Diapause induction and termination: north-south strain differences in Ostrinia nubilalis.

Diapause induction and termination responses of a northern strain (Minnesota [MN]) of Ostrinia nubilalis were compared with those of a southern strain (Georgia [GA]). A thermoperiod in constant light (12 hr at 25 degrees C alternating with 12 hr at 4 degrees C) failed to induce diapause in GA larvae, but approximately 50% diapause induction was observed in the MN population. Moreover, the 50% of MN larvae that continued their development (i.e., underwent pupation and adult development) did so at a slower rate, as measured by days to pupation, than GA larvae. In the laboratory, diapausing MN larvae responded more slowly to the optimal light-dark (LD) cycle for terminating diapause, LD 16:8, than did GA larvae. In the field MN populations are univoltine (i.e., are characterized by one generation per year). A delayed termination response in the spring, coupled with a longer critical daylength for diapause induction as daylength decreases during late summer (earlier diapause) restricts the time during which development can occur as contrasted with GA populations. In addition, it is postulated that these two phenomena, coupled with a possibly slower growth rate in the MN insects as revealed under laboratory conditions, may collectively represent the basis for univoltinism in the field.