Critical photoperiod and daylength threshold differences between northern and southern populations of the butterfly Limenitis archippus

Laboratory studies show that different photoperiods induce diapause in northern (Vermont) and southern (Maryland) larval strains of the butterfly Limenitis archippus. The northern strain responds to $\text{1}\text{2}\text{hr}$ longer photoperiod thresholds and critical ranges than does the southern one. These responses are correlated with geographic differences in the ambient photoperiod of the two localities. In this facultative diapausing species, third instar larvae construct hibernacula within the basal portions of tubular leaves spun with silk, when daylength approaches either 13·5 hr (Vermont strain) or 13·0 hr (Maryland strain). When reared in total darkness some larvae develop directly to fourth inszar without diapause, although mortality is high. Among both strains different broods exhibit different incidences of diapause. Reciprocal inter-strain hybrids show intermediate diapause responses, suggesting that larval diapause is under the control of multiple genes.