Abstract:

 In order to clarify the effects of body size and fat content on cold tolerance in adults of Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), two developmental temperatures (18℃ and 25℃) were used to induce a phenotypic variation in insect size and the relationships were analyzed among the adult size, fat reserves, the supercooling point (SCP) and the survival rate exposed to constant low temperatures (CLTs) or fluctuating thermal regimes (FTRs) in the laboratory condition. The results showed that the adults grown at the lower temperature (18℃) were significantly larger than those reared at 25℃ (P<0.01). The adult fat content was positively related to dry mass of body, indicating that larger individuals contained a higher proportion of fat. The SCP frequency distribution showed that the lower SCPs (-10℃- -9℃) were found in adults bred at the higher temperature (25℃), whereas the higher SCPs (-8℃- -6℃) in adults bred at the lower temperature (18℃). The negative linear relation was observed between SCP and adult body size. The survival rate, especially under CLT, decreased significantly with duration of coldexposure. The type of exposure (FTR versus CLT) had a dramatic impact on the survival rate, which was distinctively higher under FTR than under CLT as in the following order: FTR18℃＞FTR25℃＞CLT18℃＞CLT25℃. The results indicate that energy storage is an important factor, but the vulnerability to chill-injury is supposed to be the primary factor regulating survival at low temperature.

Key words: Harmonia axyridis, chill-injury, supercooling point, body size, fat reserves, fluctuating thermal regimes