NC State Extension Publications

Description

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The Euonymus scale, Unaspis euonymi, female secretes dark, oyster-shaped armor about 116-inch long. The armor of male nymphs is narrow (132-inch long) and white. Mature males are tiny two-winged insects that emerge from their armor to mate with the females (Females do not leave the protective covering). Eggs are tiny, yellow, oval and are only found beneath the mother's armor. Young crawlers are yellow and very small before they secrete the protective scale covering (armor). Males are usually more numerous than females, and with dense infestations, clusters of the white male armor on the leaves and twigs is noticeable from a distance.

The white armor of male euonymus scales contrasts against dark f

The white armor of male euonymus scales contrasts against dark foliage.

Euonymus scale females have oystershell-shaped armor.

Euonymus scale females have oystershell-shaped armor.

When abundant, euonymus scales tend to lay mostly male eggs.

When abundant, euonymus scales tend to lay mostly male eggs.

Biology

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Euonymus scale is found throughout North Carolina wherever its host plants grow. Euonymus scale is the most commonly reported insect pest of euonymus, pachysandra and celastrus in North Carolina. Female scale feed by inserting microscopic, thread-like mouth parts into the plant and sucking out fluids. This scale usually has two or three generations per year. The males emerge and mate with the females. The females lay eggs under their protective shell, and the tiny crawlers hatch and emerge from the mother's shell in April, May and June. They crawl along the leaves and stems before inserting their mouthparts.

Another brood hatches in late summer, and a partial third brood may appear even later so that all stages of development are present during most of the year. Although this scale is small, infestations are often dense and plainly visible particularly with heavy populations where males usually greatly outnumber female scales.

Host Plants

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Euonymus scale is found throughout North Carolina wherever euonymus, pachysandra, and staff vine (staff tree or bittersweet) grow. Yellow spots first appear on the leaves. Leaves and stems may become encrusted with the scales to such an extent that whole branches or the entire plant may die.

Euonymus scales often cause yellow spots where they feed.

Euonymus scales often cause yellow spots where they feed.

Management

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Euonymus scale is relatively easy to detect because they make yellow blotches visible from the tops of leaves. However, scale covers do not disappear when the insect dies. Therefore, the first thing to do is flip over female covers to determine if the scales are still alive. A live female is an orange blob that releases juice when you smash it. If no live scales are present, no treatment is necessary. The armor of dead scales eventually weathers away.

Timing is critical when trying to manage euonymus scale. The hard waxy cover protects adult scales from contact insecticides. Therefore, adults treated with a contact insecticide live to produce another generation later on. Crawlers can be killed easily because they are small and unprotected by a cover. Thus, nearly all products specifically target this stage. Monitor to determine when crawlers are present.

Crawlers emerge from under adult female covers and move about the plant in April or May. At this point the whole population is extremely vulnerable because the crawlers are exposed and all the adults from last year have died.

Euonymus scale has two to three generations per year. Therefore, if you miss the spring emergence and crawlers are not present you will have another opportunity later in the summer. Euonymus scale crawlers emerge about every 60 days.

Scales are often treated with broad spectrum insecticides such as pyrethroids or organophosphates that must contact scale to kill them and also kills beneficial insects. Horticultural oil is effective at killing crawlers on contact and has low impact on beneficial insects. Therefore, coverage is important with these products.

Do not spray the tender new growth of euonymus to avoid any phytotoxic effects. Spray before bud break or wait until the new growth has hardened-off somewhat. Water the plant well before spraying or spray after adequate rainfall. If the shrub is under moisture stress, the pesticide (especially oils) may damage the foliage.

Some newer products are available that offer systemic activity for longer control and are softer on beneficial insects (Table 1). Systemic or translaminar activity allows the product to be absorbed into plant tissue. Others can be applied as a drench to be taken up by the roots. Thus, it reduces the need for thorough coverage on difficult to spray plant parts such as the underside of leaves.

Another intervention option is removing problem plants. Plants that are severely damaged from years of scale infestation or that require yearly treatment to keep scale free may not be worth the work. Consider replacing with a newer Euonymus variety that is more resistant to scale or with another plant altogether. Research has found Euonymus fortunei to be less susceptible to scale infestations than E. japonicas and E. kiatschovicus species. In addition, variegated Euonymus varieties increase scale reproduction and survival more than green varieties. The lady beetle, Chilocorus kuwanae, has been shown to be highly effective in controlling euonymus scale, but it difficult to find commercially and has been hard to locate in natural settings in North Carolina.


Table 1. Some of the insecticides labeled for use on armored scale in ornamental systems that are more compatible with beneficial insects than pyrethroid and organophosphate products.
Active Ingredient Trade Name Scale Stages Affected Labeled Location Activity Signal Word IRAC MOA Group Compatible with Beneficials
acetamiprid TriStar Crawler, adult G, N, L Translaminar Systemic Caution 4A Yes
dinotefuran Safari Crawler, adult G, N, L, I Systemic Caution 4A Yes
horticultural oil* Many Crawler G, N, L, I Contact Warning - Yes
insecticidal soap* Many Crawler G, N, L, I Contact Warning - Yes
pyriproxyfen Distance Crawler G, N, L, I Translaminar Systemic Caution 7C Yes
G = greenhouse, N = nursery, L = landscape, I = interiorscape
* Suitable for homeowner use.

Chilocorus lady beetle

Chilocorus lady beetles feed voraciously on euonymus scales.

Other Resources

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For assistance with a specific problem, contact your local Cooperative Extension center.

Authors

Professor and Extension Specialist
Entomology & Plant Pathology
Professor Emeritus
Entomology & Plant Pathology
Retired Extension Specialist (Home Ornamentals/Turf)
Entomology & Plant Pathology

Publication date: April 1, 2009
Revised: Sept. 18, 2019

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