Phloem Sucrose Osmoregulation and Vector Competence in the Asian Citrus Psyllid, the Vector of Huanglongbing

Phloem-feeding insects, including the Asian citrus psyllid (Diaphorina citri), navigate one of the most osmotically challenging diets in nature, the sugar-rich phloem sap. Maintaining osmotic homeostasis is critical for survival, development, and vector competence, particularly in transmitting 'Candidatus Liberibacter asiaticus', the causal agent of citrus greening disease or huanglongbing (HLB). This review synthesizes current knowledge on the physiological, biochemical, and behavioral adaptations that enable D. citri to cope with extreme osmotic pressures. Key strategies include sucrose hydrolysis and conversion into glucose, fructose, and trehalose, as well as redistribution via the hemolymph, and efficient excretion of honeydew, which varies structurally and chemically across different developmental stages and sexes. These adaptations support prolonged phloem feeding, facilitate dual-lifestyle pathogen survival in both phloem and hemolymph, and directly enhance 'Ca. L. asiaticus' acquisition and transmission efficiency. Honeydew as an indicator for feeding behavior, host suitability, and susceptibility to insecticides is also highlighted. Finally, emerging approaches to disrupt osmoregulation, including RNAi-mediated interference with sugar metabolism and water transport, which may provide innovative avenues for integrated management of D. citri and mitigation of HLB spread, are discussed.