Cold tolerance of the aphid predator Episyrphus balteatus (DeGeer) (Diptera, Syrphidae)

Abstract. Larvae of the hoverfiy Episyrphus balteatus (DeGeer) are important predators of aphids in the U.K. A large proportion of the U.K. population migrates south to warmer climes at the end of summer, but a small number are thought to overwinter in the U.K., with the mated female being the overwintering morph. The cold tolerance of adult flies was investigated to assess the overwintering potential of E. balteatus in the U.K. The high supercooling point (SCP) of -8.3 ± 0.7°C, and lethal temperature (LTemp₃₀) of -9.1°C for acclimated females suggest that E. balteatus has limited cold hardiness. This was confirmed by experiments where, despite a strong acclimation response in both males and females, there was no long-term survival at 5, 0 or - 5°C. At 5°C, 90% of females had died after 10 days. The weak cold hardiness of adult E. balteatus was corroborated by field experiments which demonstrated a 100% mortality after 10 weeks' exposure to U.K. winter conditions. The ecological significance of this limited cold hardiness is discussed in relation to the overwintering abilities of E. balteatus in the U.K.