Effects of exposures to repeated heat stress on the survival of the pea aphid Acyrthosiphon pisum and its endoparasitoid Aphidius ervi

Organisms could be exposed to several heat waves during their life, and their ability to survive a heat wave strongly depends on the effects of the previous one. Exposure to extreme temperatures can have important effects on the outcome of host-parasitoid interactions, as the ability of the parasitoid to survive depends on the ability of its host to cope successfully with these stresses. In the present study we address the impact of repeated exposure to heat stress on the survival of the pea aphid Acyrthosiphon pisum (Harris) (Hemiptera Aphididae) and its endoparasitoid Aphidius ervi Haliday (Hymenoptera Braconidae). The first treatment consisted of a heat stress of 35 degrees C for 30 minutes performed on 4 days old aphids, the second and third heat stresses of 39 degrees C were performed on 5 days old and on adult aphids, respectively. The three treatments were applied alone or in all their combinations. We found that aphid thermal tolerance is positively influenced by heat hardening if a severe stress occurs a few days after the first event. Adult parasitized aphids show significantly higher survival than unparasitized ones; however, the effects of parasitization and hardening on host survival after heat shock are not additive. We also found that A. ervi has a lower thermotolerance capacity than its host and does not show apparent hardening effects. In addition, parasitoid survival after mummification is not affected by the previously experienced heat shock. The possible explanations of the observed phenomena are discussed.