Laboratory‐simulated naturally‐decreasing day lengths, twilight and aphid photoperiodism

. Day‐length changes, as well as periods of twilight, that occur in the course of each natural day‐night cycle, were recreated for two chosen latitudes in a computer‐controlled ‘natural‐day‐length simulator’.The photoperiodic responses of two aphid species, Aphis fabae and Megoura viciae , were examined in conditions which mimicked late summer to autumn at two simulated latidudes, 51.5d̀N (e.g.Ascot, Southern Britain) and 60°N (e.g.southern tip of Shetland Islands, Northern Britain), with temperatures between 16°C (‘night’) and 18°C (‘day’).The responses under simulated natural photoperiodic conditions were similar to those observed under conventional experimental conditions of squarewave light‐dark cycles (with abrupt lights‐on and lights‐off and constant light intensities during the light phase): both aphid species responded to civil twilight as light, and the critical day lengths (including civil twilight) for the induction of sexual morphs by the two aphid species observed in the simulator were the same as those found in squarewave light‐dark cycles. Autumn field experiments (51.5°N) with the same clones of A. fabae and M.viciae revealed much longer critical day lengths for gynopara and male induction in A. fabae compared with those in the laboratory, but the same critical day length for ovipara induction in M.viciue. Minimal night temperatures in the field were on average 6°C, whereas maximal day temperatures declined from around 30°C in early September to 12°C at the end of October; it seems that the critical day lengths in A.fabae are temperature dependent, whereas the findings for M.viciae confirm that the critical day length is temperature compensated.