Although deleterious effects of solar UVB radiation on mites have been brought to light, how it affects the population dynamics remains largely unknown. Brevipalpus obovatus (Tenuipalpidae), an herbivorous mite, occurs on Viburnum erosum var. punctatum (VEP; a deciduous shrub) only in autumn. One third of B. obovatus eggs were laid on upper surfaces of leaves. Oviposition on upper surfaces is beneficial for avoiding predators residing on lower surfaces. Instead, the eggs are exposed to solar UVB radiation as well as radiant heat. Therefore, we tested the effects of UVB radiation and temperature (radiant heat) on egg hatchability under near-ambient and UV-attenuated sunlight conditions from May to October 2012. Although the UV-attenuation significantly improved egg hatchability, most eggs died regardless with UV treatments due to heat stress in July and August. A deterministic model based on UVB dose–egg hatchability response, egg-development parameters and meteorological observation data showed two survivability peaks, which was higher in October (hatchability = 0.424) than that in June (0.220). This predicted pattern roughly corresponds with field observation, meaning that solar UVB radiation and temperature are major determinants of seasonal existence of B. obovatus on VEP.