Euonymus scale

Overview

The euonymus scale, Unaspis euonymi, is an armored scale that can cause complete defoliation or even death of the evergreen Euonymus. This scale also attacks a number of other plants such as English ivy, hibiscus, holly, honeysuckle, lilac, and pachysandra. Individual scales are small and are often overlooked until serious damage has occurred. In general, plants growing close to buildings are more susceptible to damage because there is less air circulation and greater moisture stress in that situation.

Symptoms and Diagnosis

Light infestations produce yellow or whitish spots on the upper surfaces of leaves. The scales are typically concentrated along the stems and lower leaf veins of the plant. In the case of heavy infestations, plants look water-stressed, leaves develop yellow spots and fall off, and twigs and limbs may die. Plants may become bare by midsummer.

Scales are small (less than 1/8 inch long), and with close examination, you can see the white, longitudinal ridges of the males and the dark oystershell-shaped females. A scale cover will remain on the plant after the insect has matured or died. These covers are an identification sign, but they should be checked to see whether the scales are alive or dead. This can be determined by sliding your thumb over the scales. If alive, the crushed scales will be wet from body fluids. If dead, the scale cover will be hollow and fall off.

Life Cycle

Euonymus scales overwinter as fertilized, grayish brown adult females that have oyster-shell shaped covers. Eggs are laid in early spring beneath the female's covering. Eggs hatch during a 2- to 3-week period and the pale yellow nymphs ("crawlers") move to other parts of the plant or are blown onto nearby plants. After a few days, the crawlers molt and lose their legs. At this time they become stationary and begin secreting their protective cover. The scales spend almost all of their life feeding on the same spot while being protected by their hard covering. When mature, winged adult males leave behind their covers and mate with the still-covered females. There are generally two overlapping generations per year in Missouri.

Integrated Pest Management Strategies

1. Maintain plant vigor. Scales thrive on stressed plants so keep trees and shrubs well watered and fertilized.

2. Conserve natural enemies. Natural enemies can help control light infestations. Immature and adult lady beetles and small parasitic wasps are very helpful. Lady beetles are easy to identify but evidence of the wasps is harder to find. Parasitic wasps leave small round exit holes in the scale cover of their host. These holes can be seen easily with a magnifying glass. Plants should be examined to see if natural enemies are already at work reducing scale numbers before considering the use of an insecticide.

3. Remove scales manually. If caught early, minor scale infestations may be scraped off with a fingernail or gently scrubbed off with a soft brush or toothbrush. By rubbing the scales off of the plant, their mouthparts are injured and they are unable to re-infest the plant.

4. Suffocate scales with oil sprays. Superior-type horticultural oil sprays can be used to kill scales by literally suffocating them. These sprays can be applied at any time during the year but check the label and follow instructions to avoid injuring plants. Ideally, an oil spray should be applied during the dormant season to kill overwintering scales. Repeated applications during the season may be necessary with heavy infestations and should be targeted when the crawlers are most active.

5. Use insecticides. Pesticides registered for use include insecticidal soap, permethrin, cyfluthrin, malathion, and pyrethrins.

6. Plant resistant varieties. Replace susceptible Euonymus with resistant varieties. Most evergreen species of Euonymus (for example, E. japonicus and E. fortunei) are susceptible to attack from scale, although an exception to this is the relatively resistant E. kiautschovicus. Deciduous Euonymus species are also resistant.

7. Prune. For heavy infestations on E. japonicus and E. fortunei, plants may be cut to the ground in early to mid-spring. Remove and destroy infested material.

Organic Strategies

Strategies 1, 2, 3, 6, and 7 are strictly organic approaches. For an organic approach to Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate pyrethrin products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against scale. Not following the pesticide label before usage is a violation of federal law.