Behavioral thermoregulation in a small herbivore avoids direct UVB damage

Direct damage of increased solar ultraviolet-B (UVB) on organism fitness has attracted attention due to stratospheric ozone depletion. Although most ectotherms are not capable of detecting and avoiding solar UVB, they may avoid direct exposure to solar UVB via thermoregulation behavior. However, it is still not clear whether organisms are harmed by ambient UVB radiation before escaping to shaded microhabitats. In this study we used the English grain aphid, Sitobion avenae (Hemiptera: Aphididae), to test whether sunlight-avoidance behavior was caused by heat stress rather than UVB, and whether behavioral thermoregulation in shaded microhabitats contributes to avoidance or reduction of direct UVB damage. Our results showed that S. avenae tended to inhabit exposed adaxial leaf surfaces in mid-May in Mongolia, but inhabited shaded leaf surfaces in mid-June, thereby avoiding strong sunlight. Heat exposure rather than solar UVB was the primary reason for such avoidance behavior. The average and extreme temperatures of shaded leaf surfaces were several degrees lower than sunlight-exposed surfaces at midday, suggesting that movement to shaded leaf surfaces represents a form of behavioral thermoregulation. Such responses occurred before UVB radiation reached harmful levels, and contributed to avoiding direct UVB damage. As future climate warming is expected to lead to harmful UVB radiation as well as increasing temperatures, this may represent a case where responses to one stressor inadvertently protect against the harmful effects of a different stressor.