Ash trees and emerald ash borer: should you treat or remove? Plus hemlock woolly adelgid
Updated · KeystoneProGuide editorial team
Should you treat an ash tree or take it down?
Pennsylvania's Department of Conservation and Natural Resources (DCNR) gives a clear rule. "Only ash trees that are healthy and are of value to the homeowner should be considered for treatment. Other ash trees should be removed before they become a hazard." It also says insecticide applications should begin before the tree is attacked. So the question is less "can it be saved" than "is it still healthy, and do you want to keep it."
What is emerald ash borer, and how fast does it kill?
A small beetle from Asia whose larvae feed under the bark of ash trees. DCNR says the half-inch, metallic green beetle can now be found in nearly every county of the state, that it was first identified in North America in 2002 and in western Pennsylvania in 2007, and that its larvae kill ash trees three to five years after infestation. All native North American ash species and cultivars are susceptible, as is the white fringetree.
DCNR lists the signs of infestation:
- dieback in the upper crown;
- shoots sprouting along the trunk (epicormic branching);
- splits and flaking in the bark;
- damage from woodpeckers feeding on the larvae;
- D-shaped exit holes in the bark;
- S-shaped feeding tunnels just under the bark.
Other warning signs to watch for on any tree are in signs a tree needs an arborist.
What treatments are there?
DCNR names four kinds of insecticide treatment for landscape ash: systemic products applied as soil injections or drenches, systemic trunk injections, systemic sprays on the lower trunk, and protective cover sprays on the trunk, main branches and foliage. It cautions that controlling insects that feed under the bark is challenging, and that preventing an infestation means choosing the product and method with care and applying it at the right rate and time.
Some of these products are sold for homeowners to use. DCNR says the most effective option, though, is a Restricted Use Pesticide that only certified pesticide applicators may use. On the cost site Angi's arborist price list, tree injections run $50 to $200.
Who is allowed to treat your trees?
Anyone who applies a pesticide for you needs a state licence. The Department of Agriculture says a commercial applicator licence is required to use pesticides on another person's property, that applicators must pass a core exam and a category exam, and that one of the categories is "Ornamental and shade trees". The applicator must work for a business holding a pesticide business licence, which requires at least one certified applicator and proof of insurance. A business may also use registered technicians who have been trained under a certified applicator. The details are in what Pennsylvania and its towns require.
Why not wait and see?
Because the tree gets harder and more dangerous to deal with. DCNR's advice is to remove untreated ash "before they become a hazard". Penn State Extension's Master Gardeners say removing a live tree is usually less expensive than removing a dead one, since dead wood becomes brittle and dangerous and needs extra safety measures. If an ash near the house is already thinning badly, get it assessed sooner rather than later.
What about the wood once the tree is down?
Keep it local and check the rules. The Department of Agriculture's spotted lanternfly quarantine regulates moving "Logs, stumps, or any tree parts" and firewood of any species, and it applies to residents as well as businesses. Ask your tree company how it handles wood and whether it holds the permit.
Is my town doing anything about ash trees?
Some are. DCNR lists completed emerald ash borer management plans for several Pennsylvania communities, among them the City of Philadelphia and the Borough of West Chester.
What is hemlock woolly adelgid?
A tiny sap-feeding insect that attacks the eastern hemlock, Pennsylvania's state tree. DCNR says its feeding causes needle drop, branch dieback and death, and that its egg sacs look like the tips of cotton swabs clinging to the undersides of the current year's twigs. It was reported in southeastern Pennsylvania by the late 1960s and had been found in all 67 counties by the end of 2023.
Can a hemlock be saved?
Sometimes. DCNR says research shows systemic insecticides injected into the trunk or applied to the soil kill the adelgid and can prevent new infestations for up to five to seven years. For homeowners, it says ornamental and landscape hemlocks can be treated, but that the treatments "can be expensive and results can vary." The same licence rules apply to anyone treating them for you.
How should treatments be applied safely?
By the label, and with pollinators in mind. Penn State Extension, writing about lanternfly treatments, says never to spray any insecticide on plants that are in bloom, and that soil drenches should go on after a tree's flowers have faded. The International Society of Arboriculture says plant health care programs rely on routine monitoring and preventive treatment, and that some problems need treatments spread over several years. Price ranges for removal, if it comes to that, are in what tree work costs. Businesses that serve your area are on the tree services directory page.
Sources
- Agrilus planipennis (Emerald Ash Borer), Pennsylvania Department of Conservation and Natural Resources
- Adelges tsugae (Hemlock Woolly Adelgid), Pennsylvania Department of Conservation and Natural Resources
- Pesticide Applicators, Registered Technicians, and Certification Exams, Pennsylvania Department of Agriculture
- Pesticide Application Businesses, Pennsylvania Department of Agriculture
- When to Call an Arborist, Penn State Extension (Allegheny County Master Gardeners)
- Spotted Lanternfly Management Guide, Penn State Extension
- How Much Does an Arborist Cost?, Angi
- Tree Health, International Society of Arboriculture (TreesAreGood.org)
- Spotted Lanternfly Quarantine and Permitting, Pennsylvania Department of Agriculture