The caterpillar, the larval stage of butterflies and moths, is primarily an eating machine focused on rapid growth before metamorphosis. This intense feeding phase requires a highly specialized diet to acquire the energy needed for transformation into an adult insect. What these larvae consume is not random; the adult female selects a specific plant for her offspring, recognizing that the young caterpillar’s survival depends on consuming the correct host. This precise relationship between insect and plant is foundational to understanding why caterpillars feed on certain flowering plants and not others.
Why Caterpillars Are Picky Eaters
The extreme dietary selectivity observed in most caterpillars results from sophisticated chemical ecology and evolutionary history. Plants are not passive food sources; they produce diverse chemical compounds, known as secondary metabolites, as a defense mechanism against herbivores. These chemicals, such as alkaloids, terpenes, and cardenolides, are often toxic to insects lacking specific counter-adaptations.
Caterpillars have specialized sensory organs, particularly gustatory receptor neurons on their mouthparts, to detect and process these secondary metabolites. For specialized caterpillars, these defensive chemicals function as feeding stimulants and host-recognition cues, signaling the appropriate food source. This mechanism explains host specificity, which ranges from monophagy (feeding on a single plant species) to oligophagy (feeding on a few related species) and polyphagy (feeding on a broad range of plants).
The relationship between plants and their specialized insect herbivores is a constant evolutionary “arms race,” known as co-evolution. As a plant evolves a new toxin, the caterpillar must evolve a corresponding mechanism to detoxify or sequester the compound. For example, the monarch caterpillar consumes cardenolides from milkweed, which are toxic to most other animals. It then sequesters these toxins for defense against predators, meaning the caterpillar cannot survive on any plant outside of the family or genus to which it is chemically adapted.
Common Garden Flowers Targeted for Feeding
When a caterpillar consumes a flower in a garden, it is often a pest species using a cultivated plant that shares the defensive chemistry of its natural host. The European cabbage white butterfly lays eggs on plants in the Brassicaceae family, including ornamental cabbage and kale. The larvae, commonly called cabbage worms, also consume the leaves of the ornamental flower nasturtium, which contains defensive compounds similar to those found in brassicas.
Another common example involves the tobacco hornworm or its close relative, the tomato hornworm, whose larvae feed on flowering plants within the Solanaceae family. While known for feeding on tomato and tobacco, these large caterpillars also target flowering nightshades such as petunias, datura, and nicotiana. Damage to these ornamentals often focuses on the flowers and buds, where the larvae tunnel into the soft tissue, causing the blooms to fail or appear ragged.
The tobacco budworm is a separate pest that specifically feeds on the buds and petals of many cultivated flowers, including petunias, geraniums, and snapdragons. Consumption by these generalist or oligophagous species is usually viewed as undesirable damage because it destroys the aesthetic value of the plant.
Flowers to Cultivate for Butterfly Support
Conversely, many gardeners intentionally cultivate specific flowering plants to support the life cycle of native butterflies, viewing the resulting consumption as a successful outcome. These plants are known as host plants, and they are grown with the expectation that they will be eaten by the larvae. The most widely recognized pairing is the Monarch butterfly and the various species of milkweed, which is the only plant the Monarch caterpillar can consume.
Another pairing involves the Black Swallowtail butterfly, whose caterpillars are often called “parsley worms” due to their preference for plants in the carrot family (Apiaceae). Gardeners support this species by planting cultivated herbs like dill, fennel, and parsley, which provide the necessary chemical cues and nutrition for the larvae to develop. Native host plants for the Black Swallowtail include flowering species like Queen Anne’s Lace and Golden Alexander.
The Gulf Fritillary and Zebra Longwing butterflies rely exclusively on the passionflower vine, a flowering plant in the genus Passiflora, as their host. The larvae feed on the foliage of this vine and, like the Monarch, sequester the plant’s defensive chemicals to make themselves unpalatable to predators. For conservation-minded gardeners, the flowers of these host plants are a byproduct of the primary goal: providing the essential food source needed for the caterpillar to complete its development.