Oystershell scale—Lepidosaphes ulmi
This armored scale (family Diaspididae) generally damages only poplars and especially willows. Populations are generally innocuous on its numerous other hosts.
Identification
The cover of nymphs is silver gray and that of adult females is coppery brown to purplish. The covers are elongate and narrow at one end and wider and rounded at the other end, resembling miniature oysters. Mature females are about 1/8 inch long. Nymphs and adult females are found on bark, commonly in crowded clusters. Males of this species are not known to occur.
At least nine similar-looking Lepidosaphes species occur in California. The look-alike purple scale, Lepidosaphes beckii, generally is common only on bark and leaves of citrus in coastal areas. Both oystershell scale and purple scale can infest broadleaf evergreens, such as boxwood, citrus, and holly. But unlike purple scale, oystershell scale does not infest leaves.
The similar fig scale, Lepidosaphes conchiformis, generally is common only on Ficus carica. Fig scale and oystershell scale can both occur on elm, fig, and walnut. But fig scale occurs on bark and leaves and mature females are 1/12 to 1/16 inch long, noticeably smaller than mature females of oystershell scale, which do not infest leaves.
Females of euonymus scale, Unaspis euonymi, and oystershell scale also resemble each other and both can infest euonymus. However euonymus scale infests bark, green stems, and leaves and the elongate, white covers of males are common; oystershell scale lacks males.
Life cycle
Oystershell scale overwinters as whitish eggs under the cover of mature females on bark. The crawlers (mobile first instars) emerge in the spring after buds have burst. Nymphs develop through two increasingly larger instars. The scale usually has one generation per year in Northern California and two generations in Southern California.
Damage
The scale sucks and feeds on bark. High populations over several years can kill twigs and branches on young poplars and especially willows. Prolonged, abundant scales can stunt the growth of mature poplars and willows and cause them to decline.
Except for poplars and willows, oystershell scale is generally not a significant pest in California on about 150 other plant species it feeds on including alder, almond, ash, aspen, box elder, boxwood, ceanothus, fruit trees, holly, maple, sycamore, and walnut. See ScaleNet from the U.S. Department of Agriculture for a complete list of hosts of this species and references for more information.
Solutions
Horticultural or narrow-range oil provide control if plants are small enough to be sprayed. Thoroughly spray infested bark when crawlers are active in late winter or spring. If plants are tall or located where spray may drift to adjacent properties, a soil drench or bark spray of certain systemic insecticides (e.g., dinotefuran) may be the best option. See the section "Monitoring" in Pest Notes: Scales for how to effectively time applications by monitoring scale crawlers using sticky tape traps.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM) and The Scale Insects of California Part 3: The Armored Scales (Homoptera: Diaspididae), California Department of Food and Agriculture. |
Adult female oystershell scales.
Adult female oystershell scales.
Adult female oystershell scales.
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