The great caterpillar factory

Turn the calendar back two months to early May, and the great caterpillar factories of eastern North America are in full production. If you stand quietly at night in any mature forest or woodlot, you'll hear it: a soft crackling and hissing in the dead leaves underneath the trees, an invisible sleet storm. It's the sound of frass hitting the ground — millions and millions of tiny black fecal pellets from hundreds of thousands of caterpillars munching away high in the leafy canopy.

For wildflowers that live beneath the trees (trilliums, bluebells, Dutchman's breeches, violets) it's the sound of an intravenous drip of nutrients. For the migrating songbirds overhead (warblers, vireos, tanagers, thrushes), it signals how much food — caterpillars — they can count on to fuel the rest of their flights to breeding grounds in the north.

But these great eastern caterpillar factories are failing.

Partly this is happening as the diversity of species in the forests drops. When chestnut blight raged through the forests early last century, it wiped out the most prominent of the forests’ citizens, the American chestnut, leaving gaping holes even in those forests where lumbering had not yet occurred. At the same time, a host of non-native tree species started colonizing the suburbs and roadsides of America, eventually claiming the now-empty holes in the forest patchwork.

Caterpillars are usually picky eaters, sometimes able to eat only one species or a few closely related species of trees. The health of the caterpillar factories depends on a patchwork quilt of native tree species. An ocean of Bradford pears, escapees from ornamental horticulture, blowing white in the spring breeze is a veritable food desert.

To this unintended pressure on the caterpillar factories we must add direct assault on the caterpillars themselves from hundreds of tons of pesticides applied annually by airplane to control forest pests such as the gypsy moth. Few of these poisonous potions are selective; most kill every insect that ingests or touches them, whether butterfly, beetle, bee or moth.

Increasingly, when the hordes of hungry warblers make their way up the Appalachian spine in spring, they face a forest famine. They’ve used up every calorie they stored gorging in Central and South America to wing across the open waters of the Gulf of Mexico or across the arid scrublands of Texas; they don’t have the energy to continue without refueling in the caterpillar factories. If the birds are species that stay in the Eastern forests to nest and raise their young — wood thrushes, ovenbirds, cuckoos, vireos— there’s scant food for the nestlings.

The bankruptcy of the great caterpillar factories impoverishes entire regions and endangers some bird species wholesale. Loss of habitat and loss of food sources presage dire times ahead for many of the nation’s bird migrants — the great feathered river that flows northward with the spring in fewer and fewer numbers each year, threatening to slow to a trickle and finally cease altogether.

This week, citizen scientists of all stripes have turned their attention to the moths the caterpillars have become. The occasion is National Moth Week, the last full week of July, when parks, summer camps and nature centers across America set up special lights and paint tree trunks with sugary syrup to attract these beautiful and diverse night fliers.

We should use the occasion to start looking at the caterpillar pipeline that gives us these moths and the birds and other wildlife that depend on them. Let’s reinvest in these great caterpillar factories by managing state and federal forests for greater biodiversity. Let’s spend the extra dollars it takes to use safer, more targeted pesticides in forest settings. And when a tree dies in our yard, on our street, in our local park — let’s replace it with a native tree.

Rick Borchelt is a science writer who lives and blogs about natural history in College Park. He is moderator of the local butterfly observation and research site, leplog.wordpress.com.

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