Caterpillars are the larval stage of butterflies and moths (order Lepidoptera), and their primary purpose is to eat and grow. A caterpillar may increase its body mass by a factor of 1,000 or more during its larval life, and almost all of that growth is fuelled by plant material. The specific plants that caterpillars eat — known as host plants or food plants — vary enormously between species, and understanding these relationships is fundamental to butterfly and moth conservation.
Specialist vs. Generalist Feeders
Caterpillars fall broadly into two dietary categories:
| Type | Description | Examples |
|---|---|---|
| Specialists (monophagous/oligophagous) | Feed on one species or a small group of related plants | Monarch (milkweed only), large blue (wild thyme), holly blue (holly/ivy) |
| Generalists (polyphagous) | Feed on a wide range of unrelated plants | Garden tiger moth, angle shades moth, buff-tip moth |
The majority of butterfly species are specialists, with females carefully selecting specific host plants on which to lay their eggs. This specificity has evolved because many caterpillars have developed the ability to tolerate or even sequester the defensive chemicals produced by their host plants, turning the plant's defences into their own protection against predators.
Common UK Butterfly Caterpillars and Their Food Plants
- Peacock: Common nettle (Urtica dioica)
- Red admiral: Common nettle
- Small tortoiseshell: Common nettle
- Comma: Common nettle, hop, elm
- Large white: Brassicas (cabbages, nasturtiums)
- Small white: Brassicas, garlic mustard
- Orange-tip: Garlic mustard, lady's smock (cuckooflower)
- Brimstone: Buckthorn and alder buckthorn
- Common blue: Bird's-foot trefoil and other legumes
- Speckled wood: Various grasses (cock's-foot, Yorkshire fog)
The Importance of Nettles
The common nettle (Urtica dioica) is arguably the single most important food plant for British butterflies. At least five common species depend on it: the peacock, red admiral, small tortoiseshell, comma, and painted lady. Despite its reputation as a weed, leaving a patch of nettles in a sunny, sheltered corner of the garden is one of the simplest and most effective ways to support butterfly populations.
For best results, nettles should be:
- In a warm, sunny position — caterpillars develop faster in warmth
- Sheltered from strong winds
- Cut back by half in June to produce a flush of fresh growth for second-brood caterpillars
- Left undisturbed from July onwards to allow late-season caterpillars to pupate
Did you know? Caterpillars of the large white butterfly can strip a cabbage plant bare in days. A single female can lay over 100 eggs on one plant, and the gregarious caterpillars feed together in groups, consuming leaves at an astonishing rate. Gardeners have waged war on these caterpillars for centuries.
Unusual Caterpillar Diets
While the vast majority of caterpillars are herbivores, some have evolved remarkably different diets:
- Carnivorous caterpillars: The large blue butterfly (Phengaris arion) larvae trick red ants into carrying them into the nest, where they spend months feeding on ant grubs
- Lichen feeders: Several moth species feed on lichens growing on rocks, walls, and tree bark
- Wax moths: Larvae of the greater wax moth (Galleria mellonella) feed on beeswax in honeybee hives
- Clothes moth larvae: Feed on wool, silk, and other animal-derived fibres using specialised gut enzymes
- Detritivores: Some caterpillars feed on dead leaves, decaying wood, or fungi rather than living plants
How Caterpillars Find Food
It is the adult female butterfly or moth that selects the food plant, not the caterpillar. Females use a combination of visual cues (leaf shape, colour) and chemical cues (detected by sensory receptors on their feet and antennae) to identify the correct host plant. Once an egg is laid on the right plant, the tiny caterpillar that hatches typically begins eating its own eggshell before moving on to the surrounding leaf tissue.
Supporting Caterpillars in Your Garden
- Grow native plants: Native wildflowers and grasses support far more caterpillar species than exotic garden plants.
- Leave a nettle patch: A patch of nettles in a sunny corner supports at least five butterfly species.
- Plant a native hedge: Hawthorn, blackthorn, buckthorn, and holly are food plants for numerous moth and butterfly species.
- Tolerate some damage: If caterpillars are eating your plants, consider it a success — your garden is supporting wildlife.
- Avoid pesticides: Insecticides kill caterpillars indiscriminately, including those of beneficial and beautiful species.
- Allow grass to grow: Many grassland butterflies (meadow brown, ringlet, speckled wood) depend on long grasses for their caterpillars.
Key Takeaway
Caterpillars feed primarily on plants, with most butterfly species being specialists that depend on one or a few specific host plants. The common nettle alone supports five of Britain's most familiar butterflies. By growing native plants, leaving nettle patches, and avoiding pesticides, gardeners can make a significant contribution to supporting caterpillar populations and, by extension, the butterflies and moths they become.