Birch Trees, Caterpillars, and Chickadees

Most gardeners are delighted to see a butterfly sipping nectar from a flower in their garden, a delight that may be accompanied by a sense of accomplishment: “My garden is a butterfly garden!” Yet many of these same gardeners become alarmed by the sight of a caterpillar chewing the leaves of a garden plant, a sight that sends some to the garden shed for the chemical solution.

There is no such thing as an “adults only” butterfly garden. While occasional visits by passing butterflies do occur, a female butterfly is not likely to linger in a garden unless there are plenty of larval host plants, those plants that caterpillars of a specific species must eat until they pupate. If she finds these plants, she will deposit eggs on them, and perhaps stay awhile to sip nectar from the garden’s flowers.

This male monarch butterfly is nectaring on tropical milkweed in Marjorie's Garden.

This male monarch butterfly is nectaring on tropical milkweed in Marjorie’s Garden.

Our recent experience with monarch butterflies serves as a good example. In twelve years of gardening together, monarchs were absent from Marjorie’s Garden. Then, in early June of this year, we planted several seedlings of a tropical annual milkweed (Asclepias curassavica), some in the ground and others in a large tub. A month later, two monarchs fluttered over the garden fence and, two weeks after that, two dozen tiny caterpillars were munching away on milkweed leaves. Lesson learned: plant the larval host and they will come.

This larva of the eastern black swallowtail butterfly is munching on carrots.

This larva of the eastern black swallowtail butterfly is munching on carrots.

Most butterfly species have a relatively narrow range of larval host plant species: Baltimore checkerspots on turtlehead; black swallowtails on dill, fennel, parsley, and rue; eastern tiger swallowtails on tulip tree, wild cherry, and lilac; field skippers on grasses; gulf fritillaries on passion flower; monarch butterflies on milkweed; mourning cloaks on willow, aspen, and elm; pipevine swallowtails on pipevine; silver spotted skippers on locust and wisteria; spicebush swallowtails on spicebush and sassafras; viceroys on willow and cottonwood; and western tiger swallowtails on wild cherry, sycamore, and willow (Grissell, Eric, 2001, Insects and Gardens, Timber Press). Thus the true butterfly garden should contain a variety of these host plants.

Monarch butterfly caterpillar, shown here eating the flower buds of a tropical milkweed.

Monarch butterfly caterpillar, shown here eating the flower buds of a tropical milkweed.

Equally important, the gardener must welcome the munching of caterpillars. In essence, a portion of any butterfly garden, the larval host plants, will be eaten. For example, by the end of this past garden season, most of the tropical milkweed plants that we planted were reduced to leafless stems as we enjoyed at the beauty of both adult monarchs and the caterpillars.

Well, says the gardener who would rather not host caterpillars, my garden will be a songbird garden, filled with the calls of chickadees, warblers, cardinals, thrushes, and other songbirds. But no, in order for songbirds to nest in and around your garden, there must be an abundance of caterpillars. For example, each pair of chickadees, which raise their young exclusively on lepidopteran larvae, requires between 6,000 and 10,000 caterpillars for a single clutch of fledglings. The same picture can be painted for other songbirds: insects, notably butterfly and moth larvae, are a major component of each nestling’s diet.

Where do all these caterpillars come from? Perhaps a better question would be: what kind of garden can produce caterpillars in such abundance? The answer: a garden planted with a wide variety of plants, woody and herbaceous, that are known larval hosts for butterflies and moths.

A white admiral butterfly sipping nectar from the flowers of summersweet clethra.

A white admiral butterfly sipping nectar from the flowers of summersweet clethra.

Native trees and shrubs are important larval hosts, especially for moth species. In his groundbreaking book, Bringing Nature Home (2007, Timber Press), entomologist Douglas Tallamy ranks oaks (Quercus sp.) at the top of the list of woody plants that function as lepidopteran larval hosts. According to Tallamy, the 80 Quercus species in North America are hosts to 534 species of moths and butterflies, including at least 20 species of dagger moths, 18 species of underwings, 8 species of hairstreaks, 44 species of inchworms, and 15 species of giant silk moths. Willows (Salix sp.) share second place ranking with cherries and plums (Prunus sp.), each group hosting 456 species of moths and butterflies. Birches (Betula sp.) are another important group, hosting 413 species, while poplars and cottonwoods (Populus sp.) serve as hosts to 368 species. Other woody larval host plants include crabapples (Malus sp.), blueberries (Vaccinium sp.), maples (Acer sp.), pines (Pinus sp.), and spruce (Picea sp.).

