Modelling the spread of Popillia japonica Newman (Coleoptera: Scarabaeidae) from a recently infested area

The Japanese beetle (JB) Popillia japonica (Newman), has been detected for the first time in Northern Italy in 2014. This alien species is spreading fast Eastwards and Westwards, and this calls for mandatory restrictions within infested areas. To forecast spreading dynamics of P. japonica over years, an iterative spatially based model was conceived and developed. Five years of trapping data by the Regional Plant Protection Organization (RPPO) of Lombardy region were used to build up the model: 2015–2016 observations were used for model parametrization; 2017–2019 for validation. Model includes: a source dilution factor (SF) describing the rate of P. japonica moving away from a given position, based upon variograms; a value G, quantifying the increase in P. japonica population at the same location along years; a mortal- ity factor (MF) based on literature data; three land cover classes quantifying the probability of colonization of new areas. The model proved to perform from fair to good; validation showed that determination coefficient (R2) ranged between 0.39 and 0.87. Variograms from both trapping data and model simulations, suggested a range of spatial autocorrelation of P. japonica ranging from 7.5 to 15 km. Prediction maps indicate a potential spread of P. japonica mainly Southwards and South-eastwards, matching the distribution of suitable land covers (e.g. meadows, croplands, woods). As a result, this model can be retained operationally useful to predict the spread of P. japonica year-per-year, although a periodic/continu- ous calibration with new field data is suggested.