Phenotypic plasticity and geographical variation in the pre‐reproductive period of Autographa gamma (Lepidoptera: Noctuidae) and its implications for migration in this species

Adults of the noctuid moth Autographa gamma undertake seasonal migrations into areas where they are unable to breed continuously. Individuals migrate into Britain each spring, and offspring of these migrants probably return in autumn to over‐wintering areas in North Africa and the Middle East. However, the existence of these return migrations has been questioned. Insects usually migrate during the adults' pre‐reproductive period (PRP). The length of this period is therefore an index of migratory potential since individuals with longer PRPs have more time to express their potential for flight and to travel further before becoming sexually mature. Offspring of insects from Morocco, Britain and Germany showed that, under both long and short photoperiods, PRP increased with latitude over the range 34–53°N. There were significant genotype—environment interactions in the effect of photoperiod on length of PRP. However, PRPs of offspring from insects collected in Sweden were no different under a long photoperiod from those of offspring collected further south, and were significantly shorter under a short photoperiod than those of insects from Morocco. With the exception of Swedish moths, offspring from insects collected at higher latitudes had longer PRPs and, therefore, the potential to travel further during migrations. The ability of some individuals to respond to environmental cues signalling habitat deterioration will further increase their potential to return to over‐wintering areas further south.