Behaviour and cold hardiness of the purple stem borer in winter, colonizing more northerly latitudes

To escape or alleviate low temperatures in winter, insects have evolved many behavioral and physiological strategies. The rice pest insect, the purple stem borer, Sesamia inferens (Walker) is currently reported to be expanding their northern distributions and causing damage to summer maize in Xinxiang, China. However, their method of coping with the lower temperature in the new northern breeding area in winter is largely unknown. This paper investigates the overwinter site of S. inferens, and identifies the cold hardiness of larvae collected from a new breeding area in winter and explores a potential distribution based on low temperature threshold and on species distribution model, MaxEnt. The results show that the overwintering location of the S. inferens population is more likely to be underground with increasing latitude and,in the north, with the temperature decreasing, the larvae gradually moved down the corn stalk and drilled completely underground by February 18th. Those who were still above ground were all winterkilled. The cold hardiness test shows the species is a moderate freeze-tolerant one, and Supercooling Points (SCP), Freezing Points (FP) and mortality rate during the middle of winter (January, SCP: -7.653, FP: -6.596) were significantly lower than early winter (October) or late winter (March). Distribution in the new expansion area was predicted and the survival probability area was below N 35{degrees} for the Air Lower Lethal Temperature (ALLT50) and below N 40{degrees} for the Underground Lower Lethal Temperature (ULLT50), The suitable habitat areas for S. inferens with MaxEnt were also below N 40{degrees}. This study suggests the overwinter strategies have led to the colonization of up to a five degree more northerly overwintering latitude. This behavior of S. inferens could help maize producers to propose a control method to increase pest mortality by extracting the maize stubble after harvest.