Oystershell scale

What it is

Lepidosaphes ulmi, an armored scale insect

Trees affected

Ash, aspen, poplar and cottonwood, willow, lilac, dogwood, maple

Where in our area

Planted trees on valley lots and in the Tahoe basin, about 4,000 to 7,100 feet

Key sign

Crusty, oyster-shaped scales on bark, about 1/8 inch long

Damage

Branch dieback; severe infestations kill trees

The scales hide in plain sight because they match the bark. Colorado State Extension says it attacks more than 50 plant species and that heavy feeding can kill a branch or an entire tree. This page covers identification, which trees, what an arborist checks and when pruning or removal is the answer.

What does oystershell scale look like?

The scale is brown or gray, about an eighth of an inch long, and shaped like a tiny oyster shell, according to Colorado State Extension. They sit on the bark of twigs, branches and trunks, often in dense groups that make the bark look scaly. Utah State University says they blend in with bark, so infestations often go unnoticed until symptoms appear.

Symptoms are the real alarm: branches encrusted with scales, bark that looks scaly and cracks, small off-color leaves, reduced vigor and dieback of individual branches. A dead branch with a crust of scales on its twigs is the typical find.

• Gray-brown oyster-shaped scales, about 1/8 inch
• Bark that looks scaly and cracks
• Small, pale or sparse leaves
• Branch dieback

Which trees and which elevations?

Colorado State lists ash, cotoneaster, aspen, dogwood, lilac, poplar and willow as the most commonly infested; Utah State adds maple and cottonwood to its Utah list. In our area that includes ash, quaking aspen, Fremont cottonwood, weeping willow and silver maple.

Planted trees on lots from Reno (4,505 feet) and Carson City (4,680 feet) up to Incline Village (6,437 feet) are host trees, so elevation does not settle it. We have not found a Nevada survey of how common the scale is, so we make no claim on that.

How does it spread, and what does it do?

Colorado State describes the cycle: eggs overwinter under the dead mother's shell, hatch in late May or early June in Colorado and the crawlers settle within hours on the bark. They develop a waxy covering within a week and stay in place for life. The scales suck sap from the bark and weaken the plant, and the damage is usually branch by branch before it is whole-tree.

A crust on twigs does not by itself mean the tree is dying; the dieback is what counts. See drought stress and dieback.

What does an arborist check?

The arborist looks at how far the scale has spread along the tree, whether it is on small twigs or the trunk, how many branches are dead and how much of the crown is still leafing out well. They look for signs of other problems such as Cytospora canker.

• Where on the tree the scale is
• Number of dead branches
• Crown density and leaf size
• Other bark problems
• Neighboring host trees

When is pruning or removal the answer?

Utah State says damage is usually limited to individual branches, and that localized infestations can be removed by pruning. Colorado State likewise recommends pruning heavy infestations, especially when neighboring trees are clean. That covers most yard trees: remove the encrusted branches and dead wood.

Removal is the answer when the main trunk and scaffold limbs are coated, a majority of the crown is dead or the tree is a hazard over a house. Ash and cottonwood can be large; see large tree removal.

What does Battle Born Tree Service do about oystershell scale?

Start with a free estimate. A certified arborist assesses the tree before any work begins, and the estimate says whether the answer is pruning out infested branches or removal. Two of our crew are ISA Certified Arborists, and credential WE-11728A belongs to one of them. We identify the problem, prune out dead or hazardous wood and remove trees that cannot be saved; we do not spray, inject, fertilize or apply products.