Assessment of the suitability of the Finnish climate for Popillia japonica
Popillia japonica is a priority pest which all EU countries must survey annually. However, surveys are not required if the climate in the country is unsuitable for the pest. In previous research, it was unclear whether the climate in Finland was suitable for P. japonica.
Based on a literature review, we identified soil moisture and temperature as critical factors for the establishment of P. japonica. The suitability of soil moisture for P. japonica in Finland was assessed by comparing the summer rainfall across Finland to the corresponding rainfall within the known range of the pest. The suitability of the winter soil temperature was assessed by comparing the winter soil temperature across Finland with the minimum temperatures that P. japonica has been reported to endure. The suitability of summer soil temperatures was assessed by comparing the annual growing degree days (GDD) across Finland with a the annual GDD which P. japonica has, in previous assessments, been considered to require to complete its life cycle (i.e., 711 above 10 °C).
The results indicate that the soil moisture and winter soil temperature are suitable for P. japonica throughout Finland. However, the summers appear to be too cool for the pest throughout the country since the threshold GDD was not reached every year anywhere, and even in the warmest areas, it was reached only in 75% of the years.
The greatest sources of uncertainty in the assessment are the uncertainty of the annual GDD that P. japonica needs to complete its life cycle and the lack of research on the pest’s cold tolerance.
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