VARIATION IN CARDENOLIDE CONTENT OF LYGAEID BUGS, ONCOPELTUS-FASCIATUS AND LYGAEUS-KALMII-KALMII AND OF THEIR MILKWEED HOSTS (ASCLEPIAS SPP) IN CENTRAL CALIFORNIA

A colorimetric assay was used to quantify the amount of cardenolides in O. fasciatus and L. kalmii kalmii and their milkweed host plants (Asclepias spp.) in central California, USA. The cardenolide content of individual insects, determined in microgram equivalents of digitoxin, varied from zero to over 300 .mu.g/insect. Sources of variation of cardenolide content in the insects included interspecific and intraspecific differences in the content of the host plant species and also differences in the content of plant organs on which insects were feeding. This last source of variability may explain temporal variation in the cardenolide content of the insects. Adults of O. fasciatus, which migrate into California in the late spring and early summer, and adults of L. k. kalmii, which emerge from winter hibernacula in the early spring, contained small to immeasurable amounts of cardenolides. The colonization pattern of O. fasciatus on species of Asclepias in north central California suggests that this species does not maximize its opportunities to sequester large quantities of cardenolides from potential hosts. The emetic potential of lygaeids in California to vertebrate predators is briefly discussed.