Japanese maple scale

What is Japanese maple scale?

Japanese maple scale is an armored scale insect that was probably brought into the US from Asia. It was first found in Connecticut in 1914 and is now in Pennsylvania and many other eastern states. Each female lives under a tiny, grayish-white, oystershell-shaped wax cover on the bark.

The scales pierce plant cells and drain fluids. That causes leaf yellowing, early leaf drop, a thin crown, and dieback; heavy infestations can kill twigs and sometimes whole plants. Unlike soft scales, it does not make honeydew.

It is a serious nursery pest because it causes plant losses and rejected stock (UMass), and Penn State calls it difficult to manage. At low numbers it blends into bark crevices, so it is easy to miss.

Signs and damage

• Tiny grayish-white, oyster-shell-shaped covers on bark of trunks, branches and twigs, often packed at branch collars
• Purple body and eggs under the white cover
• Yellowing leaves, early leaf drop and a thin crown
• Twig and branch dieback on heavily infested plants
• No honeydew or sooty mold

Japanese maple scale identification: what to look for

• Adult: Female cover is elongate, oystershell-shaped, grayish white, 1 to 2 mm long; the body under it is purple. Males have smaller covers of the same color and become tiny winged insects.
• Eggs: About 25 per female; Penn State calls them white with a purple hue, while UMass and the University of Kentucky call them purple.
• Larva: Crawlers are tiny, oval and six-legged; UMass describes them as purple to lavender, Penn State as white.
• Damage: Leaf discoloration, early leaf drop, thin canopy, dieback; no honeydew.
• Timing: In PA, one generation; males are present late April to late May and crawlers are active late May through early August.

Life cycle

In Pennsylvania there is one generation a year, and mated females spend the winter on the bark. Males are out from late April to late May. Each female lays about 25 eggs, and crawlers are active over a long stretch, from late May through early August, which makes timing control hard. UMass says Maryland, and possibly Virginia, have two generations, and University of Kentucky surveys found two in central Kentucky, with crawlers able to start forming a wax cover within about three days of hatching.

When to look: Inspect bark in the dormant season, when leaves don't block the view. Watch for crawlers from late May through early August (Penn State) with electrical tape wrapped sticky side out around a twig or trunk, set in May and replaced every 2 to 3 weeks (University of Kentucky).

Japanese maple scale look-alikes

A few things get mixed up with it:

• White prunicola scale (Pseudaulacaspis prunicola): Female covers are round, like a fried egg, and masses of males make bark look woolly white.
• White peach scale (Pseudaulacaspis pentagona): Also has round, fried-egg-like female covers rather than JMS's narrow oystershell shape.
• Oystershell scale (Lepidosaphes ulmi): Also has an oystershell-shaped cover; UMass says it can easily be confused with JMS, and the University of Kentucky says JMS superficially resembles it.

How to control Japanese maple scale

Cultural control

• Scout trunks, branches and twigs, especially branch collars.
• Prune out and destroy heavily infested branches or twigs where practical.
• Keep plants healthy and vigorous.
• Monitor often during the long crawler period.

Biological control

• Parasitic wasps, lady beetles and fungal diseases attack it, but UMass says their effect in landscapes isn't well understood.

Chemical control

Penn State says to treat when crawlers are active, from late May through mid-August, and notes soil-applied systemics may work less well because the scale lives on twigs, branches and trunks. UMass says dormant oil may reduce crawler numbers the next season, recommends reduced-risk products to protect natural enemies, and lists options such as horticultural oil, insecticidal soap, buprofezin and pyriproxyfen among others. Apply after bloom when pollinators are least active. Use only products labeled for this use; the label is the law. Have an ISA Certified Arborist or licensed applicator treat trees. Muthler Landscaping does not treat trees or apply pesticides; see our pesticide applicator license guide.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Check plants in the dormant season, when bark is easy to see.
• Look closely at branch collars and bark crevices on maples, dogwoods, zelkova, holly and pear.

When to call a pro

Call an ISA Certified Arborist if a maple or other landscape tree has thinning foliage and bark crusted with tiny white scales. Penn State calls this scale difficult to manage.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about Japanese maple scale

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