COLD TOLERANCE OF THE APHID PREDATOR EPISYRPHUS-BALTEATUS (DEGEER) (DIPTERA, SYRPHIDAE)

Larvae of the hoverfly Episyrphus balteatus (DeGeer) are important pre dators of aphids in the U.K. A large proportion of the U.K. population migrates south to warmer dimes at the end of summer, but a small numb er are thought to overwinter in the U.K., with the mated female being the overwintering morph. The cold tolerance of adult flies was investi gated to assess the overwintering potential of E. balteatus in the U.K . The high supercooling point (SCP) of -8.3 +/- 0.7 degrees C, and let hal temperature (LTemp(50)) of -9.1 degrees C for acclimated females s uggest that E. balteatus has limited cold hardiness. This was confirme d by experiments where, despite a strong acclimation response in both males and females, there was no long-term survival at 5, 0 or -5 degre es C. At 5 degrees C, 90% of females had died after 10 days. The weak cold hardiness of adult E. balteatus was corroborated by field experim ents which demonstrated a 100% mortality after 10 weeks' exposure to U .K. winter conditions. The ecological significance of this limited col d hardiness is discussed in relation to the overwintering abilities of E. balteatus in the U.K.