Cold hardiness and sugar content in photo-insensitive individuals of the cotton bollworm Helicoverpa armigera

A proportion of Helicoverpa armigera collected from fields in Okayama Prefecture (Western Japan; 34.6°N, 134.1°E) does not enter diapause when reared under a short days at 20°C during the larval stages. However, diapause in such photo-insensitive individuals can be induced when they are reared at moderately low temperatures, such as 15°C, regardless of photoperiod. To determine whether such photo-insensitive individuals can survive overwintering in fields, the present study compares the cold hardiness and sugar content between nondiapausing and diapausing pupae of photo-insensitive individuals selected over several generations at 20°C under a short day photoperiod (LD 10 : 14 h). Diapausing and nondiapausing pupae are obtained under the short days by rearing at 15 and 20°C, respectively, during larval and pupal stages. These pupae are stepwise acclimated at a reduction of 5°C every 5 days to 0°C. Maximum survival periods of nondiapausing and diapausing pupae at 0°C are approximately 30 and 90 days, respectively. Trehalose content in diapausing pupae increases, reaches a maximum level (1.95 mg 100 mg^-1 in males and 2.1 mg 100 mg^-1 in females) 28 days after exposure to 0°C and then decreases. On the other hand, glucose content in diapausing pupae increases (maximum level: 0.32 mg 100 mg ^-1 in males and 0.21 mg 100 mg^-1 in females) with decreasing trehalose content 42 days after exposure to 0°C. The decrease in trehalose content and the increase in glucose content may be linked to termination of diapause in H. armigera. These results suggest that, in Japan, the photo-insensitive individuals can only survive in the mild winters of southern regions, and not in the severe winters of northern regions.