Why do two-spotted spider mites (Tetranychus urticae) inhabit on the underside of leaves? Why are diapause females orange? A possible key to answering these questions is ultraviolet (UV) radiation. UV radiation directly damages DNA and produces reactive oxygen species (ROS). ROS also damages DNA, dissociates proteins, and oxidizes lipids. Because mites are small, the UV/ROS-induced damage would be lethal to mites. In non-diapause females, mortalities under UV-C (250 nm) at 0.6 W m–2 and UV-B (300 nm) at 2.4 W m–2 were significantly higher than those under continuous darkness (DD; control). Oviposition rates in such treatments were significantly lower than those under DD. No significant effects for mortality and oviposition rate were observed under UV-A (350 nm) even when the intensity was as high as 2.4 W m–2. In diapause females, the differences in mortalities between all types of UV radiation and DD were not significant. Interestingly, more than half of diapause females escaped from leaf disks under all types of UV radiation, and the escape rates were significantly higher than those under DD and those for non-diapause females. This suggests that diapause females exhibit negative phototaxis. Mites inhabit the underside of leaves in summer with dense vegetation. Most of UV radiation is absorbed and reflected by leaves. Therefore, the underside of leaves is considered a suitable environment for mites to avoid UV radiation, particularly UV-B (UV-C is completely absorbed by the ozone layer). In autumn, leaves start turning yellow and red as winter approaches and finally, fall. During this phenological event, the UV-B level in the plant canopy would increases dramatically while female mites enter diapause with a change in their body color from yellow-green to orange. It is known that the orange color is mainly due to the accumulation of β-carotene, which plays a role as a scavenger of ROS. Therefore, low mortalities observed in diapause females under UV-C and UV-B may be a result of β-carotene accumulation or merely due to the increase in the escape rate. Therefore, whether the escaped diapause females are resistant to UV-C and UV-B damage needs to be confirmed. Our findings suggest that UV radiation is utilized as an effective non-chemical measure to reduce the mite population and that the selection of habitat and change in body color is the mite’s strategy to reduce the deleterious effects of UV-B.