Impact of Young Citrus Shoot Flush Nutrients and Phloem Sap Composition on Asian Citrus Psyllid Populations

Asian Citrus Psyllid (ACP) Diaphornia citri Kuwayama, 1908 (Hemiptera: Liviidae) is a phytophagous pest on citrus and other rutaceae host plants. Psyllids feed on phloem sap and reproduce exclusively when young expanding flush shoots are present. As they feed on young leaves theyextract  phloem sap which results in a leaf deformation known as epinapsis and can lead to flush shoot death.  While feeding on plant tissues they transmit the phloem inhibiting bacterium Candidatus Liberibacter asiaticus, putative causal pathogen of the deadly citrus greening disease or Huanglongbing (HLB).  Presently there is no cure for the disease, however, vector control plays a very important role in the management of ACP and in turn reduces the spread of HLB. Understanding the nutritional ecology of a pest is very effective in controlling the pest population. In this study, flush shoots of citrus at different growth stages were analyzed for their nutritional composition. Phloem sap was extracted from leaves of each developmental stage and different free amino acid contents were recorded. The results were then used to study the interactions of D. citri densities on flush shoots of different growth stages. The juvenile flush stages showed several major macro-nutrients (N, P, K) and some micro-nutrients (Na, Zn, and Cu) in higher concentrations, while mature flush shoots contained larger amounts of Ca, S, Mg, Fe, Mn, and B. Higher quantities of amino acids were also present in in younger flush stages, and their content decreased as the leaves matured. The higher concentrations nutrients and amino acids found in in juvenile flush stages were positively related to psyllid densities and development, as eggs are laid solely on newly emerged flush shoots and nymph development occurs only on younger flush stages. This indicates that the chemical composition of mature leaves and phloem sap are not conducive to oviposition and immature development.