It has been known that diapause is not induced in populations of the two-spotted spider mite, Tetranychus urticae, under continuous darkness (DD). When the light intensity was changed under LD 8:16 at 18°C, the threshold intensity for blue light (475 nm) to induce 50% of diapause was below 50 mW/m2; and the blue light was most effective in photoperiodic induction of diapause. The threshold intensities for green (572 nm) and orange (612 nm) light were 50–500 and 500–2,500 mW/m2, respectively. T. urticae showed no sensitivity to red light (658 nm) even at 2,500 mW/m2 intensity. Low diapause incidence under long wavelengths was probably due to the absence of photoreception by photoreceptor pigments for photoperiodic time measurement, or for circadian entrainment required for photoperiodic induction.