Cardenolides in the defensive fluid of adult large milkweed bugs have differential potency on vertebrate and invertebrate predator Na ⁺ /K ⁺ -ATPases

Abstract

Aposematic animals rely on diverse secondary metabolites for defence. Various hypotheses, such as competition, life history and multifunctionality, have been posited to explain defence variability and diversity. We investigate the compound selectivity hypothesis using large milkweed bugs, Oncopeltus fasciatus , to determine if distinct cardenolides vary in toxicity to different predators. We quantify cardenolides in the bug’s defensive secretions and body tissues and test the individual compounds against predator target sites, the Na + /K + -ATPases, that are predicted to differ in sensitivity. Frugoside, gofruside, glucopyranosyl frugoside and glucopyranosyl gofruside were the dominant cardenolides in the body tissues of the insects, whereas the two monoglycosidic cardenolides—frugoside and gofruside—were the most abundant in the defensive fluid. These monoglycosidic cardenolides were highly toxic (IC 50 < 1 μM) to an invertebrate and a sensitive vertebrate enzyme, in comparison to the glucosylated compounds. Gofruside was the weakest inhibitor for a putatively resistant vertebrate predator. Glucopyranosyl calotropin, found in only 60% of bugs, was also an effective inhibitor of sensitive vertebrate enzymes. Our results suggest that the compounds sequestered by O. fasciatus probably provide consistency in protection against a range of predators and underscore the need to consider predator communities in prey defence evolution.