How Aphids Choose Leaves: The Roles of Phenolics in Host Selection by a Galling Aphid

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Abstract
The aphid Pemphigus betae does not induce galls at random on leaves of its host plant, Populus angustifolia (narrowleaf cottonwood). Probability of stem mother success in forming a gall, body mass of stem mother and progeny, number of progeny, development rate of progeny to maturity and numbers of embryos in mature progeny are all correlated positively with leaf size at maturity. Leaf size at maturity is predictable from leaf position in the shoot at bud burst (Whitham 1978, 1980). The major secondary compounds of the host plant are known phenol glycosides. Using the Folin Denis assay for total phenol determination, a strong inverse correlation has been established between the concentration of total phenols and suitability of a tree, leaf, or leaf section for galling at the time of bud burst, when the colonizing stem mothers are at the peak of their activity. Expanding leaves within the same shoot have different phenol concentrations; the potentially largest leaves have lower phenol levels. Leaves containing one or more incipient galls have lower phenols levels than leaves without galls. Phenol concentrations in leaves are not uniform; the existence of phenol gradients in leaves is demonstrated. In a comparison between two neighboring trees that are differentially infested with colonizing aphids, the tree with the higher aphid numbers per leaf has lower phenol levels in its leaves. These same leaves were shown to posses different average phenol gradients, suggesting that phenol gradients play a role in microhabitat selection. These findings are consistent with the known biology and territorial behavior of the aphid and provide a chemical basis for continuing studies on aphid habitat selection and fitness.