A model for the spatial population dynamics of the European invasion of the Japanese beetle (Popillia japonica)

Popillia japonica is an invasive beetle first detected in Northern Italy in 2014 and considered as one of the most dangerous pests for crops due to the economic, environmental, and social costs by European phytosanitary authorities. Here we develop a process-based model for quantifying the invasion dynamics of P. japonica with the goal to elaborate risk maps for the design of surveillance and control strategies. The proposed spatially heterogeneous model captures the different behaviors of both adults, which drive the invasion through flight and short-distance human-mediated transport, and ovo-larval stages, which affect demography due to their susceptibility to environmental conditions. Both processes are biologically informed through variables that are known to influence population dynamics, namely the number of summer days, affecting the flight ability of the adults, and the environmental suitability for larval survival. The simulated presence of adult beetles is then contrasted to pest observation data at the municipality scale from Northern Italy and Switzerland from the initial invasion to December 2023. Results reveal that the ongoing outbreak is occurring in one of the most favorable areas for the beetle, the Po Valley, which favors rapid spread and can sustain high population densities. Mountain regions emerge as obstacles to the progression of the invasion, with the Alps, excluding the valleys, being an almost insurmountable defense, whereas the Apennines only slow down the southward advancement without stopping it altogether. The projected speed of invasion, within and outside the outbreak area, ranges between 0 and almost 20 km/year and it is modulated by environmental conditions. The predicted speed of invasion at continental level constitutes an actionable tool for invasion management, since it provides phytosanitary services with a critical evidence base for optimizing the geometry and extent of delimitation and containment areas.