Crystallization temperature and chilling injury during overwintering in a feral face fly (Diptera: Muscidae) population

Adult face flies, Musca autumnalis DeGeer, overwinter in reproductive diapause. In this study, we monitored temperature in a hibernaculum, and periodically assessed cold hardiness of flies from this site. Of flies collected in Keats, KS (39 degrees 13' N, 96 degrees 42' W) in late December and January, approximately 40% exposed to -15 degrees C for 5 h froze internally, whereas, the remainder successfully supercooled. Of flies collected in February, approximately 70% froze internally. All flies that froze internally died. In December and January, all flies that did not freeze recovered fully and immediately, whereas in February nearly 80% of the flies that avoided freezing were killed or impaired. Unfed flies held at room temperature (23 degrees C) for 8 h and then subjected to -15 degrees C for 5 h had low crystallization temperatures, but were killed by chilling injury. Flies that were warmed, allowed food, and then held at 4 degrees C overnight before being exposed to -15 degrees C had higher crystallization temperatures than unfed flies. These data indicate that fully acclimatized face flies in midwinter are susceptible to freezing but not direct chilling injury, and that susceptibility to freezing and chilling injury in overwintering flies increases progressively with time. The laboratory studies show that, when warmed, diapause-induced and cold-acclimatized flies will take available food. Results from the laboratory studies suggest that flies warmed by exceptionally mild temperatures in midwinter are more susceptible to chilling injury from subsequent sudden cold weather, and that feeding when warmed increases crystallization temperatures. Higher crystallization temperatures could cause mortality under more mild conditions conditions that those in our test