Cardiac Glycosides

The first example of the collection of toxic plant compounds to protect insects was demonstrated by Miriam Rothschild. She showed that the larvae of the Monarch butterfly Danaus plexippus (Plate 12) feed on milkweed, Asclepias curassavica which contains cardiac glycosides like calotropin (used as an arrow poison) (Figure 10.1). The compounds are stored unchanged in the larva and through metamorphosis, making the adult butterfly unpalatable to predators like birds. Calotropin is highly toxic to vertebrates but evidently has no ill effects on the insect. In addition to calotropin, the butterfly also stores three volatile alkylmethoxy-pyrazines (Figure 10.1) from the plant. The warning odour of these volatile compounds, associated with calotropin, is enough to deter a bird on close approach from trying to eat a butterfly charged with calotropin. It does not follow that because an insect feeds on a toxic plant that it will sequester the toxins. Another danaid butterfly Danaus chrysippus does not seem to sequester calotropin from milkweed.

calotropin

N^och3 N^OCH3 N^OCH3

pyrazines from Asclepias curassavica calotropin

Figure 10.1 The structures of calotropin and the three volatile pyrazines sequestered by the Monarch butterfly

The popular flowering tropical shrub oleander (Nerium oleander) contains toxic cardenolides. The principal one is oleandrin (Figure 10.2), but the bug Aspidiotus nerii feeding on it sequesters only a minor component, adynerin. The ladybird Coccinella undecempunctata preying on Aspidiotus nerii sequesters the adynerin from its prey while another ladybird, C. septempunctata feeding on the same bugs does not. The aphid Aphis nerii (see Chapter 8, and Plate 7) on the same plant collects and stores three of its cardiac glycosides.

The formation of cardiac glycosides from non-toxic plant sterols (Figure 7.12) and of related saponins from common triterpenes (Figure 7.13) have already been described.

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