Monarch caterpillars exclusively eat milkweed. This specialized relationship is a remarkable example of coevolution. Female monarchs lay eggs almost entirely on plants in the Asclepias genus, ensuring their offspring have an immediate food source. Milkweed is the sole host plant for the larval stage. Without it, the caterpillar cannot complete its development and transform into a butterfly.
Obligate Feeding Behavior
The monarch caterpillar exhibits monophagy, a specialized feeding strategy meaning it feeds only on milkweed. This specialization resulted from millions of years of coevolution with the plant’s defenses. The caterpillar must consume massive quantities of foliage, increasing its body weight thousands of times in just two weeks as it progresses through its five larval instars.
Monarchs have evolved specific physiological mechanisms to deal with the plant’s natural toxicity. They possess a modified version of the Na+/K+-ATPase enzyme, which is the cellular target of the milkweed’s toxins in other animals. This adaptation renders the caterpillars largely insensitive to the effects of the plant’s defensive compounds, allowing them to process the leaves without suffering harm.
The caterpillar stage is the only time in the monarch’s life cycle when it requires milkweed, and its survival hinges entirely on this plant. If a caterpillar runs out of milkweed before it is ready to pupate, it will starve. This dependency makes the availability of milkweed a limiting factor for monarch populations across North America.
The Chemical Defense Mechanism
Milkweed plants defend themselves from most herbivores with a milky, sticky substance called latex, which contains a class of potent toxins known as cardiac glycosides, or cardenolides. These compounds are designed to disrupt heart function in animals that consume them. Monarch caterpillars, however, turn this defense into their own protection through a process called sequestration.
Sequestration involves the caterpillar absorbing cardenolides from the leaves and storing them in specialized tissues. This absorbed toxin is carried through metamorphosis, protecting both the larva and the adult butterfly. The cardenolides make the monarch highly unpalatable or toxic to most vertebrate predators, such as birds.
The bright, contrasting orange and black coloration of the adult monarch advertises this toxicity, a strategy known as aposematism or warning coloration. A bird that attempts to eat a monarch and gets sick quickly learns to associate those colors with an unpleasant meal, avoiding all future monarchs. This learned avoidance protects the entire population. Young caterpillars primarily avoid the toxic latex, but late-instar caterpillars actively consume the exuding latex to increase the cardenolides they sequester for defense.
Supporting Monarch Populations
Understanding the monarch’s reliance on milkweed is essential for conservation efforts. The primary way to support the declining monarch population is by providing a reliable source of milkweed. This requires planting native species appropriate for the local region where the butterflies breed.
Species such as Common Milkweed (Asclepias syriaca) and Swamp Milkweed (Asclepias incarnata) are generally preferred by female monarchs for laying their eggs and provide excellent larval habitat. It is important to avoid using pesticides on any milkweed intended for monarchs, as the caterpillars are highly susceptible to chemical contamination.
A significant concern involves the non-native Tropical Milkweed (Asclepias curassavica), which is widely sold in nurseries. Unlike native varieties, this species often does not die back in warm climates, which disrupts the monarch’s migratory cycle. The year-round presence of Tropical Milkweed can also lead to the buildup and spread of a debilitating protozoan parasite called Ophryocystis elektroscirrha (OE). Native milkweed naturally goes dormant in the fall, which acts as a seasonal cleanse, eliminating the OE spores that accumulate on the plant tissue.