A single birch tree, for example, can host thousands of caterpillars. We discovered this one summer when we located Lynne’s new trampoline under the shade of a large yellow birch (Betula alleghaniensis), one of several in Marjorie’s Garden. All was fine until mid-May when the moth eggs hatched and caterpillars began to feed on the birch leaves, resulting in a daily carpet of fras (as well as several hundred larvae) covering the jumping surface of the trampoline. We tried sweeping up the larvae and their excrement for a few days, then gave up and moved the trampoline into the open.

Caterpillars are what bird food looks like, but not all species of caterpillars are palatable to each species of bird. Chickadees, for example, feed mainly on spruce budworms, tent caterpillars, and other small larvae. Only gardens that are planted with a large variety of larval host plants, mostly trees and shrubs, can sustain a large variety of songbirds in the nesting season.

Creating a garden that sustains wildlife takes years. How many years depends on the plants that are already growing in and around your garden when you begin. A few existing mature trees from Tallamy’s list will speed things along, but if space allows you will want to plant a variety of other trees, including some of the smaller native understory trees, such as pagoda dogwood (Cornus alternifolia), and as many species of native shrubs as your garden space will allow. If you have room for only one large tree, plant a red oak or one of the native birches. If your available space for shrubs is limited, be sure to include a few summersweet clethras (Clethra alnifolia), for there is no shrub native to New England more ornamental or more functional in terms of providing nectar for butterflies and other pollinators.

Shrubs that function as butterfly larval hosts include staghorn sumac (Rhus typhina), highbush blueberry (Vaccinium corymbosum), mapleleaf viburnum (Viburnum acerifolium), spice bush (Lindera benzoin), sheep laurel (Kalmia angustifolia), gray dogwood (Cornus racemosa), flowering dogwood (Cornus florida), and serviceberry (Amelanchier sp.). Each of these shrub species functions as a larval host for one or, at best, a few species of butterflies, so plant as many different shrubs as space allows.

This is the larva of the brown hooded owlet moth.  The small white dots on its back are eggs laid by an insect predator.

This is the larva of the brown hooded owlet moth. The small white dots on its back are eggs laid by an insect predator.

Caterpillars are important links in the garden food web. Not only are they the principal food of summer song birds, they are the principal prey of many predatory insects, including hoverflies, wasps, and beetles, insects that keep garden herbivore populations at tolerable levels. Many gardeners who spend time in the garden looking for caterpillars find them among the most beautiful of garden creatures. For all of these reasons, when someone sends me an email asking how to deal with a horde of caterpillars crawling over the porch steps and railings, my answer is always the same: be careful where you step.

A paddled dagger moth caterpillar is a rare sight in any garden.

A paddled dagger moth caterpillar is a rare sight in any garden.

Sustaining a garden that supports wildlife, including caterpillars, takes a lifetime. The job is never done and the learning never stops. Each garden season brings new discoveries, new species to add to your ever-growing list of garden life, and new efforts to strengthen the ties that bind species together in the ecosystem we call the garden.

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About Reeser Manley

I was born in Laramie Wyoming but moved to the southeast at an early age. I was educated through my B.S. in Biology in the Columbus, Georgia area, then crossed the Chattahoochee River to earn my M.S. in Botany at Auburn University. For the next ten years, I worked as Horticultural Manager for the George W. Park Seed Co. in Greenwood, S.C. At 40 years of age, I decided to return to graduate school and in 1994, I earned a Ph.D. in Horticultural Science from Washington State University, Pullman, Washington. Fast forward through 10 years at university (7 at UMaine, Orono) and you find me teaching high school science in Eastport, Maine, the edge of the world, and writing a weekly garden column for the Bangor Daily News. My new book, The New England Gardener's Year, a Month-by-Month Guide for Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, and Upstate New York”, will be published later this year by Cadent Publishing. You can learn more about the book by visiting its Facebook page: http://www.facebook.com/pages/The-New-England-Gardeners-Year/187285218055676.)