Early detection of bark beetle (Ips typographus) infestations by remote sensing – A critical review of recent research

Bark beetle disturbances increasingly threaten structure and functionality of temperate and boreal forests globally. The early detection of bark beetle-infested trees, i.e. before beetles' emergence from the breeding tree, is essential for an effective outbreak mitigation. Terrestrial control surveys as traditionally employed for infestation detection, however, are resource-intensive and approach their limits in difficult terrain and during mass outbreaks. Developments in remote sensing and detection algorithms are increasingly giving hope that early-infested trees will be detectable remotely, thereby improving control success and management efficacy. Yet, a comprehensive quantitative evaluation of the approaches currently being developed is lacking to date. This review synthesises the state-of-the-art of recent research on early infestation (or green-attack) detection by remote sensing, and places it in the context with underlying biological constraints, technical opportunities and potential management applications. Since each bark beetle-host tree system has specific characteristics and detectability, we focus on the system with the greatest impact on European forests, the European spruce bark beetle (Ips typographus), which attacks Norway spruce (Picea abies). By screening research published within the period 2000–2022, we included 26 early detection studies in our analyses. All studies reviewed were purely exploratory, testing a variety of data and/or classification algorithms with a relatively limited spatial and temporal coverage. Among tested platforms and sensor types, satellite and multispectral imagery were most frequently investigated. Promising spectral wavelength range or index highly varied among studies and regions. Timeliness and accuracy of detection were found to be insufficient for efficient management, regardless of the tested platform, sensor type, and spatial resolution applied. The main reasons preventing better performance include the rapid development of I. typographus in combination with the delayed and highly variable vitality response of the spruce crown, and frequent cloud cover in spruce-dominated regions across Europe. In conclusion, current remote sensing survey methods cannot yet replace terrestrial control surveys for timely bark beetle management. Nevertheless, they might be supportive either as a back-up to regular and frequent terrestrial surveys, or in specific situations, e.g. to detect hibernation trees, in terrain with difficult accessibility, or in extensively managed forests without sufficient survey capacity. We suggest that the term 'early detection' be used consistently as a synonym for 'pre-emergence detection' to avoid ambiguity. Finally, we provide recommendations for future research based on the lessons learned from the studies analysed, namely to use a more rigorous and targeted study design, to ensure interdisciplinarity, and to communicate research results explicitly.