Sequestration of plant compounds in a leaf beetle's defensive secretion: cardenolides in Chrysochus
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Leaf beetles of the genus Chrysochus were shown to release secretions from pronotal and elytral glands when disturbed. This defensive reaction is similar to that observed in members of other, not closely related, subfamilies of the Chrysomelidae. In Chrysochus auratus and C. cobaltinus which both feed on plants of the genera Asclepias and Apocynum (Ascle piadaceae and Apocynaceae), the secretions contain cardenolides in a concentration of 77 to 358 μg/μl. Extracts of whole beetles contained cardenolides in concentrations too low for spectrophotometric quantification to a maximum of 748 μg/g dry weight. The cardenolides are apparently taken up from the food plant, since both plant genera contain these toxins and cardenolide pattern and concentration of whole beetle extracts and of the beetles' secretion depended on the local host plant. In C. asclepiadeus, which feeds on Vincetoxicum hirundinaria (Asclepiadaceae) devoid of cardenolides, phenylalanine, tryptophane, leucine and diacetyl putrescine were identified as major constituents of the secretions. The volume of secretion produced in the three Chrysochus species seems to be inversely correlated with the known toxicity of the compounds present in the secretion.
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