Red River Gorge, KY

Red River Gorge, KY

Tuesday, September 16, 2014

Backyard Ragnarök: The Emerald Ash Borer

Naturalists of all ages know that trees are cornerstones of the environment. Trees provide food and habitat for wildlife, play a major role in global oxygen and carbon cycles, help to stabilize soils and prevent erosion, and perform many other ecological tasks.

"The Ash Yggdrasil" (1886) by Friedrich Wilhelm Heine
Trees can also hold great emotional, aesthetic, and cultural value. From the ancient Babylonian and Greek societies to Native American tribes, many cultures revere trees to some extent. Trees were central in many tales of the Norse. According to the Prose Edda compiled by Snorri Sturluson, the nine worlds were held within the boughs of Yggdrasil, the World Tree. Yggdrasil was a giant ash tree where the gods held court. This immense, cosmic tree was a link that served as connective tissue for the Norse cosmology. Although the fate of Yggdrasil after Ragnarök, the Norse end-times, is never explicitly stated it is surmised that the survival of humankind rests solely upon this gigantic tree1.

I find this legend especially interesting given the serious threat that native ash trees face throughout the eastern United States. An exotic invasive insect called the emerald ash borer (Agrilus planipennis) has been rapidly dispersing throughout the eastern and midwestern United States, aggressively attacking native ash (Fraxinus) species. As adults, these beetles are metallic green, and feed on ash foliage. They do most of their damage as larvae, burrowing through the vascular tissue and disrupting the tree's ability to transfer food downward from the leaves.

Top: Emerald ash borer larva.
Bottom: Emerald ash borer adult.
Images from

There are confirmed populations of emerald ash borer in Kentucky, and according to the Kentucky Division of Forestry, there are approximately 130 million individual ash trees in the Commonwealth. How might this invasion affect Kentucky? Native ash trees support a number of native insect species, mostly Lepidopterans (butterflies & moths) which subsist on ash foliage as caterpillars2.  Localized mortality of native ash trees in forests can cause canopy gaps to open, changing light levels within those forests, leading to increased temperature and reduced moisture on the forest floor. This can affect the composition of the forest floor plant community. One ash species, the blue ash (Fraxinus quadrangulata) is a tree indicative of a functionally extinct ecotype found only in Kentucky - the Bluegrass Savanna. The emerald ash borer invasion leads to an overall net reduction in biodiversity in our forests.

From an economic standpoint, Louisville Slugger baseball bats are made from northern white ash grown in Pennsylvania. Emerald ash borer is moving closer to these trees, and the company is evaluating other woods to use for bat production. Ash is also a popular urban tree in Kentucky, and is planted in neighborhoods and along roadsides to improve local aesthetics. Loss of these trees and the associated cost of removal represents an economic loss as well.

Recently, we have been discussing the importance of biodiversity in BIO 1164: Biology & Human Concerns. The threat that invasive species pose to native biota is alarming, and I will explore this topic in more detail later. But for now, let's return to our Norse myth. Each native species, from plants to insects to vertebrates, could each be represented by a branch on the World Tree, Yggdrasil. Since human society cannot be separated from nature, do we have an obligation to protect and conserve biodiversity to ensure a higher quality of life for future generations? If so, the parallel of the old Norse myth is an effective one, and it is in our best interest to protect Yggdrasil, or biodiversity, and stave off ecological Ragnarök for as long as we can!

For more information on emerald ash borer in Kentucky visit

Until next time!


1. Larrington, Carolyne (Translator) (1999). The Poetic Edda. Oxford World's Classics. ISBN 0-19-283946-2
2. Wagner DL. 2007. Emerald ash borer threatens ash-feeding Lepidoptera. News Lepid Soc 49:10–11

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