Maryland Tree Pests: Emerald Ash Borer, Spotted Lanternfly & Oak Wilt

Emerald Ash Borer: The Ash Killer

The emerald ash borer (EAB) is a small, metallic-green beetle from Asia that has killed millions of ash trees across the eastern U.S. It's well established in Maryland — first detected in Prince George's County in 2003 and confirmed on the Eastern Shore by 2015. If you have ash trees (green ash and white ash are common landscape trees here), assume the pest is in your area.

Signs to look for: thinning and dieback starting in the upper third of the canopy is usually the first visible symptom; D-shaped exit holes in the bark where adult beetles emerged; vertical bark splits; heavy woodpecker activity (they're after the larvae); and clusters of small sprouts growing from the trunk below the dying crown (epicormic sprouting). Infested trees typically decline over 2–4 years.

What to do: a still-healthy ash (less than about a third of the crown affected) can often be protected with systemic insecticide treatments applied by a licensed professional — these need to be repeated every year or two, so it's a commitment, not a one-time fix. A heavily infested ash is a removal candidate: dead ash becomes brittle and hazardous fast, and it's considerably more dangerous (and expensive) to remove once it's fully dead. Don't move ash firewood — that's how the pest hitchhikes to new areas. The University of Maryland Extension maintains current EAB guidance for homeowners weighing treatment versus removal.

Spotted Lanternfly: The Spreading Invader

The spotted lanternfly (SLF) — a striking planthopper with spotted gray forewings and red hindwings — was first found in Maryland in Cecil County in 2018 and has since spread across much of the state. It feeds on a wide range of plants (maples, fruit trees, grapes, and especially tree-of-heaven), weakening them and coating surfaces with sticky honeydew that grows black sooty mold.

What to look for, by season: in spring and summer, the nymphs — first black with white spots, later red with white spots — cluster on stems. Adults appear in late summer. In fall and winter, look for egg masses on tree trunks and any vertical surface: they look like smears of grayish mud, about an inch long.

What to do: scrape egg masses off bark in fall and winter using a plastic card, catching them in a bag with hand sanitizer or rubbing alcohol — scraping them onto the ground isn't enough to kill them. Crush or stomp nymphs and adults on sight. Consider removing tree-of-heaven (Ailanthus altissima) from your property: it's the lanternfly's preferred host, it's itself invasive, and its presence anchors infestations. Report sightings to the Maryland Department of Agriculture — your reports help the state track the spread. And skip the home-remedy sprays: the University of Maryland Extension specifically warns against vinegar, dish soap, and similar DIY treatments, which harm beneficial insects without controlling lanternflies.

Oak Wilt: The Fungus After Your Oaks

Oak wilt is a fungal disease present in Maryland that plugs the water-conducting tissue of oaks. Red oaks (pointed leaf lobes) can die within weeks of infection; white oaks (rounded lobes) decline more slowly and sometimes recover. It spreads two ways: sap-feeding beetles carrying spores from infected trees to fresh wounds on healthy ones, and root grafts between neighboring oaks.

The timing rule is the whole game: avoid pruning or wounding oaks from mid-April through mid-July, when the beetles are active — this is the Maryland DNR's standing guidance. If storm damage forces a cut during that window, dress the wound promptly; wound paint is normally discouraged in arboriculture, and oak wilt season is the exception. Our pruning timing guide covers this in detail.

Signs of infection: rapid leaf wilt and drop starting in the upper crown in early-to-mid summer, sometimes with leaves showing odd two-tone discoloration. Suspect oak wilt should be reported — the Maryland DNR and Department of Agriculture handle diagnosis, since confirmation requires lab work. There is no cure for an infected tree; management focuses on preventing spread to neighboring oaks.

General Rules for All Three

• Report first, react second. The Maryland Department of Agriculture tracks all three pests. Your sighting report genuinely helps the state's response — and for oak wilt, professional diagnosis is the only way to confirm.
• Don't move firewood. It's the classic vector for EAB and a plausible one for SLF egg masses. Burn local wood locally.
• Keep stressed trees vigorous. None of these pests are stopped by watering and mulch alone — but a stressed tree declines faster under any pest pressure, so the basics (proper mulch ring, no volcano mulching, watering in drought) still matter.
• Know when it's a removal decision. A dead or structurally compromised tree is a hazard regardless of which pest got it there. Our tree removal page covers how declining trees are evaluated — and storm prep explains why deadwood shouldn't wait.

Frequently Asked Questions