Effects of Induced Herbivory on the Common Milkweed Plant Asclepias syriaca and its Impact on Adult Monarch Butterfly Behavior
Secondary metabolites, which are organic compounds produced by a plant in response to damage or injury, are usually described as herbivore or insect feeding deterrents, so they are not normally associated with affecting the plant's floral nectar. In milkweed plants (Ascelpias) cardenolide glycosides are key secondary defense compounds that only specialized milkweed feeders have appeared able to palate and consume. The presence of these toxic cardenolides in milkweed nectar is a "consequence of production" created by the plant's essential herbivory defense mechanism, but the consequences may also affect pollinators such as adult monarch butterflies, even though larvae of this butterfly are specialized to feed on milkweed plant leaves. In this study we manipulated nectar composition in synthetic artificial flowers using the toxic compound Digoxin to simulate nectar cardenolide compounds. We then compared three different butterfly species' (black swallowtail (Papilio polyxenes); clouded sulphur (Colias philodice), monarch (Danaus plexippus)) behaviors towards artificial flowers which contained varying levels of toxin in the nectar. These experiments could provide insights into the effects of herbivory on milkweed pollination by monarch butterflies, and also the cardenolide level preferences of both specialist and non-specialist species. While with a low number of data points the results are not statistically significant, we do find qualitative evidence pointing towards all three butterfly species being deterred from flowers whose nectar contained high levels of the toxic cardenolide compound.