Rapid cold-hardening in Culicoides variipennis sonorensis (Diptera: Ceratopogonidae)

Rapid cold-hardening was studied in the adult stage of the biting midge Culicoides variipennis sonorensis Wirth & Jones from a laboratory colony. No individuals could withstand a direct transfer from 20 degrees C (rearing temperature) to -10 degrees C for 2 h; however, an acclimation period of 1 h at 5 degrees C immediately before exposure to -10 degrees C yielded 96% survival. The cold-hardening response was very rapid; increased cold tolerance peaked after only a 1-h exposure to 5 degrees C. No fly withstood 2 h at -20 degrees C, demonstrating the limitations of the cold-hardening response. Varying degrees of cold tolerance also were induced by a 2-h exposure to 37 degrees C. The ability to rapidly cold-harden varied with the age of the adult insect. Rapid cold-hardening in adult C. v. sonorensis may be of ecological and epizootiological significance because it is a mechanism by which a nondiapausing life stage quickly can enhance its tolerance to subzero temperatures.