Physiological correlates of chill susceptibility in Lepidoptera

The majority of insects enter a state of reversible coma if temperature is lowered sufficiently. If the cold treatment is not too severe these insects recover gradually when returned to benign temperatures in a time-dependent manner that often depends on the duration and intensity of the cold exposure. Previous studies have associated these phenotypes to changes in membrane potential (V_m) and ion balance, and especially hemolymph [K⁺] is known to be of importance for the recovery time. In the present study we examined this link in three species of Lepidoptera as insects from this order are known to possess resting hemolymph [K⁺] that would severely compromise V_m in other insects. Specifically, we exposed larval and adult Manduca sexta, larval Bombyx mori, and adult Heliconius cydno to stressful cold (0°C) for extended periods of time. Subsequently we measured chill coma recovery time (CCRT), ion- and water balance, and muscle V_m. As expected we find that resting hemolymph [K⁺] is high and that resting hemolymph [Na⁺] is low compared to most other insect species. Muscle V_m depolarised considerably during acute cold exposure, but did so in a manner that was not associated with changes in ion balance. However, prolonged cold exposure coincided with an increase of hemolymph [K⁺] and further depolarisation of V_m which correlated well with prolongation of CCRT. Combined this demonstrates how insects with different ionic compositions generally suffer from similar consequences of cold stress as other species, such that cold tolerance of chill-susceptible insects within Lepidoptera is also intimately linked to maintenance of ion balance and membrane polarisation.