Surveillance of Ash Trees Under Multiple Threats: Integrating Emerald Ash Borer and Ash Dieback Dynamics with Stakeholder Behaviour

Across the world, emerging and invasive pests and diseases are increasing in number, range, and co-occurrence with other pests. Ash trees (Fraxinus excelsior L.; Oleaceae) in Great Britain (GB) face a dual threat: 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. Kowal. (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, national 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 the spread of EAB.We developed a spatially explicit model to assess contingency strategies for EAB’s possible arrival and spread. Our model dynamically integrates: (i) A component predicting EAB arrival and spread, (ii) a static layer of ADB estimated prevalence, and (iii) a socio-dynamics component, based on a values-driven theory that simulates land-manager decision-making in relation to tree management. Interviews (n=45), a survey (n=368), and three workshops (n=27) informed the socio-dynamics model’s structure and parameterisation, with opinion dynamics capturing interactions over time. 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). In the model, if EAB is detected, national contingency measures—including tree felling and intensified monitoring—are enacted, with the potential to eradicate or slow its spread. We analysed three case-study areas with varying ash density and ADB prevalence. Results showed that if land managers are made aware of EAB, health and safety (H&S) 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 sites is similarly unlikely to succeed in early detection and subsequent eradication unless entry pathways are well-defined, and ash connectivity is low. Volunteer trapping has an important role to play in early detection and if this is subsidised EAB eradication becomes more likely.