Drought-induced rapid cold-hardening in Eurosta solidaginis: Thresholds, response strength, and tissue-level induction

Rapid cold-hardening (RCH) is a highly conserved response in insects that induces physiological changes within minutes to hours of exposure to low temperature and provides protection from chilling injury. Recently, a similar response, termed drought-induced RCH, was described following acute desiccation. In this study, we investigated the limits and mechanisms of this response in larvae of the goldenrod gall fly Eurosta solidaginis (Diptera, Tephritidae). Cold-hardiness of larvae increased markedly with as few as 2 h of desiccation and a loss of less than 1% fresh mass, as organismal survival increased from 8% to 41% following exposure to a discriminating low temperature. Tissue-level effects of desiccation were observed in as few as 1 h, as 87% of midgut cells from desiccated larvae remained viable following cold shock relative to 57% of controls. We also demonstrated that drought-induced RCH occurs independently of neuroendocrine input, as midgut tissue desiccated ex vivo displayed improved freeze-tolerance relative to control tissue (78% to 11% survival, respectively). Finally, though there was an increase in hemolymph osmolality beyond the effects of osmo-concentration of solutes during dehydration, we determined that this increase was not due to the synthesis of glycerol, glucose, sorbitol, or trehalose. Our results indicate that E. solidaginis larvae are extremely sensitive to desiccation stress, which is a triggering mechanism for one or more physiological pathways that confer enhanced freeze-tolerance.