Euonymus scale—Unaspis euonymi
When abundant, this armored scale (family Diaspididae) is a serious insect pest of Euonymus japonica.
The mature female is much wider at one end, oystershell shaped, slightly convex, and blackish, dark brown, or dark purplish. The immature male (puparia) is white and elongate with three longitudinal ridges. On the narrow end of their covers, both genders have a brownish yellow area (exuviae, skin of earlier nymphal stage). The rarely seen adult male is delicate with one pair of wings and long antennae.
At least nine similar-looking Lepidosaphes species of armored scale occur in California. These include oystershell scale, which is a pest mostly on poplars and willows, but can also occur on euonymus and numerous other hosts. Unlike euonymus scale, oystershell scale occurs only on bark and does not infest leaves.
Euonymus scale develops through three life stages: egg, nymph, and adult. The scale overwinters as mature females, which in spring produce eggs that hatch into orangish crawlers that emerge over a several-week period. Female nymphs develop through two, increasingly larger immature stages and males develop through four immature stages before maturing into adults. Euonymus scale has two or three generations per year.
This scale feeds on at least 29 genera of plants as listed on ScaleNet from the U.S. Department of Agriculture, but in California is generally a pest only on Euonymus species. The scale feeds on leaves and green stems, most importantly on E. japonica. Scale feeding causes brownish to yellow leaf spots, severe yellowing overall, and premature drop of foliage. Prolonged abundant scales cause the gradual decline and sometimes the death of E. japonica.
Where this scale has been a problem, consider replacing and do not plant E. japonica. Euonymus kiautschovica (=E. sieboldiana) tolerates this scale and E. alata remains nearly scale-free even when heavy infestations occur on nearby susceptible hosts.
Several parasitic wasps and predatory insects feed on euonymus scale as listed at ScaleNet. Predators include an introduced lady beetle (Chilocorus kuwanae) that closely resembles the twicestabbed lady beetle (Chilocorus orbus), which also preys on euonymus scale. Adults of both predators are shiny black with two reddish spots. However, the spots of C. kuwanae tend to be deep red, somewhat rectangular, and located near the center of each wing cover. The spots of C. orbus tend to be more yellowish orange, round, and located more forward toward the head of the beetle. Larvae of both species are brownish with black spines.
To conserve scale natural enemies, control ants, reduce dustiness (e.g., periodically hose off shrubs), and avoid the use of broad-spectrum, persistent insecticides. See Protecting Natural Enemies and Pollinators for more information.
Horticultural oil controls euonymus scale if foliage and shoots are thoroughly sprayed when crawlers are active in late winter or spring; one annual application during several consecutive growing seasons may be needed to sufficiently reduce high populations of this pest. See the section "Monitoring" in Pest Notes: Scales for how to effectively time applications by monitoring scale crawlers using sticky tape traps.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM) and The Scale Insects of California Part 3: The Armored Scales (Homoptera: Diaspididae), California Department of Food and Agriculture.
Abundant euonymus scales.
Male puparia (white) and mature female (purplish) euonymus scales.