Fate of host-plant iridoid glycosides in lepidopteran larvae of Nymphalidae and Arctiidae

We compared the ability of larvae of six lepidopteran species to sequester iridoid glycosides. All larvae were fed on a common host plant, Plantago lanceolata, which contains two iridoid glycosides, aucubin and catalpol. Four species of arctiids were examined: Pyrrharctia isabella, Spilosoma congrua, Spilosoma latipennis, and Spilosoma virginica. For comparison, we also examined two nymphalid species, one of which, Junonia coenia (a specialist on plants containing iridoid glycosides), was known to sequester iridoid glycosides, and the other, Vanessa cardui, was a generalist that also feeds on P. lanceolata. We found that, as expected, J. coenia larvae did contain iridoid glycosides, whereas V. cardui did not. To our surprise, Spilosoma congrua contained substantial amounts of iridoid glycosides (mean = 7.14% dry weight), whereas none of the other arctiids (S. latipennis, S. virginica, or P. isabella) contained detectable levels of iridoid glycosides. We found small amounts of iridoid glycosides in the frass of S. virginica but none in the frass of S. congrua or P. isabella.