Effect of temperature and photoperiod on the calling behaviour of a migratory insect, the oriental armyworm Mythimna separata

The pre‐calling period (PCP), between emergence and first release of pheromone, determines the number of nights over which female Mythimna separata (Walker) (Lepidoptera: Noctuidae) can express their migratory potential. In its seasonal migrations in eastern Asia, this armyworm penetrates to latitudes at which it is unable to over‐winter but at which reliable photoperiodic and temperature cues anticipate the deterioration of conditions. The effect of these environmental factors on PCP was examined in moths descended from samples collected in Nanjing (32 o N) during the summer migration. Under LD 14:10h, a reduction in temperature, from 23 o C during larval development to 18 o C from 24 h after pupation, resulted in a twofold increase in PCP compared with insects held at 23 o C. At 23 o C, short days (LD 12:12 h) extended PCP but the magnitude of the response was increased under a regime of decreasing photoperiod, from LD 15:9h, to LD 12:12 h through larval, pupal and adult development. Further experiments demonstrated that the response depended on a decrease in photoperiod from pre‐pupal stage to adult emergence (the pupal period is spent below ground). A reduction of 50–60min over the 13–15 day pupal period at 23 o C was effective. The significance of decreasing photoperiods, irrespective of absolute levels, as a cue for species whose migrations traverse substantial ranges of latitude is considered and the results are discussed in relation to the genetic determination of PCP in M.separata and to its seasonal migrations in eastern China.