Diapause and overwintering of the hoverfly Episyrphus balteatus

The overwintering biology of the hoverfly Episyrphus balteatus (DeGeer) (Diptera: Syrphidae) was studied and analysed. In Europe, females of this species can overwinter as adults in a facultative, reproductive diapause. The diapause phase is characterized by the ovaries ceasing to develop and by hypertrophy of the fat body. Diapause was induced during the second and third larval instars. The critical photoperiod for inducing diapause was between 11.8 and 11.9 h, corresponding to the day lengths that occur during mid- to late September in Hannover, Germany. When temperatures were lower, insects could be induced into diapause at longer day lengths, similar to those that occur in early September in Hannover. A semifield study was done during the winter of 1999/2000 to confirm the results obtained under laboratory conditions and to obtain additional information on the overwintering development and mortality of E. balteatus. The results suggested that mortality was correlated with the duration of the experiment and with humidity, rainfall, and temperature. The importance of these results for the regulation of cereal aphid populations by this hoverfly is discussed.