Cardenolide content of some leaf- and stem-feeding insects on temperate North American milkweeds (Asclepias spp.)

Abstract

The cardenolide content of eight species of insects which feed on the leaves and stems of milk weed (Asclepias spp.) in temperate North America was examined. Cerambycid beetles of the genus Tetraopes contained small quantities of cardenolides. while two genera of chrysomelid beetles, Chrysochus and Labidomera, contained little or no cardenolides in their bodies. All of these insects are thought to be warningly coloured. The cardenolide content of Tetraopes, in contrast with other milkweed herbivores such as the lygaeid bugs Oncopeltus fasciatus and Lygaeus kalmii, appears to be limited by physiological factors, rather than the cardenolide content of the host plant. Cardenolides were not detected in the pupae of an arctiid moth which has an aposematic larva, a cryptic grasshopper, or a cryptic plant hopper, all of which fed on milkweeds known to contain cardenolides. The implication for chemical defense and the position of these insects in mimicry complexes is briefly discussed. It appears that few of the principal milkweed-feeding insects in temperate North America may use host-derived cardenolides as a chemical defense strategy.