We developed a reaction-diffusion model to describe the spatio-temporal dynamics of the Japanese beetle (Popillia japonica Newman), based on adult abundance data collected by the Regional Phytosanitary Service during the monitoring activities in the infested area in Lombardy (northern Italy), from 2015 to 2021. The model simulated the abundance of the pest, with a discrete time step of one year along linear trajectories departing from an initial point of establishment. The model allowed to determine the asymptotic spreading speed (i.e., the speed at which the leading edge of a population wave moves over time) of the pest along 13 different trajectories, and varied from 4.5 to 13.8 km/y, with a mean value of 8.2 km/y. Finally, we developed a land suitability index that summarises the effect of land use on the trajectory-specific asymptotic spreading speed of P. japonica. Specifically, it has been observed an increase in the spreading speed of 260 meters per year for an increase of a percentage point of the land suitability index. The model presented and the knowledge acquired in this work represent an important step forward in the comprehension of P. japonica population dynamics, and they represent important elements for the development of a decision support tool for pest risk managers to design and implement scientifically-driven management actions.