Survival rate and development of the two spotted spider mite, Tetranychus urticae are decreased by UV-B irradiation (Otsuka and Osakabe, submitted). This is considered to be a reason why this mite prefers lower leaf surfaces host plants. On the contrary, the citrus red mite, Panonychus citri is known to remain on upper leaf surfaces of host plants. Such difference in the distribution was assumed to depend on the difference in UV resistance. Hatchability of P. citri eggs was decreased by UV-B irradiation, yet their eggs were more protected from the UV-B irradiation in company with T. urticae. Eggs of P. citri are reddish, while those of T. urticae are white. To test whether the reddish color invest UV resistance in eggs, pale white eggs of albino strain and reddish eggs of wild type of P. citri were simultaneously irradiated by UV-B. Reddish eggs of wild type showed higher resistant to the UV-B irradiation than the pale-white eggs of albino strain. For the next step, the wild type and the albino eggs of P. citri under UV opaque film (-UV) or under UV transparent film (+UV) were exposed to solar irradiation. Hatchability of albino eggs was signifycantly lower than that of wild type eggs under the + UV condition. Under UV –, both eggs showed higher hatchability than under UV+ conditions. From these results, the wild type eggs of P. citri were likely to have some mechanism protecting itself from ambient UV irradiation, and the red pigment possibly act as a shield againt solar UV radiation. F1 female proceeded by reciprocal crosses between the wild type and the albino strain exhibited reddish body color in their adult stage, indicating that the albino was recessive. Notwithstanding, F1 eggs from the crosses between albino female and wild type male were pale white. After hatching, the F1 larvae also exhibit pale white body color, then the females turned to reddish at their first quiescent stage. F1 progeny from the crosses between wild type female and albino male were all reddish during egg to adult stage. These revealed that the color of eggs and also larvae were conferred as a maternal effect (: eggs of P. citri might be protected by the red pigments as the first gift of their mother.)

Terrestrial animals are widely considered to be well protected from damage caused by ambient UV radiation, and thus the direct effects of solar UV-B radiation on arthropods have not attracted a great deal of attention. However, if plant-dwelling arthropods have evolved behavioral adaptations to solar UV-B radiation, but not morphological or physiological adaptations, the resources available to them would be strongly limited. Tetranychus urticae and their dominant natural enemies, phytoseiid mites, usually stay on the lower leaf surfaces. Because of the accumulation of UV-B-absorbing compounds such as flavonoids in the leaves, the upper and lower leaf surfaces provide different UV environments. We tested whether UV irradiation affects survival and reproduction of T. urticae and, if so, whether staying on the lower leaf surfaces is beneficial for their performance in solar UV radiation. Consequently, we observed lethal effects of artificial UV-B irradiation and solar UV radiation on T. urticae. However, by remaining on lower leaf surfaces, the mites could avoid such deleterious effects. These suggest that staying on the lower leaf surfaces is a behavioral adaptation of T. urticae to avoiding harmful solar UV-B radiation. Lethal effects of UV-B irradiation also exerted strong deterrent effects on the hatchability of phytoseiid mites. Additionally, preliminary experiments suggested that phytoseiid mites avoided and quickly escaped from a place irradiated by solar UV. Therefore, solar UV-B radiation may strongly affect the diurnal and spatial patterns of distribution or migration of mite communities on host plant canopies.