Mild desiccation rapidly increases freeze tolerance of the goldenrod gall fly, Eurosta solidaginis: evidence for drought-induced rapid cold-hardening

Overwintering insects may experience extreme cold and desiccation stress. Both freezing and desiccation require cells to tolerate osmotic challenge as solutes become concentrated in the hemolymph. Not surprisingly, physiological responses to low temperature and desiccation share common features and may confer cross-tolerance against these stresses. Freeze-tolerant larvae of the goldenrod gall fly, Eurosta solidaginis (Diptera: Tephritidae), experience extremely dry and cold conditions in winter. To determine whether mild desiccation can improve freeze tolerance at organismal and cellular levels, we assessed survival, hemolymph osmolality and glycerol levels of control and desiccated larvae. Larvae that lost only 6-10% of their body mass, in as little as 6 h, had markedly higher levels of freeze tolerance. Mild, rapid desiccation increased freezing tolerance at -15 degrees C in September-collected larvae (33.3 +/- 6.7 to 73.3 +/- 12%) and at -20 degrees C in October-collected larvae (16.7 +/- 6.7 to 46.7 +/- 3.3%). Similarly, 6 h of desiccation improved in vivo survival by 17-43% in fat body, Malpighian tubule, salivary gland and tracheal cells at -20 degrees C. Desiccation also enhanced intrinsic levels of cold tolerance in midgut cells frozen ex vivo (38.7 +/- 4.6 to 89.2 +/- 5.5%). Whereas hemolymph osmolality increased significantly with desiccation treatment from 544 +/- 16 to 720 +/- 26. mOsm, glycerol levels did not differ between control and desiccated groups. The rapidity with which a mild desiccation stress increased freeze tolerance closely resembles the rapid cold-hardening response, which occurs during brief sub-lethal chilling, and suggests that drought stress can induce rapid cold-hardening.