Beet armyworm, Spodoptera exigua (Hübner), is a freeze-susceptible and migratory species, but can overwinter in temperate areas without diapause. Most developmental stages reared under constant environments are able to induce a cold-hardening process in response to a brief exposure to cool temperature. In this study, we analyzed the effect of daily temperature and photoperiod cycles on the cold-hardening process. Rearing temperatures significantly affected cold tolerance of larvae. In constant temperature regimes, as rearing temperature decreased, cold tolerance increased. Cyclic temperature regimes caused a significantly higher cold tolerance than did constant temperatures with the same daily average. In cyclic regimes with three combinations of a thermophase temperature and three different cryophase temperatures below a developmental threshold (13°C), cryophase temperature had a significant effect on cold tolerance. Cold-hardening induced by rearing temperatures was associated with decreases of supercooling points and increases of hemolymph osmolalities and glycerol contents. A cyclic photoperiod produced a higher cold tolerance than did constant dark or light. There was, however, no variation in cold tolerance among different cyclic daylength regimes. These results indicate that fluctuating temperature and photoperiod significantly affected cold tolerance of S. exigua.
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