A host plant is a specific plant species on which a female butterfly must lay her eggs for her offspring to survive. The plant serves as the sole food source for the caterpillar once it hatches. The presence or absence of the correct host plant in a habitat is the primary factor determining which butterfly species can successfully reproduce and thrive in that area.
The Essential Life Cycle Function of Host Plants
Butterflies exhibit a high degree of selectivity when choosing where to deposit their eggs, a behavior rooted in the specialized diet of their larval stage. The caterpillar has evolved to feed exclusively on the leaves of its particular host plant. This narrow diet is a biological necessity because the larva’s digestive system is only equipped to process the chemical makeup of that specific plant family or species.
Plants naturally produce defensive chemical compounds, known as secondary metabolites, to deter herbivores from eating them. Each butterfly species has developed an evolutionary adaptation to not only tolerate these compounds but also to use them as a chemical signal to identify the correct plant for egg-laying. For example, some caterpillars sequester the plant’s toxins in their bodies, making themselves unpalatable or poisonous to predators.
Specific Host Plants for Common North American Butterflies
The relationship between butterfly species and their chosen larval food source is one of the most specific pairings in nature. Understanding these connections is the foundation for supporting local butterfly populations.
Monarchs
Monarch butterflies (Danaus plexippus) are strictly dependent on members of the Asclepias genus, commonly known as milkweed. Females lay their eggs only on milkweed species, such as Common Milkweed, Swamp Milkweed, or Butterfly Milkweed. The caterpillars feed exclusively on these leaves, ingesting cardenolides, a toxin that makes them and the subsequent adult butterflies toxic to many predators.
Swallowtails
Black Swallowtails (Papilio polyxenes) are drawn to plants in the carrot family (Apiaceae), which often includes garden herbs. Common host plants for this species include:
- Parsley
- Dill
- Fennel
- Rue
Eastern Tiger Swallowtails (Papilio glaucus) prefer the foliage of certain trees and shrubs, such as Ash, Wild Black Cherry, and Tulip Tree.
Painted Ladies
The widely distributed Painted Lady butterfly (Vanessa cardui) is less restrictive, utilizing a range of host plants, but still showing strong preferences. Their caterpillars feed on plants like Thistle, Mallow, and various legumes. This broader host range is why this species is found across nearly all continents except Antarctica and Australia.
Cabbage Whites
Cabbage White butterflies (Pieris rapae) lay their eggs on plants belonging to the mustard family (Brassicaceae). This includes cultivated vegetables:
- Cabbage
- Broccoli
- Cauliflower
- Kale
Their caterpillars are sometimes seen as garden pests. They also use weedy relatives such as Shepherd’s Purse and wild mustards as host plants.
Distinguishing Host Plants from Nectar Sources
A common point of confusion is the difference between a host plant and a nectar source, as these two types of plants serve entirely separate functions in the butterfly’s life cycle. The host plant is the nursery and food for the caterpillar, providing the nutritional building blocks for growth and metamorphosis.
The nectar source, by contrast, is the source of liquid energy for the adult butterfly. These flowering plants provide the sugary fuel required for flying, mating, and searching for host plants. While a plant can sometimes fulfill both roles, such as certain milkweed species, their primary purpose in the garden should be understood as distinct. A garden full of nectar-rich flowers will attract adult butterflies, but a lack of appropriate host plants means they will not stay to reproduce.
Cultivating a Successful Butterfly Laying Habitat
Creating an environment that encourages egg laying requires a shift in traditional gardening perspectives, primarily by tolerating some foliage damage. Since the goal of planting a host plant is for the leaves to be eaten, it is helpful to plant host plants in groups or clumps to provide a more abundant and visible food source for the female butterfly.
The avoidance of insecticides and herbicides is necessary, as these chemicals will kill butterfly eggs and caterpillars instantly. Optimal placement involves selecting a sunny garden spot that receives at least six hours of light daily, since butterflies need warmth to be active and lay eggs. Placing host plants near a protective windbreak, such as shrubs or a fence, also provides shelter for the delicate adults and developing larvae.