Question

A milkweed garden at a local park in NJ creates a ‘monarch rest stop’ to feed the monarchs on their journey to Mexico. The monarch butterfly (Danaus plexippus) feeds on the nectar of milkweed (Asclepias syriaca, other species of Asclepias, and many other plants). he journey usually begins in late summer (August to September) and can take several months (approximately three to four generations), with the majority of monarchs reaching Mexico in early November. Please describe the energy cycles of D. plexippus (monarch butterflies) and A. syriaca (milkweed). Outline the major steps performed in the two most important metabolic pathways, being sure to include any necessary inputs and outputs. What would happen to the monarch butterflies if milkweed were extinct? And vice versa?

Answer 1

Danaus plexippus co-evolved with milkweeds, by developing a unique adaptation that allows their caterpillars to feed on toxic compound (cardenolides) releasing milkweeds whereas, many other insects caterpillers cannot fedd on these. Milkweeds are generally avoided by many herbivores because of their toxic nature, but weed specialists such as Danaus plexippus hijack the milkweeds’ defenses mechanism to protect themselves against predators. Cardenolides toxins interfere with the Na+ /K+ -ATPase sodium pumps, but Danaus plexippus have amino acid substitution mutations at 111 and 122 residues of this protein that reduce cardenolide binding. As the caterpiller of  Danaus plexippus feeds on these toxins they accumulates such toxins and becomes an unpleasent prey for their predators and thus can escape from potential predatory birds.

Life cycle of monrch butterfly includes a four generation migratory priod as depicted in the following figure.

(Taken from - The Perilous Migration of the Monarch Butterfly, http://www.highplainsgardening.com/perilous-migration-monarch-butterfly )

During the migration period monarch butterflies accumulates large quantities of triacylglycerides and only minor amounts of glycogen. Lipids are stored in the the fat body in relation to life stages and migratory condition. The stored lipid is not all utilised during a brief pupal stage, whereas adults metabolizes most of it during migration.

If the milkweeds becomes extinct then monarch population will decrease as its caterpillar can not protect itself from predatory species.