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Trees of life

On butterflies and the urban forest

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Eastern tiger swallowtail - NANCE KLEHM
  • Nance Klehm
  • Eastern tiger swallowtail

In August we see butterflies floating in abundance in the breeze, nectaring from flowers, or gathering minerals from muddy puddles, and it is largely the monarch that comes to mind. A host plant specialist, the monarch lays its eggs on milkweed, the only genus its larval young feed on. Some 22 native species of milkweed grow in the state.

Sidelining this midwestern polar bear of insects is the eastern tiger swallowtail (Papilio glaucus). The males are white bodied with yellow-and-black wings spanning over four inches. The females vary in color and can resemble the males or take on a dark form—black with blue spotting, most likely a design strategy. They live in deciduous woods near streams and rivers. In the city, if we see them, we can largely thank the urban tree canopy for this.

Urban folks understand that food chains, like supply chains, are a linear progression of how food gets to our mouths through the hands and machines of gardeners, farmers, laborers, and brokers of agribusiness via planes, trains, trucks, and ships, and end up on the shelves of co-ops, chain groceries, and farmers' markets.

Nature's food web is infinitely more webbed, relational as opposed to transactional, dependent and interdependent at the same time, with many points of connection, interaction, and exchange of energy. Plants are the basis of food webs for humans, but humans themselves and insects are one of the glues that hold much of the web together.

Trees are one of nature's keystones of the plant kingdom. And like all plants, trees are situated. They need to call others to them, be it for minerals in exchange for the sugars they produce, the pollen and nectar of their flowers through scent and color to develop their fruit and seed. They are anchors, beckoners, and muses of life.

The connection between Lepidoptera (aka butterflies and moths) and trees lies in their less celebrated form as caterpillars. This emphasis I borrow from University of Delaware research entomologist Douglas W. Tallamy, who wrote the book Bringing Nature Home: How You Can Sustain Wildlife With Native Plants. Songbirds need thousands of caterpillar protein snacks to feed their young.

As specialized feeding machines, caterpillars are categorized by what they do with plant material—there are leaf munchers, sap suckers, web and tent spinners, and skeletonizers. The thoracic region of all caterpillars has three pairs of true legs (making them insects) with small claws for grasping; on the abdomen are between two and five pairs of proto legs, little stubby flat-bottomed pads with "crochets" or small Velcro-like "hooks" to stabilize them on plant surfaces. Caterpillars transform plant protein into higher-grade animal protein.

A female eastern tiger swallowtail scatters her eggs and, if not slurped by a predator, they hatch within four days as hungry, hungry caterpillars. The munchers eat consistently and silently, converting the sun's energy from plant into their body mass for ten days before going through five instars (latin for "likeness"), or moltings, whereby they change in color, texture, patterning, and of course size, oh my.

Each instar happens when the caterpillar secretes enzymes that allows it to digest and reabsorb most of its body. After ten days the caterpillar spins a chrysalis and, unlike the monarch, which emerges within two weeks, tiger swallowtails take 14 to 60 days to emerge as adult butterflies.

As caterpillars, swallowtails munch on more than 21 species of trees, many of them native to the eastern region and not as well represented in our urban canopy. Cottonwoods (which feed over 260 local lepidoptera), wild cherry (host to 240 species), and hickory and pecan (231) are considered too large and "messy." Ash trees (host to 130 different species including mourning cloaks, cecropia moths, and many of our sphinx moths) were once common in Chicago's parkways and parklands but are slowing being eradicated from our landscapes due to the emerald ash borer. Thousands of both green and white ash have been taken down in the city to date. How does the makeup of our urban tree canopy affect our population of eastern tiger swallowtails? Our other local butterflies and moths?

As a whole population, our ecological IQ is low. I blame the system for teaching us to be independent and separate from nature, autonomous economic creatures. It's true, ecological thinking is complicated, as there are many points of access to this web. If we can take the suggestion from Tallamy and make new conservation goals to support local pollinators and complex food webs, store carbon, manage watersheds, we also address some of our urban green infrastructure issues. If selecting and planting more trees to diversify our urban forest is the answer, what is the question?   v

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