Behavioral and Physiological Adaptation to Soil Moisture in the Overwintering Larvae of the Rice Stem Borer in the Subtropics

Diapausing larvae of the rice stem borer, Chilo suppressalis Walker, overwinter in rice stubble. During overwintering, the larvae may move to sites with suitable moisture and undergo physiological changes to prepare for the declining temperature. This study measured the behavioral and physiological adaptation to soil moisture (25%, 50%, and 75% of saturated soil water content) in the diapausing larvae at 30, 60, and 90 days of treatment. The results showed that the diapausing C. suppressalis larvae behaviorally exhibited hygrotaxis and distributed mainly (65%) in the lower part (0–10 cm above the soil level) of the rice stem where the moisture was higher. Physiologically, the insects showed significantly decreased glycogen content and weight whereas increased trehalose content with decreasing soil moisture. In the subtropics where this study was conducted, the supercooling points of the insects were lower than the lowest ambient temperature, and the soil moisture had no significant effects on the cold hardiness (supercooling point) and survival of the diapausing C. suppressalis larvae. The decreased larval weight at low soil moisture may reduce the post-diapause reproductive potential of the larvae, which may open the potential of developing agronomic measures-based management of the overwintering C. suppressalis population.