By Mark Shour
Iowa State University
A common shrub in Iowa is the burning bush or winged spindle tree, Euonymus (yew-on-ih-muhss) alatus. It can be used in mass plantings to create a shrub border, pruned to form a neat hedge or serve as a striking accent landscape plant. This species is a distinguished player in fall color display, with vibrant scarlet foliage, small orange-red fruit and corky ‘wings’ flaring out from its branches. With exception of very wet or dry sites, burning bush tolerates a variety of soil types (including being pH adaptable), grows well in sun or partial shade and is not adversely affected by air pollution.
Burning bush is a member of the staff-tree family (Celastraceae) and is native from northeastern Asia to central China. It was introduced into the United States about 1860 as an ornamental shrub. There is one native staff-tree species in Iowa, eastern wahoo (E. atropurpureus) that grows as an understory shrub or small tree in moist woodlands statewide.
E. alatus has been widely planted along interstate highways and in foundation plantings. When planted near woodlands and pastures this species can be invasive. Birds feed on the seeds and have consequently distributed the plant across the countryside.
Although it has been marketed as free of the pestiferous euonymus scale, burning bush has three pests that occasionally cause problems.
The most severe pest of E. alatus is the twospotted spider mite. This arthropod is omnivorous, feeding on more than 180 different trees and shrubs, herbaceous plants, greenhouse plants and field crops. These mites feed under host leaves, removing liquid chlorophyll and other cell components with their piercing mouthparts. Damaged leaves appear stippled with silvery or yellowish spots. On burning bush, leaves prematurely turn red, then brown and fall off the host. Fine webbing can be seen over host foliage in heavy infestations.
Twospotted spider mites are tiny and difficult to see with the naked eye. Adults (eight legs) and nymphs (six legs) are light green to greenish yellow with two lateral dark spots. The life cycle from egg to adult can occur in five days at 75 degrees F, thus several generations occur from late spring through fall. Adults overwinter in protected places away from host plants. Violets and weeds are common winter ‘hosts.’
Control measures for twospotted spider mite:
- Water host plants during dry periods. If needed, apply fertilizer at the minimal rate during early spring.
- Monitor for spider mites by shaking host foliage over white paper. Plant-feeding mites produce a green streak when smashed compared to the red streak for beneficial mites.
- If two spotted spider mites are present, spray foliage with a biorational product (e.g. horticultural oil, insecticidal soap, abamectin, bifenazate or spinosad) or a specific miticide (e.g. bifenthrin, hexythiazox, malathion, or permethrin) when mites first appear. Repeat as often as monitoring shows mites and at an interval permitted by the product label.
Another common pest of burning bush is the winged euonymus scale. This tiny insect also feeds on basswood, elm and willow. Scales remove plant sap from stems and branches with a thread-like mouthpart. Feeding can cause premature leaf drop, branch dieback, and predispose the plant to winter injury.
Winged euonymus scales have a small (2mm long), brown to gray, oystershell shape waxy cover protecting a white scale body. This scale is found on twigs between ‘wings’ of burning bush. It has 1 generation/year, overwintering either as eggs or partially mature scales. Eggs hatch in late June through July.
Control measures for winged euonymus scale:
- Water host plants during dry periods.
- Examine infested twigs when egg hatch is expected, looking for whitish, flat crawlers.
- If crawlers are observed, spray twigs/branches with a biorational product (e.g. horticultural oil or insecticidal soap) or a specific insecticide (e.g. carbaryl, cyfluthrin or malathion) when crawlers first appear. Repeat application if needed due to long crawler hatch period. Note: imidacloprid (Merit) is not effective against this type of scale.
A third possible pest of burning bush is the euonymus caterpillar. Although rarely seen in Iowa at this time, neighboring states have seen this pest. Euonymus caterpillars also feed on eastern wahoo, winter creeper, spreading euonymus, European spindle tree, Japanese euonymus and common buckthorn. Caterpillars eat the host leaves and feed in colonies within loose webs. The gossamer-like webbing and defoliation starts at branch tips and move inward, possibly engulfing the entire plant. Repeated defoliation weakens the host plant and may predispose it to decline from environmental conditions, pathogens or other insects.
Caterpillars are yellowish white with a black head. They have pairs of black spots along the top of the body. A full-grown larva is ¾ inch (20mm) long. The adult is a white moth with black spots and has a wingspan less than 1 inch (22mm). The annual life cycle involves adult moths active in mid- to late-July, and then laying eggs and covering them with a gummy material. Young larvae hatch, feed for a short period and overwinter under the protective gummy secretion. The following year, larvae begin feeding once leaves are full grown (late May to early June).
Control measures for euonymus caterpillars:
- Pull off or prune webbing and destroy caterpillars.
- If infestation is widespread, spray with high-pressure sprayer using either a biorational product (e.g. Bacillus thuringiensis or Beauveria bassiana), or a conventional insecticide (e.g. carbaryl, fluvalinate, lambda-cyhalothrin or permethrin).