By David Page, Swarthmore Tree Committee
Readers, beware. Your trees are under attack. The wooly adelgid damages your hemlocks. The gypsy moth damages your oaks. Now, your ash trees are in deadly peril, threatened by another small insect.
The emerald ash borer is coming and there is nothing you can do to stop it. But that doesn’t mean you can’t protect your trees. The emerald ash borer is a metallic green, half-inch long beetle believed to have been inadvertently brought into Michigan in the early 2000’s in packing crates from its native Asia. EAB primarily infest, damage, and eventually kill ash species (genus fraxinus), including green ash, blue ash, black ash, and white ash. In its native range,the EAB is a nuisance pest, since local predators and parasitoids keep population densities below lethal levels. In America, the mortality rate for infested trees left untreated is nearly 100%.
On their own, EAB can spread about 12 miles per year, but often they spread faster in transported firewood and wood products. This year the little green meanies were found in Gettysburg and Valley Forge; it is simply a matter of time before they reach Delaware County. Adult insects lay their larvae in crevices in the bark of ash trees, and after hatching the larvae eat their way through the bark into the cambium, phloem, and xylem where they develop. Feeding larvae create long serpentine galleries that disrupt the flow of nutrients and water through the tree, in time effectively girdling the tree and killing it. When the fully developed adult beetles emerge, they leave a distinctive “D” shaped hole in the bark. The adults then feed upon the leaves of the ash until they find a mate and thus begin the process over again. Depending upon the age, size, and condition of the tree, it will usually die within two to four years.
To date, tens of millions of American ash trees have been killed by EAB; 8.7 billion more are at risk. In Pennsylvania, that risk translates into about 300 million trees, or 3.5% of the forest cover. It also equates to billions of dollars in treatment, removal and replacement costs by municipalities across the region.
According to forester Rick Hartlieb of the Pennsylvania Department of Conservation and Natural Resources, each district is treating about 200 trees a year on state park lands, and harvesting others. Once an ash tree is infected, not only does its value as a timber tree decline quickly, it also rapidly becomes more of a safety hazard.
The best defense has proven to be systemic insecticides injected into the tree or placed in the ground to be absorbed into the tree through the roots. While effective, the treatment typically has to be performed by a professional, it can be expensive (up to $500), and is only effective for about three years. Swarthmore College is already treating some of its trees, as are some local homeowners. Hartlieb says two treatments of insecticide will buy 6 or 7 years of immunity at a cost about equal to the cost of removing the tree.
Homeowners will have to weigh their choices, and the borough will have to be prepared to bear its share of removal costs. If you are unsure about whether you have ash trees, a member of the Street Tree Committee can help with identification — get in touch through the borough office.
This is one invasion we can’t avoid. It will be prolonged, and it will be painful to some. Sadly, the day may come when the ash, like the American chestnut, is basically just a forest memory, or perhaps with just a few trees surviving in remote pockets. No more Hillerich & Bradsby white ash Louisville Slugger baseball bats. Ouch.