The impact of fluctuating thermal regimes on the survival of a cold-exposed parasitic wasp, Aphidius colemani

The impact of fluctuating thermal regimes on the cold tolerance of the parasitoid Aphidius colemani at the mummy stage is examined. The hypothesis is tested that, if a cold period is interrupted by a return to a higher temperature for a short time, a physiological recovery is possible and may lead to higher survival. Mummies are exposed to a constant temperature of 4 degrees C and, when periodic sudden transfers to 20 degrees C for 2 h are applied, survival of immature parasitoids is markedly improved, proportionally to the warming frequency. The time lag before emergence diminishes with the duration of cold exposure and with warming frequency. The sex ratio of emergent adults after cold exposure indicates that males may be more susceptible than females to cold-injury. It is suggested that the transfer to 20 degrees C allows a re-activation of the normal metabolism, leading to repair and recovery of any injuries caused by prolonged chilling. The study underlines the importance of cyclic temperature changes on insect survival.