Scale (or armored scale) describes a common family of insects that feed on various shrubs and trees. One type of scale that attacks euonymus shrubs (especially Euonymus japonica) is the euonymus scale (Unaspis euonymi). This is an armored type of scale that comes in two colors: Males are white and females are brown. Both suck on plant tissue, which can cause damage. The insects also excrete honeydew, a sticky substance that can cause sooty mold to form.
Identifying Euonymus Scale
Armored scale lives under a protective covering and feeds on plant juices. Euonymus scale appears on the stems and leaves of infested plants as tiny white lines, about 1/16 inch long, or as slightly larger, oyster-shell-like brown lumps. Heavily infested plants may be coated with white lines of scale extending along stems and leaf veins. This coating of white lines is the most easily identifiable symptom of infestation. Another symptom is white or yellow spots, which appear on leaf surfaces in response to scale feeding. In extreme cases, plants can be killed by euonymus scale.
Several varieties of euonymus are susceptible to scale, including wintercreeper (Euonymus fortunei), European euonymus (Euonymus europaeus), Pachysandra (Pachysandra terminalis), and bittersweet (Celastrus scandens).
Euonymus Scale Life Cycle
Euonymus scale spends the winter as mated adult females that then lay eggs in the spring. These eggs hatch into tiny immature scale called crawlers that, for a brief period, move along stems and leaves until they become immobile, grow a protective covering and begin to feed. The first crawlers hatch and are active from late May to early June. Another generation of crawlers is active again from late July to August. Mature male scale grows wings and travels through flight to mate with female scale.
Scale does not infest the soil around plants; it needs a branch to get established. The females have their young under their coverings, so it can be hard to take care of them for good.
Getting Rid of Scale on Euonymus Shrubs
First, you can try cutting out and destroying infected branches. For this treatment, you can prune more than the general pruning rule of one-third, but be careful not to cut too much and severely stress the plant. Pruning tends to work best when removing branches that grow close to buildings or are crowded next to a fence or other structure, which limits air circulation and promotes infestation. Pruning improves airflow and reduces the insect population at the same time.
Two natural enemies can help reduce euonymus scale: Chilocorus kuwanae, a type of lady beetle, and small parasitic wasps. Lady beetles can easily be seen on the plants, while wasps make their presence known by tiny holes made in the hard shells of the scale when the wasps leave the host insects. If you find evidence of either enemy, consider holding off on topical treatments to see whether the enemies have an effect on the scale.
One of the most popular methods for controlling euonymus scale is to apply horticultural oil. However, it is important to apply the right type of oil at the right time of year. In most climates, a dormant oil spray is suitable in late winter and early spring, provided the air is above 32 F for deciduous shrubs or above 40 F for evergreens; temperatures must be sustained for at least 24 hours. Dormant oil can damage plants if applied too late in spring when there are leaves or blooms.
Horticultural oils are most effective if you can apply them during the short period when the newborn crawlers are active, usually late May to early June and again in late July and August. One way to check for crawlers is to spread out a white sheet beneath the plant and shake the branches and leaves above; moving crawlers will fall onto the sheet. Apply a summer-weight horticultural oil during these active periods.