I determined the difference in sensitivity of immature and adult mites to host plants with regard to diapause induction. I used immature and adult polyphagous spider mites, Tetranychus kanzawai Kishida (Acari: Tetranychidae), and two host plants, Orixa japonica Thunb. (Rutaceae) and Phaseolus vulgaris L. (Fabaceae). The diapause incidence was investigated in two adult mite-host and two immature mite-host combinations of treatments. The incidence of diapause induction was much higher in mites developing on O. japonica than those on P. vulgaris, whichever host plants they fed on in adulthood. The host plants affected adult mites to a much lesser extent than the immature mites, though the mites feeding on O. japonica showed a consistently higher diapause incidence. These results suggest that host plants affect diapause induction to a greater extent in immature mites than in adult mites.