Cold-Hardiness and Supercooling Capacity in Diapausing and Nondiapausing Stages of the Cabbage Root Fly Delia radicum

Abstract

Supercooling point (SCP) values and cold-hardiness were measured in individual ontogenetic stages of Delia radicum (Diptera:Anthomyiidae) in various physiological states (winter diapause, summer quiescence, and normal development). Winter diapause-destined mature third-instar larvae had a lower SCP (-9.9°C) than their nondiapause counterparts (-5.2°C), and more of them survived exposure to -10°C for 5 h to pupariation and adult emergence. Values of SCPs were equal in both diapause and nondiapause states of prepupal and pupal stages. The lowest SCP (ca. -20°C) was found in the stage of phanerocephalic pupa (PCP) regardless of the physiological state. The cold-hardiness of PCP corresponded with a low SCP value only in diapausing pupae stored for 80 days at 3°C and in pupae which had terminated their diapause and whose further development was inhibited by storage at low temperatures (3°C). Such pupae survived exposure to temperatures close to their SCP (14 days at -17°C). However, this high cold-hardiness was only acquired after some time and/or exposure to 3°C, as the PCP at the beginning of diapause showed significantly impaired cold-hardiness despite the fact that their SCP was low. The cold-hardiness of nondiapausing PCP did not correspond at all to that of low SCP, as no pupa survived the exposure to -17°C for 1 day; survival rates at temperatures of -13.5 and -10°C were also remarkably lower than those in diapausing pupae. Cold-hardiness in D. radicum was closely connected with the diapause syndrome but the changes in SCP value corresponded rather with the ontogeny of this insect.