Abstract
Plant secondary metabolites that defend leaves from herbivores also occur in floral nectar. While specialist herbivores often have adaptations providing resistance to these compounds in leaves, many social insect pollinators are generalists, and therefore are not expected to be as resistant to such compounds. The milkweeds, Asclepias spp., contain toxic cardenolides in all tissues including floral nectar. We compared the concentrations and identities of cardenolides between tissues of the North American common milkweed Asclepias syriaca, and then studied the effect of the predominant cardenolide in nectar, glycosylated aspecioside, on an abundant pollinator. We show that a generalist bumblebee, Bombus impatiens, a common pollinator in eastern North America, consumes less nectar with experimental addition of ouabain (a standard cardenolide derived from Apocynacid plants native to east Africa) but not with addition of glycosylated aspecioside from milkweeds. At a concentration matching that of the maximum in the natural range, both cardenolides reduced activity levels of bees after four days of consumption, demonstrating toxicity despite variation in behavioral deterrence (i.e., consumption). In vitro enzymatic assays of Na+/K+-ATPase, the target site of cardenolides, showed lower toxicity of the milkweed cardenolide than ouabain for B. impatiens, indicating that the lower deterrence may be due to greater tolerance to glycosylated aspecioside. In contrast, there was no difference between the two cardenolides in toxicity to the Na+/K+-ATPase from a control insect, the fruit fly Drosophila melanogaster. Accordingly, this work reveals that even generalist pollinators such as B. impatiens may have adaptations to reduce the toxicity of specific plant secondary metabolites that occur in nectar, despite visiting flowers from a wide variety of plants over the colony’s lifespan.
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Acknowledgements
This research was funded by NSF grant #1907375 to AAA and PLJ and by Bowdoin College funding to PLJ.
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This research was funded by NSF grant #1907375 to AAA and PLJ and by Bowdoin College funding to PLJ.
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PLJ, AAA, and KRM designed the experiment. KRM conducted the bee consumption and activity level experiments. APH collected milkweed tissues. CD conducted the mass spectrometry. APH conducted the enzyme assays. SVP analyzed the behavioral videos. PLJ wrote the main manuscript text. PLJ and KRM analyzed data and prepared Figs. 2, 3, 4. CD prepared Fig. 1 and supplementary figures. All authors reviewed the manuscript.
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Jones, P.L., Martin, K.R., Prachand, S.V. et al. Compound-Specific Behavioral and Enzymatic Resistance to Toxic Milkweed Cardenolides in a Generalist Bumblebee Pollinator. J Chem Ecol 49, 418–427 (2023). https://doi.org/10.1007/s10886-023-01408-3
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DOI: https://doi.org/10.1007/s10886-023-01408-3