Effects of toxic compounds in milkweed nectar on flower visitors
Toxic plant compounds are generally considered from the perspective of defense against herbivores, however, many toxic plant chemicals are also found in floral nectar. It is often unclear whether toxins in nectar are adaptive or are byproducts of circulating levels for herbivory defense. Milkweed plants (Asclepias spp.) contain many different types of cardiac glycosides (cardenolides). Cardenolides are also present in milkweed nectar, but the effects of cardenolides on floral visitors are unknown. We examined the impact of cardenolides on the behavior of two milkweed flower visitors: the bumblebee, Bombus impatiens, which is a generalist flower visitor and likely an important pollinator for milkweeds, and the monarch butterfly, Danaus plexippus, which is a specialist herbivore on milkweed as a caterpillar, but as an adult feeds on nectar from a range of plant species. Individual bees and butterflies were tested in a foraging arena choice assay using artificial flowers of two different colors. One color was associated with a sucrose solution and the second color was associated with a sucrose solution plus cardenolides. Individuals were assigned to treatments with either natural cardenolide concentrations, unnaturally high cardenolide concentrations, or a control with quinine, which has been shown to be distasteful to bees.
Monarch butterflies showed no deterrence (visits did not significantly differ from 50%, i.e., random choice) to cardenolides at natural concentrations (Wilcoxon signed rank test: N=14, V=37, P=0.55), but were deterred by the cardenolides at unnaturally high concentrations (N=12, V=11, P=0.030). Monarchs were also significantly deterred by the quinine control (N=14, V=11.5, P=0.013). Bumblebees were neither deterred by natural cardenolide concentrations (N=9, V=27.5, P=0.59) nor unnaturally high concentrations (N=9, V=11, P=0.36). Bees did show a trend toward deterrence by the quinine (N=11, V=12.5, P=0.075). We therefore show that these two flower visitors with divergent interactions with milkweed plants, bumblebees as mutualistic pollinators versus monarchs as parasitic herbivores, are differentially affected by cardenolides toxins in nectar. While it may not be surprising that specialized herbivores are deterred by cardenolides at high concentrations, it is surprising that bumblebees are not so deterred.