Rapid cold-hardening of Drosophila melanogaster in a field setting

Although laboratory studies demonstrate that cooling to ecologically relevant temperatures and/or at ecologically relevant rates induces rapid cold-hardening (RCH) in a variety of insects, little is known of the induction of RCH in nature. In the present study, caged Drosophila melanogaster (1-2 days posteclosion) from a colony established with flies collected locally are placed in a field setting (i.e. in approximately 4-cm deep leaf litter beneath an apple tree in Mount Pleasant, Michigan) during late afternoon (18.00 h EST; 05.00 h GMT). As the cage temperature falls from 22.1 ± 0.8 to 10.1 ± 0.1°C between 18.00 and 06.00 h, the proportion of flies surviving a transfer to -6°C for 1 h increases from 10.0 ± 6.2% to 68.1 ± 7.2%. When obtained from field cages, and then cooled from 23°C at approximately 0.33°C min^-1, more female flies remain behaviourally responsive (clinging to surfaces, exhibiting an active righting response, and/or climbing) at temperatures of 8-12°C (24.00 h samples) or 7-12°C (06.00 h samples), than do those sampled from cages kept in an incubator (23°C). Field cooling reduces chill coma temperature from 8.7 ± 0.2°C at 06.00 h to 7.1 ± 0.2°C at 24.00 h, and to 6.6 ± 0.2°C at 06.00 h. These data demonstrate that, in a recently collected culture of D. melanogaster, natural changes in microenvironmental temperature induce RCH that can benefit the organism at temperatures encountered in nature.