Oystershell scale: what the extension services record

This page describes what the named sources say. It is not a diagnosis of any tree and does not say what should be done with one. None of the four pages read lists oystershell scale as regulated or notifiable. Each section names its source, all read on October 9, 2026. The scale has a row in insect pests of trees, which compares it with the other scale insects and caterpillars listed there.

Names and origin

All four sources give the scientific name Lepidosaphes ulmi. Penn State Extension says that when the insect was first described in Europe in 1758 it was called the mussel scale. NC State Extension says it likely arrived in North America with European settlers in the 1700s. The Morton Arboretum calls it an introduced species that is commonly found throughout North America. Penn State places it across the United States, with northern states seeing it more often than southern ones.

What the scale looks like

The sources describe an armored scale, one that secretes a hard cover over its body. Penn State gives the mature cover as about 2.5 mm long, grayish brown and noticeably convex, like a miniature oyster shell, developing on the bark. NC State says females sit hidden under waxy coverings, comma-shaped or oyster-shaped, from gray to brown, measuring 1/16 to 1/8 inch. UC IPM, the University of California's pest management program, says nymph covers are silver gray and adult female covers coppery brown to purplish, narrow at one end and wider and rounded at the other, with mature females about 1/8 inch long, often in crowded clusters on bark. The Arboretum says an infested plant shows oyster-shaped scales on twigs, branches or possibly the trunk, and that with an extremely heavy infestation the bark of a branch can be covered entirely.

Hosts

Penn State says the scale usually infests lilac, ash, dogwood, maple, poplar and willow and has been reported on more than 130 host plants. NC State says more than 100 plant species, including maple, ash, dogwood and apple, and notes it is less common than San Jose scale in North Carolina orchards. The Arboretum lists apple, ash, dogwood, lilac, maple, pear, poplar and willow, and says the two races of the scale can differ in host range. UC IPM says that in California it generally damages only poplars and especially willows, that populations on its other hosts are generally innocuous, and that it feeds on about 150 plant species there including alder, ash, maple, sycamore and walnut.

Life cycle as the sources state it

All four say the scale overwinters as eggs under the cover of the female. Penn State says one female may lay 20 to 100 eggs, that they hatch from late May to early June into crawlers, pale yellow and under one millimeter long, and that the scale has one generation a year in Pennsylvania. NC State gives hatching from late April through June, crawlers yellow and about 1/25 inch, and one generation a year in North Carolina. UC IPM says crawlers emerge in spring after buds burst, with one generation a year in Northern California and two in Southern California.

The Arboretum describes two races. The gray-banded race has one generation a year in the Midwest and the brown race has two. It says crawlers of the gray-banded race emerge when Vanhoutte spirea is in full flower and those of the brown race a little later, and that the brown race produces a second generation of crawlers in mid-summer. It describes crawlers as highly mobile, six-legged, uncovered and under 1/32 inch, and says that once they settle they lose their legs and antennae and secrete the shell. The sources disagree on males: Penn State says winged males emerge, mate and die, active from late June through early July, while UC IPM says males of this species are not known to occur.

Damage as the sources describe it

Penn State says the scale removes plant fluid with piercing-sucking mouthparts, that branches become encrusted, and that twig or branch dieback is common. It says a tree or shrub will occasionally die in a severe infestation. The Arboretum says armored scales burst plant cells and feed on the contents, that plants usually tolerate small populations, and that larger ones lead to yellowing leaves, twig dieback and stunted growth. It adds that armored scale inject their waste back into the plant, so no honeydew is produced. UC IPM says that after several years, dense populations can kill twigs and branches of young poplars and willows and stunt the growth of mature ones. NC State says heavily infested orchard trees may die if left untreated.

Natural enemies, status and reporting

Penn State names tiny parasitic wasps, lady beetles and predatory mites that feed on the scale, and says they are usually more noticeable when a severe population has built up. None of the four pages names a regulation, quarantine or report line. This page gives no treatment steps, product names, rates or timings. For who may apply pesticides see pesticide licences.