Changes in supercooling point and glycogen reserves in overwintering and lab-reared samples of beet armyworm, Spodoptera exigua (Lep.: Noctuidae) to determining of cold hardiness strategy

The beet armyworm, Spodoptera exigua (Hübner), is a highly polyphagus insect that feeds on many cultivated hosts and weeds. To understand how well this pest survives in cold temperature during winter, seasonal variation in the supercooling points (SCPs) were studied in field-collected and lab-reared samples. In this study, the beet armyworm overwintered as dark mature larvae in dried fallen leaves or on the soil surface. Their SCPs increased significantly (P<0.01) from -12°C in November to -6°C in March and these larvae could survive below their SCPs. However lab-reared ones could not survive below their SCPs. Supercooling points of different stages of lab-reared samples changed significantly (P<0.001) and it was lowest in eggs (-28°C). Trehalose, glucose, glycerol and sorbitol were identified as main cryoprotectants. Glycogen contents were significantly (P<0.01) decreased (more than 70%) in larvae collected in February and March rather than overwintering larvae collected in November and December. In different stages of lab-reared samples it changed significantly and was highest in pupae. So, it seems that overwintering larvae of beet armyworm are freeze tolerant and glycogen is a major reserve for overwintering during cold months.