Influence of trophic factors on the intra -plant distribution of aphids.

摘要

Feeding site selection behavior of the aphid Chaitophorous populicola on cottonwood (Populus deltoides) was studied to determine whether bottom-up or top-down trophic factors better explain intra-plant feeding location. C. populicola are often found on stems and lamina of new and rapidly expanding leaves, and on petioles of senescing leaves. Bottom-up factors associated with leaf developmental stage were examined in cottonwood phloem sap collected from cut petioles using EDTA-enhanced exudation. Sap exudation rate was relatively constant, as indicated by sucrose concentrations. HPLC analysis revealed that new leaves have a higher content of the phenolic glycoside salicin and aspartic acid than other developmental stages (20 amino acids were analyzed). Mature leaves have higher concentrations of gamma-amino butyric acid (GABA) and are least preferred by C. populicola . Also, leaf toughness, measured by vascular bundle lignin density and distance from phytodermal surface to vascular bundle, increased with leaf age and could contribute to fewer aphids selecting mature leaves.;Conversely, top-down factors also contribute to aphid feeding distribution patterns. Ant mutualists are associated with increased aphid population sizes and occurrence of aphids on mature leaves. Ants prefer sucrose solutions containing asparagine, GABA, salicin and salicortin over sucrose alone, suggesting that these may attract tending ants if excreted in aphid honeydew. Ladybugs reduced aphid populations in localized areas and did not initiate aphid migration within plants.;Phloem sap phytochemicals that vary among leaves of different developmental stages may serve as cues for the aphid C. populicola feeding on cottonwood, and for their associated mutualistic ants. Honeydew collected from aphids on leaves of different developmental stages was analyzed by HPLC. The phytochemicals studied did not vary qualitatively or quantitatively in the honeydew between leaves of different developmental stages, indicating that they are not transmitted as cues to the third trophic level. However, aphids on mature leaves produced more honeydew than on other leaf stages. Ants tending aphids may therefore respond to increases in honeydew quantity as opposed to elements of honeydew quality. Thus, aphid intra-plant feeding location is directly and indirectly driven by plant-mediated (bottom-up) factors, and influenced directly by ant mutualists to a lesser extent.