The Monarch Butterfly is protected from predation by toxic cardenolides that are sequestered by the larvae from its host plants in the milkweed family. The cardenolides inhibit an important enzyme (Sodium Potassium ATPase) that is important for nervous system function. Many cardenolides will cause poisoning symptoms in vertebrates (including predatory birds) that lead to taste aversion and avoidance of Monarch Butterflies as prey.The sequestration of cardenolides by monarchs is incompletely understood. A recent study in the Journal, Chemoecology*, compares the sequestration of cardenolides by monarch larvae, Danaus plexippus, to sequestration by a caterpillar in the same geuns, Danaus gillipus, the larva of the Queen Butterfly.
Larvae of both species were fed on Asclepias curassavica, a type of milkweed that contains the cardenolides, calotropin and calactin in about a 1:1 ratio. The Monarch Butterfly sequestered greater amounts of cardenlides than the Queen Butterfly. Both species were better at sequestration of calactin than calotropin. The Monarch seqeuestered up to 1.8 times less calotropin than calactin. The Queen Butterfly larvae effectively sequestered calactin, but over 10 fold less calotropin was sequestered.
Sequestration requires uptake of the chemical and storage in the body. The caterpillar species may differ in their uptake mechanisms, or in the ability to store the chemicals. Sequestered chemicals must be protected from metabolism. Protection of calactin, but not calotropin from metabolism and excretion by the Queen Butterfly could account for lower levels of calotropin sequestration. Further investigation into why calotropin is less effectively sequstered in the Queen Butterfly may give new insights into the sequestration process.
*Dietrich Mebs • Moritz G. Wagner • Stefan W. Toennes • Cora Wunder • Michael Boppre. Selective sequestration of cardenolide isomers by two species of Danaus butterflies. Chemoecology (2012) 22:269–272