2.5 Distribution based on low temperature threshold

Daily minimum temperatures in 47 different locations (Henan province, Hebei province, Tianjing city and Beijing city, formed a south-north belt in the north plain of China, in order to illustrate the northern overwinter site of S. inferens) from 2007 to 2014 were collected from national weather stations (https://data.cma.cn/). The winter temperatures from November to March were recorded in the air (10 cm from the soil surface) and underground (at 2 cm soil depth) by the two microclimate sensors mentioned above and it was found that there is an obvious significant linear relationship between lowest daily air and lowest underground temperature (R² = 0.9161, p<0.001, n = 100). Based on this linear model, the minimum daily air temperatures in different locations were translated into the minimum daily soil temperature at 2 cm underground because the underground data were not available at national weather stations, and the landscape and soil composites of the 47 locations were very similar. Another low temperature threshold, Underground Lower Lethal Temperature (ULLT₅₀) was also translated by linear equation and the value was -4.5°C, i.e. when the air temperature was -9.7°C the soil temperature postulated reached -4.5°C. Forty-seven location survival rates of air or underground in each year were speculated by predicting all larvae would die when the annual daily minimum temperature was lower than the ALLT₅₀ or ULLT₅₀, respectively, i.e. if the annual minimum temperature is lower than the ALLT₅₀ or ULLT₅₀, 0% of the population will survive. One hundred percent will survive if the annual minimum temperature is higher than the ALLT₅₀ or ULLT_50. The calculation of survival rates in different locations from 2007 to 2014, resulted in identification of the total proportion of insect survival over the 8 years. Based on the proportion of each location, a potential distribution based on annual minimum temperature threshold could be produced with ArcMap (version 10.2, ESRI, Redlands, CA, USA) [24].