Two weeks ago I lamented that fall color is disappointing this year but I should have waited. The trees in Schenley Park looked better last week with red maples, yellow hickories, and this small tree reminding me of what we’ve lost.
Those pale green, yellow, orange and violet leaves are on a small ash tree whose trunk diameter is too small to be plagued by emerald ash borer … and now I’ve found out why.
Before the emerald ash borer (EAB) invasion(*), mature ash trees added pastel violet to the splash of color on our hillsides but now only the saplings are left.
Just across the trail from the ash sapling stands a mature ash that’s alive, though struggling. Some upper branches have died back and there are sucker branches below them. An old emerald ash borer hole shows what the mature tree was dealing with.
The old tree is alive because it received insecticide treatments during the height of the EAB invasion from a program of the Pittsburgh Parks Conservancy and the City of Pittsburgh. Beginning in 2011, 158 specimen ash trees were treated in the city parks. This one is #76 according to the metal tag.
Years later it appears that emerald ash borer numbers have dropped and it wasn’t because we used insecticides.
Scientists working on EAB biological control in Michigan found that many factors contributed to the emerald ash borer population collapse there.
“Woodpeckers, native and introduced parasitoids, intraspecific competition, disease, innate tree defenses, and reduced ash abundance contributed to the collapse of EAB populations.”
Notice that woodpeckers are at the top of the list!
Second on the list are four tiny parasitic insects that kill emerald ash borer larvae. Two native insects target emerald ash borers through the thin bark of saplings and at Michigan study sites scientists introduced two more parasitic insects from China, the emerald ash borer’s homeland, to get through the bark of mature ash trees.
Thanks to the hard work of scientists and arborists we may hope that our ash saplings will grow into mature ash trees.
Read more about ash tree biological controls at this U.S. Forest Service webpage.
(photos by Kate St. John)
(*) Emerald ash borer was first discovered in Michigan and Ontario in the summer of 2002. It was confirmed in western Pennsylvania in June 2007.