Early naturalists believed that caterpillars and butterflies were separate, unrelated insects. How surprised they would have been by the notion of gardening to attract caterpillars! Yet, to further the cause of butterflies, providing host plants that feed the caterpillars is as important as filling flower beds with bright, nectar-rich blooms that provide nourishment for the adults.
If they find the plants they need for their offspring in your garden, female butterflies may lay eggs there, creating a new generation that grows to maturity in your yard. It's an effective way to boost local butterfly populations, and the four-part life cycle is fascinating to watch.
Finding the Right Caterpillar Host Plants
Providing the right caterpillar host plants is crucial for success, as many caterpillars are picky eaters that require specific food plants. Milbert's Tortoiseshell caterpillars only accept nettles (Urtica). American Copper larvae feed on sorrel (Rumex). Spring Azure caterpillars are partial to dogwoods (Cornus) and blueberries (Vaccinium). Clover (Trifolium) is the host plant of Eastern Tailed-Blue larvae.
Monarch caterpillars primarily feed on milkweeds (Asclepias). As they fly north from Mexico in the spring, adult females look for milkweeds on which to lay their eggs. It's a great help to the butterflies if gardeners on the migration route provide these critical caterpillar host plants. Obviously, planting a few milkweeds doesn't offset the massive loss of habitat that progresses daily as a result of urban sprawl and changes in rural land use, but every step counts.
A Black Swallowtail caterpillar (photo) and butterfly (illustration).
How do you know if caterpillars have made their home in your garden? Look for chewed leaves (a caterpillar may be on the leaf or nearby) or frass (dark pellets of excrement) on leaves or leaf axils. Some larvae feed communally; look for batches of Mourning Cloak larvae on willows. Other caterpillars may make a communal silk nest, such as Milbert's Tortoiseshells on nettles. Individual larvae, such as Painted Lady caterpillars, may rest in a silken "hammock" on a leaf, then emerge to feed at night. In winter, look for a rolled leaf that is still attached to an otherwise bare cherry (Prunus), willow (Salix), or poplar (Populus); inside may be a tiny admiral caterpillar.
Creating a Caterpillar-Friendly Environment
To emulate nature, locate favored host plants in different parts of the garden. In nature, caterpillars are usually scattered over an area, instead of clustering together on one bush. Spread out, they are less vulnerable to predators and other dangers. An adult female lays eggs singly or in clusters, on leaves or buds of host plants. It's vital to her offspring's survival that she choose well. She scratches the leaf surface with her feet, giving the plant a kind of chemical taste test for compatibility. She determines if the plant supplies enough tender new growth and visually checks the plant for other eggs or larvae to ensure that there will be enough greenery for the growing caterpillars. If there's too much activity already, she looks for an unoccupied plant.
How you site caterpillar host plants in your garden depends largely on where you live and what butterflies you are trying to interest in your offerings. In general it's best to approximate the plants' native habitat. It's a good way to create an environment that is appealing to the female butterflies that search out the plants.
Providing leaf mulch or low-growing plants—instead of leaving the ground bare or spreading coarse bark—is important to protect caterpillars that hide out during the day and feed at night when they are better protected against predators. Fritillary caterpillars, for example, do not spend all of their time on the foliage of their host plant, violet (Viola), instead hiding in leaf litter or low vegetation during the daytime. The caterpillars of skippers need leaf mulch when they are ready to pupate.
Be mindful of caterpillars when you clean up the garden in fall, disturbing the ground as little as possible and leaving stalks of herbaceous perennials and grasses in place. Tiger swallowtails, for example, look for rough bark, fenceposts, or sturdy plant stalks when they are ready to pupate in fall. Skippers and moths pupate in ground litter and are very vulnerable to any kind of disturbance.
Caterpillar Appetites
If you're a bit reluctant to invite hordes of munching arthropods to lunch, keep in mind that nature has a plan: Larvae are an important, protein-rich link in the food web. They attract many birds, lizards, and small mammals, as well as wasps and spiders, to your garden: Survival rate from butterfly egg to adult may be as low as one in a hundred; that's why female butterflies lay so many eggs. And should some caterpillars become too voracious, relocate some individuals to another plant of the same species. Most plants will survive moderately heavy defoliation; keeping caterpillars in check is more of an aesthetic consideration.
Also consider preferred caterpillar diets: In most cases, they won't be interested in prized ornamental shrubs or flowers. They would much rather eat "weedy" plants, such as clovers (Trifolium), nettles (Urtica) or plantains (Plantago), which are found in most yards.
And what about the so-called pests? The green-, yellow-, and black-striped larvae of the Black Swallowtail that you may see in your vegetable garden feeding on parsley (Petroselinum crispum), dill (Anethum graveolens), and carrot tops (Daucus carota) are often considered a nuisance. These caterpillars originally fed only on native plants such as angelica (Angelica), but they have also adapted to eat nonnative herbs in the parsley family, Apiaceae. So all a gardener needs to do is scatter a few extra patches of parsley and dill throughout the garden to disperse the caterpillars and allow for their huge appetites.
The Gray Hairstreak chooses a lot of host plants, one of which is green bean. Larvae may work their way through one large bean, but are not likely to consume your whole bean crop. The beautiful Long-Tailed Skipper uses green beans as well; its larva, the bean leaf roller, feeds on the leaves, but it is not voracious. Try to identify larvae and destroy only the offspring of destructive pests, such as gypsy moths, cutworms, and tent caterpillars.
The familiar Cabbage White, which feeds on plants in the mustard family (Brassicaceae), often gets unfairly blamed for damage by the Cabbage Looper moth. You can lure the Cabbage White away from your arugula with nasturtium (Tropaeolum majus); its leaves contain oils that are chemically similar.
One small yard may not seem too promising as caterpillar habitat, but don't forget that there may be environments nearby that feed and shelter caterpillars as well.
