Effects of the larval host plant on the supercooling capacity and physiological characteristics of beet armyworm pupae, Spodoptera exigua (Lepidoptera: Noctuidae)

Abstract

The beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), is a serious economic pest worldwide that overwinters as pupae in temperate regions. There is no information on the relationship between the larval host plant and the cold hardiness of S. exigua pupae (pre-overwintering feeding), whereas the acclimation temperature, thermoperiod, photoperiod, and soil moisture level are known to influence cold hardiness. Larvae incubated at 20, 23, and 26°C with a L12:D12 photoperiod were fed one of four host plants (pakchoi, cabbage, shallot and spinach) to determine the effect of larval host plant on energy reserves, supercooling capacity, and cryoprotectant level of pupae. The interaction between temperature and larval host plant was significant in regard to pupal fresh weight (FW), glycogen, lipid, supercooling point (SCP), body water content (BWC), and glycerol levels. Higher FW and glycerol content and lower SCP of pupae that were fed as larvae on pakchoi were observed at the lowest rearing temperature. Moreover, higher glycogen content of pupae from larvae reared on pakchoi was also observed at the highest rearing temperature. Glycerol and glycogen contents of pupae from larvae fed on pakchoi were negatively correlated with temperature. Our results suggest that S. exigua pupae enhance their low temperature tolerance by decreasing their SCP and accumulating cryoprotectant (i.e., glycerol) allowing the animals to tolerate the harsh environmental conditions of winter, provided that the overwintering generation of larvae fed on suitable host plants.