CHARACTERIZATION AND LIMITATIONS OF THE RAPID COLD-HARDENING RESPONSE IN THE HOUSEFLY MUSCA-DOMESTICA (DIPTERA, MUSCIDAE)

The characteristics and limitations of the rapid cold-hardening response in the housefly Musca domestica were investigated in studies on 4-day-old pupae (pharate adults). The mean supercooling point of the pupae was -16.2°C, but none survived direct transfer from 27 to -7°C for 2 h. In contrast, when pupae were acclimated at 0°C for 3 h, 88% survived after 2 h at -7°C. The cold tolerance gained after 3 h at 0°C was rapidly lost when pupae were returned to 27°C for 2 h. Cold tolerance was enhanced by increasing the period of exposure at 0°C to 24 h (32% survived after 2 h at -11°C), but mortality increased in acclimation periods of more than 24 h at 0°C, in the absence of any exposure to lower temperatures. Increased cold tolerance was induced in pupae cooled from 27°C to 0 or -7°C at 0.1°C min-1, but not at 0.5°C min-1 or higher rates of cooling; the rapid cold-hardening response is therefore a function of the duration of exposure at, or close to, the acclimating temperature of 0°C, rather than the abrupt change in temperature which accompanies the direct transfer of pupae from the culture (27°C) to the acclimating temperature. © 1990.