Emerald ash borer
Agrilus planipennis
The insect that is killing the ash trees of North America. We carried the European ash disease and not this — which is the wrong way round, and this entry fixes it.
How to know it
The adult is a narrow, brilliant metallic-green beetle 8–14 mm long, seen only briefly in early summer. What identifies the infestation is the exit hole: a crisp capital D, three to four millimeters across, flat on one side. Under the bark, the galleries wander in tight S-curves packed with fine frass. Nothing else makes that D.
What you'll see
- D-shaped exit holes in the bark, three to four millimeters wide, flat along one edge.
- Winding S-shaped galleries packed with sawdust under a lifted flap of bark.
- Crown thinning and dying from the top down, over two to four years.
- Sprouts breaking out of the trunk and the base below the dying crown.
- Woodpeckers stripping patches of bark off in winter, leaving a pale blonded look to the upper trunk.
Where it comes from
One generation a year, sometimes two years in a healthy tree. Adults emerge in late spring, feed lightly on ash leaves, and lay in bark crevices. The grubs tunnel in the tissue that carries food down the trunk, and a few years of accumulated galleries girdle the tree completely. It moves slowly on its own wings and very fast on firewood.
Introduced to Michigan around 2002 and now across most of the eastern and central United States and Canada, killing essentially every untreated ash it reaches. Mortality approaches 99 per cent within a decade of arrival.
Dealing with it
- Decide early: treat or remove. A tree that has lost more than about a third of its crown cannot be saved.
- Have a valued, healthy ash treated by an arborist with emamectin benzoate, injected every two or three years — this works, and it works indefinitely if started before the decline.
- Have declining ash removed while the wood is still sound; a dead ash becomes brittle very quickly and is dangerous to fell.
- Chip or burn the wood on site and never move it.
- Report a first county detection to the state agriculture department.
Keeping it out
- Do not move firewood. This is how it crossed the continent and it is the one thing an individual can control.
- Plant something other than ash, and diversify any planting that is heavy on one genus.
- Inspect ash trees each spring for D-shaped holes and woodpecker blonding.
- Start treatment on a valued tree before the borer arrives in the county, not after the crown thins.
- Support the parasitic wasp releases where they are running; they are the long-term answer.
What it goes for
All North American ash species — white, green, black and blue — and, lightly, white fringetree.
Who hunts it for you
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com