By FRANCES LAMBERTS
In 2015, the magazine of the Nature Conservancy described the white ash as an American icon, one of four trees that have “helped shape a cultural identity” but are now being decimated by “killer insects.”
Its great strength while being light, pliant and resistant to splintering made the wood of white ash ideal for very many uses – in sports, gardening and agricultural tools, flooring and furniture, boat building and oars, church pews and others. Since “it was of white ash, and white ash only, that good baseball bats [were] made,” its best known use was for these, making it “the tree that hits home runs.”
A deciduous member of the olive family, white ash (scientific name Fraxinus americana) is considered the largest and also finest of all North American ashes.
Its leaves are arrayed opposite to each other, each having 7 short-stemmed leaflets, dark green above but much paler on the underside. Its flowers are inconspicuous, pollinated by wind and appearing before the leaves emerge, male and female flowers on different trees.
The arboretum’s white ash was fully grown and had seemed ailing, its foliage thinning on some branches, for about 18 months. A round larval exit hole revealed, to the experienced eye of the Department of Agriculture inspector, the source of its distress. A clearwing borer had managed to invade it. Although adults of this native, wasp-like insect live only a few days and do not eat, their larvae spend a year tunneling through the sapwood under the bark. As they mine the sapwood tissue for food, they block or at least hinder the tree’s ability to send its food and water efficiently to the roots and crown and branches.
Over eons of evolution, trees have come up with remarkably effective ways, through biochemistry, to defend themselves from herbivorous predators or larval or fungal invaders. In a book titled “Tree, a Life Story,” David Suzuki describes one of their larva-killing methods. “When an insect larva bores into a tree, it risks puncturing” one of the ducts through which many trees send terpene-based resin products up and down the stem. If punctured, “resin pours into the insect’s feeding chamber,” killing it. For added protection and to prevent disease-causing fungi to get in, “the resin then hardens, sealing the (larva-entry) wound.”
Our white ash had to be taken down, even while a companion green ash is unaffected. A sapling-size replacement white ash can now be found in a sunny spot up-slope from the Britt Drive walkway.
The “killer insects” threat noted by the Nature Conservancy refers to a different borer specific to ash trees. An iridescent green beetle, called Emerald Ash Borer, was inadvertently introduced to the United States, in a shipment from Asia, in the 1990’s.
Lacking predators and evolutionary tree defense here, it has become a devastating pest, killing more than 500 million ash trees since then.
The Emerald borer larva leaves a differently shaped exit hole – like an upside-down letter D – through which it may be identified.