Diurnal variation in phloem sap composition has a strong influence on aphid performance.The sugar-rich phloem sap serves as the sole diet for aphids and a suite of physiological mechanisms and behaviors allow them to tolerate the high osmotic stress.Here,we tested the hypothesis that night-time feeding by aphids is a behavior that takes advantage of the low sugar diet in the night to compensate for osmotic stress incurred while feeding on high sugar diet during the day.Using the electrical penetration graph(EPG)technique,we examined the effects of diurnal rhythm on feeding behaviors of bird cherry-oat aphid(Rhopalosiphum padi L.)on wheat.A strong diurnal rhythm in aphids as indicated by the presence of a cyclical pattern of expression in a core clock gene did not impact aphid feeding and similar feeding behaviors were observed during day and night.The major difference observed between day and night feeding was that aphids spent significantly longer time in phloem salivation during the night compared to the day.In contrast,aphid hydration was reduced at the end of the day-time feeding compared to end of the night-time feeding.Gene expression analysis of R.padi osmoregulatory genes indicated that sugar breakdown and water transport into the aphid gut was reduced at night.These data suggest that while diurnal variation occurs in phloem sap composition,aphids use night-time feeding to overcome the high osmotic stress incurred while feeding on sugar-rich phloem sap during the day.