Global estimation of invasion risk zones for the western corn rootworm Diabrotica virgifera virgifera: integrating distribution models and physiological thresholds to assess climatic favourability.
Biological invasions represent a major threat to human health, ecosystem functioning and global biodiversity. Insect pests affecting agriculture and forestry are of special importance. Estimations of climatic similarity between a species' native range and potential zones of invasion can be useful for preventing new invasions, spreads and ulterior contacts among populations from multiple invasions. We estimated areas climatically favourable for the establishment of the western corn rootworm (WCR), an insect pest of maize Zea mays in North America that has recently invaded Central Europe through multiple invasions, and it has the potential of invasion mainly in the Northern Hemisphere. We used complementary techniques to assess the biological relevance of predictors and obtain areas of climatic favourability. The biological relevance of variables was first assessed accounting for two main components of the WCR's environmental niche (marginality and specialisation). Then, the most relevant predictors were used to obtain either climatic envelopes or environmental distances regarding the WCR's native range. Model outputs and predictor relevance were independently assessed in the currently invaded region of Europe and through the spatial projection of proposed physiological thresholds from previous empirical studies. Lastly, as examples of application for given time periods, we fed back results of environmental distances with maize data for a 10-year period in Europe, and refined global risk maps with the main maize zones for the year 2000. We present global zones of climatic favourability and invasion risk for the WCR, with emphasis on the Northern Hemisphere. The northern and north-west range limits predicted by the climatic envelope in the WCR's native range mirrored the independently characterised physiological limits. Also, our model outputs explained some of the patterns observed in Europe supporting the validity of our procedures. Synthesis and applications. Assessments of climatic favourability for the western corn rootworm can provide information on areas of invasion risk. Our study highlights the combination of holistic and reductionist approaches as a useful protocol to evaluate models and/or infer causality. Our methodology can be an efficient tool in combating future potential invasions, spreads and secondary contact zones of insect pests by reducing uncertainty regarding where to allocate prevention and/or eradication efforts.