Milkweed serves a dual purpose for butterflies, rooted in the life cycle and chemistry of the Asclepias genus. It acts as the host plant for the larval stage of the Monarch butterfly and provides a rich nectar source for a wide variety of adult butterflies. This distinction between a reproductive necessity for one species and a general food resource for many helps explain the plant’s unique role. The relationship is highly specialized, based on the plant’s chemical composition, which provides both sustenance and a powerful defense mechanism.
The Monarch’s Absolute Dependence on Milkweed
The Monarch butterfly (Danaus plexippus) has an obligate relationship with milkweed, meaning it cannot complete its life cycle without this plant. The female Monarch lays her eggs exclusively on milkweed leaves because the resulting caterpillars cannot survive on any other food source. For the Monarch larva, milkweed leaves are the sole diet and the foundation for its development into an adult butterfly.
This dependency is a direct result of a long co-evolutionary history between the insect and the plant. Milkweed contains potent toxins called cardiac glycosides, or cardenolides, which are normally toxic to most animals. Monarch caterpillars have evolved the ability to ingest these toxins without harm. Instead of metabolizing the poisons, the caterpillar sequesters the cardenolides within its body tissues, storing them for its own protection.
The stored toxins are retained through the pupal stage and into the adult butterfly, making the Monarch unpalatable or poisonous to predators like birds. This chemical defense is visually advertised through the butterfly’s bright orange and black coloration, a strategy known as aposematism. A bird that attempts to eat a Monarch quickly learns to avoid any butterfly with that distinct color pattern. The Monarch’s exclusive diet of milkweed transforms it from a defenseless larva into a chemically protected adult.
Milkweed as a General Nectar Source
While only Monarch caterpillars consume milkweed leaves, the plant’s vibrant flowers are a popular and abundant nectar source for many adult butterflies. Nectar, a sugar-rich liquid, provides the necessary energy for adult butterflies to fly, mate, and sustain their migrations. Milkweed blooms produce copious amounts of nectar, making them a high-value refueling stop for a diverse range of pollinators.
The plant’s flowers attract numerous species beyond the Monarch. These include:
- Swallowtails
- Fritillaries
- Skippers
- Various species of bees and moths
These adult insects are seeking a readily available source of sugar to power their flight. This makes milkweed a valuable generalist food source within the ecosystem, supporting a broad swath of insect life. An adult Monarch feeds on milkweed nectar just as readily as it feeds on nectar from other flowering plants.
The nectar flow is particularly important for Monarchs preparing for their fall migration, as they must build up fat reserves to survive the journey to their overwintering grounds. During this phase, the butterflies are in a state of reproductive diapause, shifting their focus from laying eggs to fueling flight.
Choosing the Right Milkweed Varieties
Choosing the correct milkweed species is important for supporting butterflies and the health of the Monarch population. It is recommended to plant only milkweed species that are native to your specific geographic region. Native varieties, such as Common Milkweed (Asclepias syriaca), Swamp Milkweed (Asclepias incarnata), or Butterfly Weed (Asclepias tuberosa), are adapted to the local climate and die back naturally in the fall.
The natural die-back of native milkweed is an important ecological cue for Monarchs to begin their migration south. This seasonal dormancy is also a natural cleansing mechanism that prevents the year-round buildup of a protozoan parasite, Ophryocystis elektroscirrha, or OE.
A significant warning exists regarding Tropical Milkweed (Asclepias curassavica), a non-native variety that does not die back in mild climates. The year-round persistence of Tropical Milkweed can disrupt the Monarch’s natural migratory cycle, encouraging them to breed late into the fall instead of migrating. More concerningly, the constant presence of this plant fosters the rapid spread of the OE parasite, leading to high infection rates in local Monarch populations. Infected butterflies often emerge with deformed wings, reduced lifespan, or fail to emerge from the chrysalis at all.