Surveillance of ash trees under multiple threats: Integrating emerald ash borer and ash dieback dynamics with stakeholder behaviour

1.Across the world, emerging pests and diseases are increasing in number, range and co-occurring with other invasive biotic factors. Ash trees (Fraxinus excelsior L.; Oleaceae) in Great Britain face the potential invasion of the emerald ash borer (EAB; Agrilus planipennis Fairmaire; Coleoptera: Buprestidae) and the ongoing impact of ash dieback (ADB; Hymenoscyphus fraxineus T. Kowalski (Helotiales: Helotiaceae)). Surveillance and management strategies accounting for land manager behaviour are crucial for improved control. 2.We developed a spatially explicit model that integrates (i) the estimated prevalence of ADB, (ii) the dynamics of EAB arrival and spread and (iii) a socio-dynamics model, based on a values-driven theory that simulates land manager decision-making in relation to surveillance and tree management. In the model, if EAB is detected, contingency measures—including tree felling and intensified monitoring—are enacted, with the potential to eradicate or slow its spread. We used the model to assess whether targeting high-risk sites with traps, using routine tree inspections by land managers, or encouraging volunteer surveillance (with or without subsidised trapping) could significantly slow EAB spread. 3.Interviews (n=45), a survey (n=368), and three workshops (n=27) informed the socio-dynamics model's structure and parameterisation. The interaction between EAB and ADB is complex, with potential positive effects (e.g. increased perceived value of ash) and negative effects (e.g. belief that ash cannot be saved, misidentification of decline causes). 4.Results showed that if land managers are made aware of EAB, health and safety inspections have a substantial role to play in slowing the spread but are unlikely to lead to eradication due to the pest's cryptic larval phase. Official trapping at a limited number of locations is similarly unlikely to succeed in early detection and eradication unless entry pathways are well-defined, and ash connectivity is low. Volunteer trapping is important for early detection and if this is subsidised, EAB eradication becomes more likely. 5.Synthesis and applications. Tree-health policies must balance identifying likely entry points and deployment of traps, targeted information campaigns and surveillance subsidies for land managers. Our unique, cross-disciplinary approach can be applied to other pest/pathogen systems to inform tree-health plans and how to balance resources.