RAPID COLD-HARDENING IN CULICOIDES-VARIIPENNIS SONORENSIS (DIPTERA, CERATOPOGONIDAE)

Rapid cold-hardening was studied in the adult stage of the biting midg e Culicoides variipennis sonorensis Wirth & Jones from a laboratory co lony. No individuals could withstand a direct transfer from 20-degrees -C (rearing temperature) to -10-degrees-C for 2 h; however, an acclima tion period of 1 h at 5-degrees-C immediately before exposure to -10-d egrees-C yielded 96% survival. The cold-hardening response was very ra pid; increased cold tolerance peaked after only a 1-h exposure to 5-de grees-C. No fly withstood 2 h at -20-degrees-C, demonstrating the limi tations of the cold-hardening response. Varying degrees of cold tolera nce also were induced by a 2-h exposure to 37-degrees-C. The ability t o rapidly cold-harden varied with the age of the adult insect. Rapid c old-hardening in adult C. v. sonorensis may be of ecological and epizo otiological significance because it is a mechanism by which a nondiapa using life stage quickly can enhance its tolerance to subzero temperat ures.