Diapause induction
The photoperiods experienced during the larval period strongly affected
the induction of pupal diapause (analysis of deviance; treatment
χ25=312; p<0.001), with
asymmetric results of switching daylength regimes. Constant exposure to
15-hour days resulted in 100% diapause induction, whereas 0% diapause
induction was attained in all three treatments that ended with larvae
experiencing 21-hour days (Fig. 2). In other words, exposure to long
days in the fourth and final instar was sufficient for consistently
activating nondiapause development, regardless of previously experienced
daylength. Meanwhile, the opposite change in daylength, from 21 hours to
15 hours, only resulted in approximately half of individuals entering
pupal diapause, even when switched as early as the third instar. Two
thirds of the diapausing individuals in this treatment were males,
reflecting an overall effect of sex on diapause propensity (analysis of
deviance; sex χ21=8.83; p=0.003).
Diapause patterns were similar across populations; there were no
significant interactions with other explanatory factors. A single male
from the univoltine Stockholm population was the only individual that
entered diapause upon being switched from long to short days in the
fourth instar, and Stockholm pupae also showed higher diapause rates
when switched to short days in the third instar (Fig. 2), although this
population difference was not quite statistically significant (analysis
of deviance; population χ22=5.31;
p=0.07).