Have you ever noticed what appears to be a dusting of white flecks along the stems and leaves of a euonymus shrub or winter creeper vine? You were likely looking at euonymus scale, Unaspis euonymi (Comstock). This armored scale is one of the most common and damaging pests of ornamental euonymus in the Midwest, and heavy infestations can turn a healthy plant into a thinning, yellowed eyesore in a single season. It is native to Japan and China but is now well established throughout Indiana landscapes. A second and very similar-looking species, the winged euonymus scale, Lepidosaphes yanagicola Kuwana, is sometimes found onbranches of burning bush (Euonymus alatus). Both of these scales can be easily confused with oystershell scale but are about half the size.
Spotting the problem
Euonymus scale is one of the easier scales to notice because the males and females look strikingly different. Males produce a narrow, white, ridged cover about 1 mm long that stands out sharply against green stems and foliage — when populations are high, leaves can look like they have been dusted with flour (Fig. 1). Females are harder to spot; their covers are pear-shaped, grayish-brown, and about 2 mm long, blending in with the bark of older stems (Fig. 2). IF the waxy cover is removed from the plant, a yellow-orange elongate scale is revealed (Fig. 3). Damage begins as stunted growth that might resemble water stress. As populations build, leaves yellow entirely and drop mid-summer, twigs die back, and the plant thins out. Vine-type euonymus such as winter creeper (Euonymus fortunei) are extremely susceptible, and severe infestations can kill plants outright. Other hosts include pachysandra, bittersweet, and occasionally English ivy, holly, and lilac.
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- Figure 1. Flaky white covers of male euonymus scales on a discolored Euonymus leaf
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- Figure 2. Euonymus stem with female scales attached, showing their oystershell-like appearance.
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- Figure 3. Female Euonymus scales removed from the plant, showing their brightly colored, soft bodies
Life cycle and timing
Euonymus scale overwinters as fertilized adult females on stems and branches. In spring, females lay eggs beneath their protective covers in late April through May, and eggs hatch over two to three weeks into tiny, yellowish-orange crawlers that disperse across the plant to find feeding sites. This first generation of crawlers is active from late May through June and develops into adults in four to six weeks. A second generation emerges in late July through August, and it is often this summer generation that tips a moderate infestation into a damaging one.
Management
The crawler stage — late May through June and again in late July through August — is when euonymus scale is most vulnerable. Once a crawler settles and begins building its waxy armor, contact treatments become much less effective. During crawler activity, horticultural oil, insecticidal soap, or pyriproxifen applied with thorough coverage can provide effective control. For persistent infestations, systemic insecticides such as dinotefuran applied to the soil in fall or early spring offer season-long suppression, though systemic products should be used with care on flowering plants to avoid harming pollinators. Pruning heavily encrusted branches before crawler emergence reduces the source population and improves the effectiveness of follow-up treatments. A full guide on scale management is available in the Purdue Extension fact sheet Scale Insects on Shade Trees and Shrubs (E-29).
Keep an eye on your euonymus
Euonymus scale is common in Indiana and we have already seen samples submitted for diagnosis this year. If you are unsure whether you have euonymus scale or another armored scale species, submit a sample to the Purdue Plant & Pest Diagnostic Lab — an accurate identification is the first step toward choosing the right management approach.
