EFFECT OF GROSS CARDIAC GLYCOSIDE CONTENT OF SEEDS OF COMMON MILKWEED, ASCLEPIAS-SYRIACA, ON CARDIAC GLYCOSIDE UPTAKE BY THE MILKWEED BUG ONCOPELTUS-FASCIATUS

Milkweed bugs, Oncopeltus fasciatus, were fed seeds of common milkweed, Asclepias syriaca, that contained differing concentrations of cardiac glycoside. Whole seeds had a mean cardiac glycoside concentration of 4.01 mg equivalents to digitoxin per g dry weight, and seed embryos had a mean concentration of 5.56 mg/g dry weight. Bugs fed these seeds concentrated cardiac glycoside: their mean concentration was 6.85 mg/g dry weight. Milkweed bugs fed seeds of lower cardiac glycoside content sequestered a greater percent of the available glycoside than bugs fed seeds of high glycoside content. The quantitative variation of cardiac glycoside content of the seeds of this single species did not significantly affect the growth of bugs. In a separate feeding preference experiment, bugs were offered seeds of both high (5.18 mg/g dry weight) and low (2.30 mg/g dry weight) cardiac glycoside content. The bugs showed no indication of selecting seeds of either high or low glycoside content. © 1979 Plenum Publishing Corporation.