A caterpillar is the larval stage of a butterfly or moth, focused almost entirely on consuming plant matter to fuel its transformation. Unlike the adult stage, which may sip nectar from various flowers, the young insect is an extremely picky eater. This intense selectivity means the female adult will only lay her eggs on specific vegetation to ensure her offspring’s survival. The caterpillar will starve if it hatches onto the wrong species.
Host Plants Versus Nectar Sources
Host plants, also called larval food plants, are the specific vegetation a caterpillar must consume to grow and develop. The female butterfly or moth uses chemical cues to locate these plants.
Nectar sources, conversely, are flowering plants that provide the adult butterfly or moth with energy-rich liquid fuel for flight and reproduction. These plants serve as fuel for the adults but offer no nutritional value for the growing caterpillar. A garden full of nectar-rich blooms attracts adult butterflies, but a garden without host plants will not support the next generation. A true butterfly habitat must provide both the nursery (host plant) for the larvae and the fuel (nectar source) for the adults.
The Science of Specificity: Chemical Co-evolution
Caterpillars are specialized in their diet due to an evolutionary arms race with plants. Plants defend themselves from being eaten by producing toxic chemical compounds called secondary metabolites. These compounds make their leaves unpalatable or poisonous to most herbivores.
Over millions of years, certain insect species have evolved specialized enzymes that allow them to detoxify or sequester these plant toxins. This tolerance narrows the caterpillar’s diet to a few plant families, or sometimes just a single species, which it can safely consume. The Monarch butterfly is a classic example, having evolved the ability to eat milkweed, which contains cardiac glycosides.
The caterpillar stores these toxins in its body tissues, which makes the insect unappetizing or poisonous to predators like birds. This process, known as co-evolution, means the caterpillar must eat the specific plant containing the chemicals it has evolved to handle and utilize for defense.
Identifying Common Host Plant Groups
To identify which plants caterpillars favor, look at specific plant families that have co-evolved with certain insect groups. The number of caterpillar species a plant supports measures its value in a local ecosystem. Native oak trees (genus Quercus) are a prime example, supporting more types of moth and butterfly caterpillars than almost any other plant.
Milkweed (Asclepias species) is the singular host plant for the Monarch caterpillar, a relationship widely recognized due to conservation efforts. The Black Swallowtail butterfly relies on plants in the carrot family (Apiaceae), which includes common garden herbs like parsley, dill, and fennel. Planting these aromatic herbs attracts the female to lay eggs.
Many species of Sulphur and Blue butterflies, such as the Common Blue, use plants in the legume family (Fabaceae) as their host, including clover, alfalfa, and bird’s-foot trefoil. The leaves of nettles (Urtica) are the sole food source for several butterfly larvae, including the Red Admiral, Peacock, and Small Tortoiseshell.
Trees such as Willow (Salix), Poplar (Populus), and Cherry (Prunus) host numerous species, including the Mourning Cloak and Viceroy butterflies. Planting these diverse native groups ensures food for a wide variety of caterpillars.
Supporting Caterpillars in a Habitat
Creating a caterpillar-friendly habitat requires embracing the fact that some foliage will be chewed. When planting host species, plant extras to ensure enough food for the larvae to complete their growth cycle. Female butterflies often lay multiple eggs, and a shortage of food can lead to starvation.
Avoid the use of broad-spectrum pesticides in any habitat intended to support caterpillars. Even organic insecticides, such as those containing Bacillus thuringiensis (Bt), are toxic to caterpillars and must be used with caution.
Before removing a caterpillar, identify the species to determine if it is a beneficial insect or a true pest. Many caterpillars, especially those of large moths and butterflies, cause significant damage but are worth tolerating to encourage a healthy insect population. Planting host plants in less visible areas or among nectar plants can help camouflage the inevitable leaf damage.