Association of cold acclimation with freezing tolerance in the beet armyworm, Spodoptera exigua (Lep.: Noctuidae)

The beet armyworm, Spodoptera exigua (Hubner), is one of the most important pests of sugar beet that overwinter as mature larvae or pupae in the soil. In this research, the influence of cold acclimation in full-grown larvae and tolerance at temperatures below their body supercooling point (SCP) was studied. Moreover, the influence of soil humidity was investigated on temperature that pest may expose. After six different temperature treatments on lab-reared mature larvae, cold tolerance of the larvae was measured under two different programs executed on them. In the first program, larvae were acclimated by cooling rate of 1°C/min and reached to -5°C or -15 °C, for 24h then it was raised to +25°C. During second program the temperature was decreased with much lower cooling rate (0.01°C /min) to -15°C and kept for 2h then it was increased again to +25°C. These two temperature programs were also performed on different soil humidity (dry, humid and humid covered with an ice layer). The results showed that these pest larvae will only be able to tolerate -15°C when they were acclimated with slow cooling rate to reach below their SCP. These findings indicated that cold acclimation at sub-zero may induce expression of freezing tolerance in these larvae. It was shown that humid soils especially those covered with an ice layer, are more resistant against temperature decreasing rather than dry soils and could play an important role in the cold-hardiness strategy and mortality of this pest under low temperatures.