Cardenolides from Gomphocarpus sinaicus and Pergularia tomentosa (Apocynaceae: Asclepiadoideae) deter the feeding of Spodoptera littoralis
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- Green, P.W.C., Veitch, N.C., Stevenson, P.C. et al. Arthropod-Plant Interactions (2011) 5: 219. doi:10.1007/s11829-011-9131-x
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Methanol extracts of Gomphocarpus sinaicus, Pergularia tomentosa and Cynanchum acutum (Apocynaceae, sub-family Asclepiadoideae) deterred feeding of Spodoptera littoralis in a binary-choice bioassay. Analyses of extracts using high-performance liquid chromatography with photodiode array detection indicated that methanol extracts of P. tomentosa and G. sinaicus contained cardenolides, while these compounds were not detected in extracts of C. acutum. Activity-guided fractionation of the methanol extracts of G. sinaicus and P. tomentosa resulted in the isolation of six cardenolides: 7,8-dehydrocalotropin, calotropin and coroglaucigenin 3-(6-deoxy-β-allopyranoside)-19-acetate (frugoside 19-acetate) from G. sinaicus, and coroglaucigenin, 16α-acetoxycalotropin and calactin from P. tomentosa. The isolation of 16α-acetoxycalotropin was a new report from P. tomentosa. Each of the 6 cardenolides deterred feeding by S. littoralis, while two cardenolide standards, digoxin and digitoxin, did not affect feeding. Differences among cardenolides in their effect on feeding were associated with specific structural features. C. acutum is the only one of the three species tested that is known to support the development of S. littoralis, although the development of larvae was delayed. The observed feeding deterrent activity of the cardenolide-free methanol extract of C. acutum would suggest that compounds other than cardenolides are responsible for the deterrent activity. These compounds, although deterrent in a short-term feeding assay, might not prevent long term feeding, thus allowing the larvae to develop on the plant.