I received this emailed question the other day: “I don’t know if you can help me, but I noticed that all of my euonymus shrubs were losing leaves. It looks like something might be attacking them. I took some pictures and have attached them. What’s eating them?”
Whodunit? The culprit is euonymus scale (Unaspis euonymi), an armored scale from Asia that is the “Achilles Heel” of euonymus throughout the U.S. and Canada, especially the Japanese euonymus (E. japonicus) and European euonymus (E. europaeus); however, it also attacks American bittersweet (Celastrus scandens), Japanese pachysandra (Pachysandra terminalis), Paxistima canbyi, and other plants.
Where’d it come from? This critter has an interesting life cycle. Fertilized females overwinter on the stems and leaves of host plants. In early spring when new leaves emerge, they lay yellow elliptical eggs and die. The eggs hatch over a two to three week period and the young scales or crawlers, which look like specks of yellow-orange dust, travel along the stems and leaves of the host plant or are windblown. They eventually settle down inserting their long, needlelike mouthparts into plant tissues to suck sap and develop a waxy protective covering. The brown oystershell-shaped females are typically found on stems, while the fuzzy white elongated males are typically found on leaves. They reach adulthood within 40 to 60 days. Males mate with the immobile females to produce a second generation. Generally, there are two generations per year in the in northern and central areas of the U.S., and three or more in southern areas.
Do my euonymuses (perhaps “euonymi”) have euonymus scale? Look for yellowish or whitish spots or halos on the upper surface of the leaves, which are caused by the feeding males. The gray to brown females are often clustered on the leaf petioles and stems. Leaves of infested plants turn yellow and drop prematurely, often leaving tufts of foliage at the ends of stunted branches. Heavy infestations lead to plant death. Like other armored scales, euonymus scale does not excrete honeydew.
How do I control them? Apply horticultural oils to dormant shrubs in late fall or early spring when temperatures will remain above freezing until the spray dries. During thegrowing season apply horticultural oil or insecticidal soap when the mobile crawlers are present and when they have recently settled. To determine when the crawlers are active, attach double-sided sticky tape to twigs or branches and and examine the tape with a hand lens for the presence of orange-yellow crawlers.
Change the tape at weekly intervals. Alternatively, shake a branch over a white sheet of paper and look for moving orange specks of dust. Certain systemic insecticides may help to reduce population of these pests such as imidacloprid (Bayer Advanced Garden® Tree and Shrub Insect Control). See “Euonymus Diseases & Insect Pests“, HGIC 2054, for more information.
Several natural predators have been introduced. In 1984, the Agricultural Research Service (USDA/APHIS), imported and established two Korean predatory beetles, Chilocorus kuwanae and Cybocephalus nipponicus in the eastern U.S.
Instead of combatting the euonymus scale, consider shovel-pruning it (removing it completely at soil level) and replacing it with other more pest resistant shrubs. Consider Japanese anise tree (Illicium anisatum) or Florida anise (Illicium floridanum), Mexican orange blossom (Choisya ternata ‘Sundance’), glossy abelia (Abeila x grandiflora), Soft Caress threadleaf mahonia (Mahonia ‘Soft Caress’), or variegated winter daphne (Daphne odora).