Cardenolide content and thin-layer chromatography profiles of monarch butterflies,danaus plexippus L., and their larval host-plant milkweed,asclepias viridis walt., in northwestern louisiana

Abstract

This paper is the first in a series on cardenolide fingerprinting of monarch butterflies and their host-plant milkweeds in the eastern United States. Spectrophotometric determinations of the gross cardenolide content of 60Asclepias viridis plants in northwestern Louisiana indicate a positively skewed variation ranging from 95 to 432 υg/0.1 g dry weight with a mean of 245 υg/0.1 g. Butterflies reared individually on these plants contained a normal cardenolide distribution ranging from 73 to 591 υg/0.1 g dry weight with a mean of 337 υg/0.1 g. The uptake of cardenolide by the butterflies best fit a logarithmic function of the plant concentration. Female monarchs (385 υg/0.l g) contained significantly greater mean cardenolide concentrations than did males (287 υg/0.1 g). No indications of a metabolic cost for either cardenolide ingestion or storage were adduced from size or dry weight data. Thin-layer chromatograms of 24 individual plant-butterfly pairs developed in two solvent systems resolved 21 individual spots in the plants and 15 in the butterflies.A. viridis plants appear to contain several relatively nonpolar cardenolides of the calotropagenin series which are metabolized to the more polar 3'-hydroxy derivatives calactin and calotropin as well as to calotropagenin in the butterflies. The epoxy cardenolides labriformin and labriformidin were absent, although desglucosyrioside (a 3'-hydroxy derivative) appeared present in both plants and butterflies. Quantitative evaluation of theR f values, spot intensities, and probabilities of occurrence in the chloroform-methanol-formamide TLC system produced a cardenolide fingerprint clearly distinct from those previously established for monarchs reared on otherAsclepias species, supporting the use of fingerprints to make ecological predictions concerning larval host-plant utilization.A. viridis is the predominant early spring milkweed throughout most of the south central United States and may be important in providing chemical protection to spring and early summer generation monarchs in the eastern United States.