Hardening trumps acclimation in improving cold tolerance of Drosophila melanogaster larvae

Chill-susceptible insects are able to improve their survival of acute cold exposure over both the short term (i.e. hardening at a relatively severe temperature) and longer term (i.e. acclimation responses at milder temperatures over a longer time frame). However, the mechanistic overlap of these responses is not clear. Four larval stages of four different strains of Drosophila melanogaster are used to test whether low temperature acclimation (10 degrees C for 48 h) improves the acute cold tolerance (LT90, similar to 2 h) of larvae, and whether acclimated larvae still show hardening responses after brief exposures to nonlethal cold or heat, or a combination of the two. Acclimation results in increased cold tolerance in three of four strains, with variation among instars. However, if acclimation is followed by hardening pre-treatments, there is no improvement in acute cold survival. It is concluded that short-term thermal responses (e.g. hardening) may be of more ecological relevance to short-lived life stages such as larvae, and that the mechanisms of low temperature hardening and acclimation in D. melanogaster may be antagonistic, rather than complementary.