FATE OF IRIDOID GLYCOSIDES IN DIFFERENT LIFE STAGES OF THE BUCKEYE, JUNONIA-COENIA (LEPIDOPTERA, NYMPHALIDAE)

The buckeye butterfly, Junonia coenia (Lepidoptera:Nymphalidae), specializes on plants that contain iridoid glycosides. To determine the fate of these compounds in larvae, pupae, and adults of this species, we reared larvae on artificial diets with and without iridoid glycosides, and on leaves of a host plant, Plantago lanceolata (Plantaginaceae). Quantification by gas chromatography showed that newly molted third-, fourth-, and fifth-instar larvae reared on leaves of P. lanceolata contained means of 5.13, 2.88, and 6.83% dry weight iridoid glycoside. In contrast, the mean iridoid glycoside concentration of actively feeding fifth-instar larvae was 0.28% dry weight, that of pupae was 0.19% dry weight iridoids, and adults contained no detectable iridoids. Feeding experiments suggested that this reduction in actively feeding larvae was due to the metabolism of iridoid glycosides. P. lanceolata leaves in these experiments contained a mean of 1.00% dry weight iridoid glycoside, with a 2:1 ratio of aucubin to catalpol. Calculation of iridoid consumption and utilization indices showed that larvae fed artificial diets consumed, digested, and sequestered aucubin and catalpol in similar ways. When these indices were calculated for larvae fed leaves of P. lanceolata, catalpol was sequestered twice as efficiently as aucubin.