Interspecific variation in the response to low temperature storage in different aphid parasitoids

Cold storage of natural enemies usually involves placing insects under constant subambient temperatures. Even at non-freezing temperatures, a reduction in survival is the norm. Using fluctuating thermal regimes (FTR) instead of constant low temperature (CLT) has shown that mortality due to accumulation of chilling injuries was significantly reduced in Aphidius colemani. Whether this phenomenon can be generalised to other parasitoid species is not known. The aim of this study was to analyse interspecific variation in the ability to tolerate cold storage under CLT (continuous 2 degrees C) versus FTR (daily cycle: 2 degrees C for 22 h and 20 degrees C for 2 h) for various durations (0-20 days). Survival, sex ratio and development of five different Aphidiine parasitoids were analysed: A. colemani, Aphidius ervi, Aphidius matricariae, Ephedrus cerasicola and Praon volucre. A marked interspecific variation in the ability to tolerate cold storage was observed: A. matricariae and A. ervi were most chill tolerant, P. volucre and E. cerasicola had an intermediate chill sensitivity and A. colemani was most chill sensitive. In all species tested, FTR significantly reduced cold-induced mortality. This phenomenon was manifested more in chill-sensitive species as they probably accumulate chilling injuries more rapidly. The sex ratio remained unaffected in all the species. Interspecific variation was also observed in developmental responses to cold storage. Under CLT, time to adult emergence of A. matricariae, A. colemani, A. ervi and P. volucre was temporarily stopped and in E. cerasicola it increased. Under FTR, the short daily intervals at 20 degrees C for 2 h allowed parasitoids to continue development in all the species. Interspecific differences are discussed. This study suggests that positive impact of FTR may apply to a wide range of species.