Oystershell Scale

Lift an oyster-shaped bump on the bark of a lilac, maple, or apple. White eggs underneath, or a moist yellow body, confirm oystershell scale, so cut the twigs that are crusted wall to wall and then wait for pale crawlers in late spring. Wet the remaining bark with labeled horticultural oil while those crawlers are moving, because a spray on settled shells does not reach the eggs under the dead female cover. Empty covers can stay on the wood for years after the insect is dead, so the crust is not the same as a live outbreak.

People still say mussel scale. Then the real name: oystershell scale, Lepidosaphes ulmi. It is an armored scale insect, not a houseplant bump and not the tropical foliage sold as oyster plant. Females sit under a gray-brown cover about an eighth of an inch long, shaped like a tiny oyster, on trunks and twigs. They feed by bursting cells under the bark, so they do not drip honeydew and they do not grow sooty mold. A light coating on a vigorous mature tree can wait. A lilac or small maple whose twigs are fully encrusted can lose those branches, and a small shrub can die if live crawlers keep coming back.

Prune the crusted twigs, then oil the crawlers

The only stage that walks is the crawler, a pale yellow speck smaller than a pinhead. Once it settles, it loses its legs and secretes the oyster-shaped cover, so contact sprays bounce off and dormant oil often fails to reach the eggs packed under last year's dead female. Start by cutting out twigs that are already a solid crust, because those stems are usually dying and the spray cannot reach every cover in a layered pile. On a small lilac or cotoneaster you can reach, a plastic dish pad used gently before hatch scrapes covers and exposes eggs. Then watch the remaining bark in late spring. In much of the northern United States that hatch runs from late May into early June, often as lilacs finish bloom, but weather moves it, so confirm crawlers instead of spraying by the calendar. Farther south the hatch can start in April. Wrap double-sided tape around an infested twig, or tap a branch over white paper, and treat when you see the specks, then repeat if the label allows and tape still catches new crawlers, because eggs hatch over one to three weeks. Some brown-race populations put out a second hatch in midsummer, so check again in July or August before you assume the year is done.

Use the labeled horticultural oil at the growing-season rate, or labeled insecticidal soap, and wet the bark until it drips. Kitchen dish liquid can burn foliage, especially on Japanese maple, and it has no plant directions. Skip a wilted plant and skip bloom, so bees are not walking a treated surface. A soil drench of imidacloprid is the wrong chemistry for this armored scale. Dinotefuran can move far enough to hit it, but that is a last option on a declining shrub after bloom, not the first numbered step. Pyrethroid and carbaryl sprays kill crawlers and also kill the wasps and lady beetles that already punch holes in covers, so they stay off a light infestation you can prune.

1. Confirm. Lift a cover. White eggs or a moist yellow body mean the insect is alive. A dry flake with nothing under it is an old shell.
2. Prune heavily crusted, dying twigs and bag them. On a small shrub, scrub remaining covers with a plastic dish pad before hatch.
3. Watch for crawlers in late spring with tape or a tap over paper, then again in midsummer if the crust was brown oystershell rather than gray-banded.
4. Oil or labeled soap on the bark while crawlers are moving. Coverage is the method. Repeat if hatch continues. Do not treat settled shells as if they were crawlers.

Empty covers stay after the insect is dead

Old oystershell covers glue themselves to bark and can sit there for several years, so a branch can look worse than the live population. Mash a cover. Live scale smears or bleeds a little moisture. Dead scale is dry and flakes. If late-spring tape stays clean and the shrub is still pushing normal leaves, you are looking at leftover armor, not a reason to fog the canopy. A healthy mature maple or ash with a few live crawlers and no dieback can wait. Natural enemies — tiny parasitic wasps that leave pinholes in covers, and some lady beetles — already work that bark, and a broad spray in June undoes them.

Act when crawlers are abundant and the plant is already showing yellow, undersized leaves or twig dieback, especially on lilac, dogwood, young maple, aspen, and willow. Feeding kills cells under the bark. Bark can crack later, and on aspen that injury often opens the door to canker fungi, so the wood keeps looking bad after the insect is gone. No spray greens a yellowed leaf or closes a cracked stem. Protect new growth by hitting the next crawler hatch. Replace a small shrub that has lost most of its live wood, because treating a skeleton for another season does not bring the canopy back.

Flip a cover: oyster shells on the bark

Look on twigs, branch crotches, and trunks, not on the leaf surface. Mature female covers are about an eighth of an inch long, gray or brown, convex, and tapered like a tiny oyster or mussel. They blend into bark, which is why a limb can be encrusted before you notice. Lift one cover. Overwintering eggs are pearly white under the dead mother. Newly hatched crawlers are pale yellow and walk for a few days, then settle, molt, and lose their legs. Males, when they appear, are tiny two-winged insects; they are easy to mistake for wasps walking the twig. There is usually one generation a year in the North. A brown race can run a second generation in midsummer. Nickname mussel scale once if that is the name on an old card, then keep saying oystershell scale.

Sticky film is a different scale

Oystershell scale is armored. It injects waste back into the plant, so there is no honeydew and no sooty mold under the canopy. If the lilac is sticky, or a black film is growing on the leaves, you have a soft scale or another honeydew insect, not this one. See scale insects for that fork, and sooty mold for the film. Do not run the houseplant swab calendar here. This insect lives on landscape wood.

Mealybugs look like walking cotton. They are not oyster shells glued to bark. See mealybugs. Euonymus scale females can look oyster-shaped, but the giveaway on euonymus and pachysandra is the white, ridged males spotting the leaves. San Jose scale is a round cone with a central nipple and can mark apple fruit with red halos. Round gray covers that bubble aspen trunks are poplar scale, a different armored species. A corky lenticel does not lift off a moist body. The plant sold as oyster plant, or Moses-in-the-cradle, is a Tradescantia. It is not this pest.

Related reading

• Scale insects — soft scale leaves honeydew; this armored cover does not.
• Mealybugs — cotton that walks, not oyster shells on bark.
• Sooty mold — black film on honeydew. Oystershell scale does not cause it.
• How to grow lilacs — lilac is a common host; keep the shrub vigorous so a light coating is easier to wait out.
• How to grow dogwood — dogwood twigs take this crust; prune dieback on the same wood.