Bark beetles
Scolytinae — Ips, Dendroctonus and Scolytus spp.
also called Engraver beetles, Ips beetles, Shothole borers
Pinhead holes in the bark and engraved galleries beneath it. They attack trees that are already stressed, which is why watering is the real control.
How to know it
The beetles are cylindrical, dark and only 2–5 mm long and you will rarely see one. Identify from the bark: round holes the size of a pinhead, often with fine sawdust caught in the crevices below them, and — the decisive sign — a pattern of galleries engraved into the wood and the inner bark when a flap is lifted, with a central tunnel and side branches like a centipede.
What you'll see
- Neat round exit holes, one to two millimeters across, peppering the bark of a trunk or branch.
- Fine reddish-brown boring dust caught in bark crevices and around the base of the tree.
- Popcorn-shaped blobs of resin on pines where the tree has tried to pitch the beetle out.
- Foliage fading from green through yellow to red-brown, usually top-down or one branch at a time.
- A pattern of engraved galleries under a lifted flap of bark, which is the confirmation.
Where it comes from
Adults bore through the bark and excavate a gallery in the living tissue beneath, where the female lays along its sides; the grubs tunnel outward at right angles, making the engraving. One to several generations a year. Many species carry fungi that stain and block the wood, and elm bark beetles carry Dutch elm disease from tree to tree — the beetle is often less important than what it brings.
Almost entirely a disease of stressed trees. Drought, root damage from construction, sunscald, fire, transplant shock and overcrowding are what let them in; a vigorous conifer pitches them straight back out. Regional outbreaks follow multi-year droughts.
Dealing with it
- There is nothing to be done for a tree already colonized. Remove it and chip, burn or debark the wood before the beetles emerge.
- Do not store infested logs as firewood near living trees, and never move them to another area.
- Water the surrounding trees deeply and mulch them; keeping the neighbors vigorous is the only real control.
- Prune only in winter, and never leave fresh cut stumps or slash piles through the flight season.
- Have a valued tree assessed early; by the time the crown fades the beetles have usually already left.
Keeping it out
- Water trees deeply through droughts. This is the single most effective thing a gardener can do about bark beetles.
- Avoid root damage from trenching, paving and compaction around established trees.
- Thin overcrowded conifer plantings so each tree has room.
- Remove dying and storm-damaged trees promptly rather than letting them become a breeding site.
- Do not move firewood between areas.
What it goes for
Pines, spruces, firs and larch above all; also elm, birch, peach and other stressed broad-leaves.
Trouble it brings with it
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
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com