PLANTÐINSECT INTERACTIONS Variation in Eastern Cottonwood (Populus deltoides Bartr.) Phloem Sap Content Caused by Leaf Development May Affect Feeding Site Selection Behavior of the Aphid, Chaitophorous populicola Thomas (Homoptera: Aphididae)

Apterous populations of Chaitophorous populicola Thomas (Homoptera: Aphididae) appear to track Eastern cottonwood (Populus deltoides Bartr.) leaf development. Few aphids occur onmature leaves.Marked individual aphids on leaves of different developmental stageswereobserved through a period of new leaf initiation. Nymph and adult C. populicola frequently track leaf development bymoving up to younger leaves. A comparison of phloem sap constituents and leaf toughness among leaf developmental stages revealed some differences that could be used by C. populicola to determine leaf age. Phloem sap exudates, collected from P. deltoides leaves of different developmental stages, were analyzed by high-performance liquid chromatography for free amino acids and the phenolic glycoside salicin. Sucrose concentration in exudates, indicative of phloem sap exudation rate, was uniform among leaf stages. Of 20 amino acids examined, only aspartic acid and -amino-n-butyric acid (GABA)concentrationsdiffered signiÞcantlybetween leaf stages. Forward stepwisediscriminant function analysis showed that seven of the amino acids analyzed are useful for classifying leafmaturity groupings. Aphid-infested cottonwoods had lower cystine concentrations in phloem sap than aphidfree plants. Salicin concentration was signiÞcantly higher in new leaves. Leaf toughness was assessed by lignin density and distance measurements in petiole cross-sections. Rapidly expanding leaves had signiÞcantly less ligniÞcation and new leaves had shorter distances to the vascular bundles than senescent leaves. These physiological and phytochemical differences among P. deltoides leaf developmental stages may contribute to the leaf stage selection patterns exhibited by the aphid, C. populicola.