Exploring the main factors influencing habitat preference of Popillia japonica in an area of recent introduction

The possibility of establishment of an invasive species in an area of new introduction is influenced by several factors, including weather conditions, soil characteristics, and species spread dynamics. Disentangling the role of multiple variables on habitat preference can provide important elements supporting the management of invasive species. In this work, we present a modelling framework that allows i) the identification of the main drivers influencing the probability of the presence of the invasive pest Japanese beetle Popillia japonica, in an area of recent introduction in Northern Italy, and ii) the evaluation of bi-dimensional niches to investigate the non-linear combined effects of couples of variables on the potential distribution of this species. Occurrence records of larval populations sampled between 2017 and 2019 in 8488 sites in Northern Italy were used in the present study. The sample sites were characterised based on a set of explanatory variables related to land use, soil characteristics, meteorological conditions, and infestation history. We performed a stepwise logistic regression analysis for testing the statistical significance of explanatory variables on the occurrence of local larval populations. Then, we implemented a Generalized Additive Model to investigate the bi-dimensional ecological niche determined by the interaction of two variables on the probability of the presence of P. japonica. The results highlight the prominent role of soil and weather variables and the infestation history of the species in driving the presence of P. japonica larvae. Less acidic soils, especially with sandy-skeletal particles, are preferred. A high density of P. japonica larvae is associated with medium content of soil organic carbon in the soil. Long drought periods or high precipitation levels reduce the probability of the presence of the species. Non-linear effects are observed between soil temperature variables and habitat preferences of P. japonica. The probability of the presence of P. japonica is also related to the number of years since the first occurrence of the species at the site. The results of the present work provide fundamental elements for guiding the assessment of the risk and the management of P. japonica.