastern Europe, it remains unclear how these species interact within the same host. We investigated whether recent ADB stem infections affect EAB performance, and whether such effects are mediated by changes in ash phytochemistry. Our findings indicate that neither larval nor adult EAB performance was affected when feeding on ADB-infected trees. Phloem chemistry showed localized changes near the ADB infection front but remained unaltered 18–20 cm away. Also leaf chemistry was only minimally affected by an ADB stem infection. These limited systemic responses suggest that F. excelsior relies primarily on constitutive phytochemical defenses against ADB. Therefore, ADB-induced phytochemical changes in recently infected trees do not affect EAB performance. We conclude that freshly ADB-infected trees neither promote nor inhibit EAB development. From a pest management standpoint, immediate sanitary removal of ADB-infected trees may not be necessary in stands only recently affected by the pathogen, as potential beneficial ADB infection effects on EAB are unlikely. However, the long-term impact of chronic ADB stress on EAB dynamics remains uncertain and warrants further investigation.