White scale on a black pine

You are cleaning a Japanese black pine in late summer, working a pad of foliage between your fingers, and there is a white fleck on a needle that does not brush off. Then another. Then you tilt the tree toward the light and the whole interior of the pad is frosted with them. That is pine needle scale, it is the most common pest a black pine gets, and the good news is that it is a slow, dumb insect that loses to anyone who understands its calendar.

This is how I deal with it. The order matters more than the products.

A scale insect spends eleven months of the year under armour, where nothing you spray can reach it. It is vulnerable for a few weeks — twice — and the whole method is knowing when.

Make sure it is scale

Three white things turn up on a black pine, and they are dealt with differently, so the first job is a fingernail.

Pine needle scale is the one this post is about. An armoured scale: each insect lives its adult life under a hard, waxy, elongated white shell about three millimetres long, glued flat to the needle, often with a yellow-brown point at one end. Press it and it resists; scrape it and it leaves a scar on the needle. It does not move, and unlike soft scales it produces no honeydew, so there is no sooty mould and no ants — which is why an infestation goes unnoticed until the pad looks dusted.

Pine bark adelgid is loose white cotton on the bark and around the bases of the needles, not on the needle blades. Wipe it and it smears into fluff. Mealybugs are cottony ovals that sit in the crotches of branches and, if you watch, walk. Both are dealt with by the same oil and soap that follow, but neither has the armour, so with those the timing hardly matters. With scale it is everything.

Why the timing is everything

The female lays her eggs under her own shell in autumn and dies; the shell stays on the needle all winter with the eggs beneath it, and nothing you can safely put on a pine gets under it. In spring the eggs hatch into crawlers — tiny, orange, mobile, unarmoured — which wander for a few days, pick a needle, settle, push in a mouthpart, and begin growing a shell of their own. From that point they are armoured again. Contact sprays only work on crawlers, and crawlers exist for a couple of weeks at a time.

In central Kentucky the first hatch is in May — a good marker is lilac in full bloom, about three weeks earlier, which puts crawlers on the needles from early May. There is a second generation from late July into August. The whole year, with the black pine work I already do laid on top of it:

Notice where the pine's own calendar lands. Decandling in mid-June comes right after the first crawler window; the new needles that follow it are soft and burn easily, which rules out oil on that growth for the rest of the summer. Needle-pulling in autumn opens the tree up, which the scale hates, and the worst needles can simply leave with the pull. The treatments below are slotted into those gaps rather than fought against them.

Right now: scrub, then watch

Any day the tree is dry, the most effective thing I own is a soft toothbrush. Hold the pad from behind, work along each needle from base to tip, and the shells come off. A bonsai is small enough that you can actually do this to every needle, which is the one advantage over a landscape pine. Old shells are dead insects anyway — they stay stuck on for a year or more after the female under them has gone — but each one may be sitting on a clutch of eggs, and a scrubbed needle has nothing to hatch from.

Then check what you removed. Flick a shell over: a live female underneath is plump and yellow-orange; a dead one is a dry hollow. If they are mostly hollow you are looking at last year's war, not this year's, and the tree has probably already beaten it — a black pine in good health sheds a light infestation on its own with help from the twice-stabbed lady beetle and a couple of parasitic wasps, which is one of two reasons to be slow to reach for anything broad-spectrum.

Then watch for crawlers, because that is when the sprays work. Wrap a piece of double-sided tape around a branch in the infested pad and look at it every few days with a loupe; or tap a pad over a sheet of dark paper. Orange specks that move are crawlers, and the clock starts.

The crawler window: soap or summer oil

When crawlers are on the tape, spray. Two things work, both by contact, both harmless to the beetles once dry:

Coverage is the entire game with contact sprays. Scale live on the undersides and in the interior of the pad, exactly where a spray from above does not go; turn the tree, spray up into it, and get the trunk side of every needle. Three applications a week apart across a window is normal, because the hatch is staggered.

The dormant oil is the one I would not skip. Late winter, before the candles move, a heavier rate goes on the whole tree, and it gets at the eggs under the old shells in a way nothing during the growing season can. On a pine that has had scale it is an annual job, whether or not you can see any.

What I do not do

• Spray on sight. A spray in August at a pad full of armoured adults kills nothing and the beetles. Scrub, tape, wait.
• Judge by the shells. The shells stay on for a year after the insect is dead. Judge by crawlers on the tape and by the colour of the new growth.
• Oil a stressed tree. A black pine that is dry, freshly repotted, freshly decandled, or sitting in a heatwave gets scrubbed and nothing else until it is not.
• Reach for a systemic first. Armoured scale are the one group systemics are least reliable against, and a drench in a bonsai pot is a strong dose in a small root mass. It has a place — a tree too dense to cover — but it is the last thing, not the first.

The cultural half, which is most of it

Scale is a stress pest. It arrives on nursery stock — check every new pine before it goes on the bench, and quarantine it for a season regardless — and it establishes in trees that are crowded, shaded, under-watered or over-fed with nitrogen, which describes a lot of black pines kept dense for display. The things that make a black pine a good bonsai are also the things that make it a good home for scale: tight pads, interior needles, no air moving through.

So the work you already do is the treatment. Needle-pulling in autumn to balance the tree also opens it to light and air and takes the worst needles away in your hand. Keeping pads thinned so you can see through them gives the beetles somewhere to hunt and a spray somewhere to land. Full sun. Water when it needs it and not before. A black pine that is grown hard is a black pine that scale does not get much of a foothold on, and every step in this post is easier on a tree that was healthy to begin with.

If you found the frosting today, in early September: scrub it, put the tape on, get the tree into more sun, and write "dormant oil" on the calendar for the first mild week of March. That is usually the whole fix.