The photoperiodic response of 10 strains of the two-spotted spider mite (Tetranychus urticae), originating between 40.5° and 60°N in Western and Central Europe, was found to be highly variable. The critical nightlength for photoperiodic induction of diapause was strongly correlated with latitude for the lowland populations and varied from 7.75 hr in the north to 13.25 hr in the south. The length of the circadian period, taken as the peak-to-peak interval in response curves of resonance experiments done with T. urticae, varied between 17.75 and 21.5 hr and appeared weakly correlated with latitude. Only a very weak correlation was observed between critical nightlength and circadian period. These results do not provide evidence in favor of a circadian-based photoperiodic clock in T. urticae. On the other hand, they also do not refute this possibility, as there may be other circadian or noncircadian factors affecting the critical nightlength, which could mask the influence of circadian period.
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