The monarch caterpillar starts as a tiny, striped larva that must rapidly consume enough biomass to fuel its complete metamorphosis into a butterfly. This journey requires a specialized and highly specific nutritional source to build the body that will eventually fly thousands of miles. The caterpillar stage, which lasts only about two weeks, is dedicated almost entirely to eating and growing at an exponential rate. Its diet must provide the necessary energy, nutrients, and biological components needed for defense later in life.
The Exclusive Diet Milkweed
Monarch caterpillars are obligate herbivores, meaning their survival is tied exclusively to one type of plant: milkweed, which belongs to the genus Asclepias. A female monarch butterfly will only lay her eggs on milkweed because her offspring cannot digest or survive on any other foliage. If a caterpillar were to hatch on a non-milkweed plant, it would quickly perish from starvation.
There are over 100 species of milkweed across North America, and the monarch will utilize various types depending on geography, including common milkweed, swamp milkweed, and butterfly weed. The availability of these species aligns with the monarch’s multi-generational migration patterns, with different varieties emerging at different times and locations across the continent.
How Milkweed Protects the Monarch
The milkweed plant’s chemistry is the reason for this specialized diet, offering the monarch a powerful defense mechanism against predators. Milkweed contains a milky, sticky sap (latex) laced with toxic compounds called cardiac glycosides, or cardenolides. These substances are poisonous to most other animals, serving as the plant’s natural defense.
The monarch caterpillar has evolved to consume these toxins without suffering harm. Instead of excreting the cardenolides, the caterpillar sequesters them, storing the toxins within its body tissues. This assimilation makes the caterpillar and the adult butterfly highly distasteful and poisonous to vertebrate predators, particularly birds. The monarch’s bright yellow, black, and white stripe pattern acts as a warning signal, advertising its accumulated toxicity.
Growth Stages and Feeding Habits
The caterpillar’s development is divided into five distinct larval stages, or instars, separated by molting events where it sheds its skin. The first instar is tiny and begins its life by consuming its own eggshell before moving on to the milkweed leaf. During the entire larval phase, which lasts between 10 and 14 days, the caterpillar will increase its mass by nearly 2,000 times.
This rapid growth demands relentless eating, facilitated by specialized mouthparts used to clip and chew the tough leaf tissue. As a defense against the sticky latex, a caterpillar in its later instars will sometimes chew a circular trench or cut the leaf’s petiole. This action causes the leaf to droop and the flow of the pressurized sap to slow or stop. The majority of the caterpillar’s feeding occurs during the final stage, the fifth instar (L5). A single fifth-instar caterpillar can consume an entire large milkweed leaf in a 24-hour period before it begins the process of pupation.
Providing Food for Growing Caterpillars
For those interested in supporting monarch populations, providing a safe and sufficient supply of milkweed is the primary action. Source milkweed from reputable native plant nurseries and ask specifically if the plants have been treated with systemic pesticides, such as neonicotinoids. These insecticides are absorbed into the plant’s tissues and can remain toxic to the caterpillars for months, often resulting in death after just a few bites.
Native milkweed species are the preferred choice because they naturally die back in the fall, which prevents the buildup of the protozoan parasite OE. Non-native tropical milkweed does not die back in warm climates, allowing the parasite to persist year-round. This persistence can lead to infected butterflies with reduced lifespan and poor flight ability. If raising caterpillars indoors, a single larva will require several large milkweed leaves daily during the final instar.