Auxins in Honeydew of Toxoptera graminum, Therioaphis maculata, and Macrosiphum pisi, and Their Relation to Degree of Tolerance in Host Plants1

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Abstract
Using paper chromatography in conjunction with a modified Avena coleoptile bioassay method, auxins were detected in honeydews in significant amounts and identified by Rf values, as follows: (1) from greenbugs, T. graminum (Rond.), on susceptible Reno barley, 3-indoleacetic acid, indolepyruvic acid, ethyl 1-3-indoleacetate, plus two unknown growth substances; (2) from greenbugs on tolerant Dicktoo barley, no known auxins in significant amounts, but general growth inhibition was observed in the plants; (3) from greenbugs on susceptible Pawnee wheat, 3-indoleacetic acid, indolebutyric acid, and 3-indoleacetonitrile; (4) from greenbugs on tolerant Dickinson wheat, indolebutyric acid; (5) from spotted alfalfa aphids, T. maculata (Buck.) and pea apliids, M. pisi (Harr.), on susceptible alfalfa clone (50-1266), 3-indoleacetic acid and indolebutyric acid; (6) from pea aphids on susceptible broad bean, 3-indoleacetic acid. A striking relationship found between auxins present in the honeydews and auxins in the various host plants suggested that aphids were effective in removing auxins from the plant sap and concentrating them in the honeydew; their presence in the honeydew may explain why certain aphids stunt plant growth so quickly. Intraconversion probably accounted for variation in auxins noted between honeydews and host plants. Fewer auxins were found in aphids feeding on tolerant plants than in the same aphid species feeding on susceptible plants of the same species. The results here confirmed the correlation between tolerance and auxin levels in host plants that the authors are reporting elsewhere.

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