Euonymus Scale| Unaspis euonymi | The Morton Arboretum

Common name: Euonymus scale

Scientific name: Unaspis euonymi

Hosts: Evergreen shrubs and vines of the Euonymus genus (sometimes broadly referred to as spindle trees) are primary hosts to these armored scales which feed on the sap. Other hosts may include hibiscus, holly, English ivy, honeysuckle, lilac, and pachysandra.

The euonymus scale originated in Japan, but can now be found around the world. In the United States, it is present in most states, since its hosts reach across North America. Euonymus scale is fairly serious, since it is so widespread and host species are commonly planted in home landscapes. Damage can occur quickly or kill the host if left untreated.

Scale insects are unique and look quite different from other insects. In their juvenile growth stage, they are referred to as crawlers. As crawlers, they are highly mobile, six-legged, have no protective cover, and are very small (<1/32 inch). At maturity, scale insects are immobile, have a covering over their body and are large enough to see without a magnifying glass.

Scale insects fall into two categories, soft scales and armored scales. Euonymus scale is an armored scale. The armored scales are usually smaller than the soft scales. They secrete a hard, protective cover over their bodies. They usually overwinter as eggs beneath the female cover.

Damage

All scale insects have sucking mouthparts and feed on sap from the tree.

Armored scale, like euonymus scale, burst plant cells and feed on the contents. Scale can remove large quantities of sap, stressing the host tree.

Plants can usually tolerate small populations of scale. The extensive feeding by a larger population often leads to yellowing of leaves and dieback of twigs. Over time, an untreated population of scale may lead to the decline of the plant.

Armored scale inject their waste back into the plant’s tissues, so no honeydew is produced.

Euonymus scale causes damage to the host by using needle-like mouth parts to feed on the sap or other plant fluids.

Signs of infestation are found on the leaves, twigs, branches, and stems where the scales attach. Less severe damage results in yellow or white dots on the upper sides of leaves, while more serious damage is characterized by leaves falling off in midsummer, water stress and desiccation, and twig and branch dieback and death.

Hosts may be killed when a severe infestation is not managed. If a plant is suspected to be infested with euonymus scale, closer inspection may reveal white, oval, or brown, oyster-shaped scales attached to leaves, leaf stems, twigs, or possibly the trunk.

With an extremely severe infestation, the leaves and twigs will be covered with a layer of scales killing plant stems and possibly the plant.

Life Cycle

Euonymus scale generally has two generations per year in the Midwest.

The brown, oyster-shaped adult females overwinter after being fertilized. They lay their eggs in early to mid-spring under their protective shell and then die.

The eggs hatch in mid to late spring, starting around 500-700 growing degree days, base 50 (GDD 50), when Japanese tree lilac (Syringa reticulata) and northern catalpa (Catalpa speciosa) are in early bloom.

The pale-yellow crawlers migrate around their host plant finding a place to settle. Some crawlers may be transported to nearby plants via wind or may crawl onto a neighboring plant.

When the crawlers lose their legs a few days after hatching, they usually settle on leaf veins or the plant stem, secrete their waxy protective shell, and spend most of their lives there feeding via their needle-like mouth parts.

After their crawler stage ends, it takes four to six weeks for scales to mature to adults and mate.

Adult females lack wings and remain immobile under their armor, while adult males emerge from their shells with wings, mate with the females, and then quickly die. The females lay eggs under their shells, the crawlers hatch, and are active from mid to late summer. They then settle and mature into adults and mate. The fertilized females overwinter.

Adult euonymus scale males are only 1/32 inch long and stand out on the host plant as tiny elongated white ovals. Females in this stage are 1/16-inch long, have an oyster-shaped protective shell, and are grayish brown and may be more difficult to spot. Newly hatched crawlers are pale yellow.

Management

Cultural Management

In cases of less serious infestations, euonymus scale can be scraped or brushed off, preventing them from being able to reattach and survive on the host.

Infested parts of a plant can be removed whether the infestation is small or large.

Pruning small infestations may solve the problem, while pruning larger infestations can make chemical treatment simpler and more effective.

If heavily infested and severely damaged, the plant may need to be completely removed. Be sure to destroy removed plant material to prevent spread.

Many plants are at risk for euonymus scale, but there are resistant species that can be planted. Evergreen Euonymus species are most at risk, while deciduous species in the Euonymus genus are more resistant to scale infestations. Cultivars with variegated leaves are more susceptible than those with green leaves.

Biological Management

Natural enemies of euonymus scale include lady beetles, predaceous mites, and parasitoids. These beneficial organisms can be effective in controlling small infestations, but are not able to control heavy infestations.

Beetles and mites can usually be seen easily, but the presence of parasitic wasps is shown by small holes through the protective shell of the scales.

Instead of using chemicals to treat a light infestation, the lady beetle (Cybocephalus nipponicus) can be purchased for biological control. These beetles can be found at some garden centers or ordered online.

Chemical Management

As with any form of management, the level of infestation and life stage of the pest should be monitored, but this is especially important when dealing with chemical treatment because effectiveness relies heavily on the appropriate timing of application when the product targets a certain life stage.

Euonymus scale crawlers can be treated directly with insecticides. This is the most vulnerable stage of their life, when they lack their protective armor. Monitoring is important so treatments can occur when the crawlers emerge in late spring and again in mid to late summer.

Horticultural oil sprays are another option that target overwintering scales, but these are less effective than crawler sprays. Horticultural oils and insecticidal soaps are the least toxic chemical options, so they may be best when trying to conserve and protect beneficial natural enemies. Some products may need to be applied several times, so always read the label to ensure optimal effectiveness.

Some, but not all systemic insecticides, used in the fall or spring may be used against this species of scale. They are absorbed internally by the host plant to target the scales, reducing the risk of harm to beneficial insects or the applicator, as compared with foliar sprays.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. Use pesticides safely and wisely; read and follow label directions. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.