In 1967, a new pest caterpillar made its appearance in North America. The euonymus caterpillar (Yponomeuta cagnagella), also called euonymus webworm, feeds on the European spindle tree (Euonymus europaea) in Europe, the Middle East and parts of Asia. Infestations first occurred in the eastern U.S. and Canada, and after 1989 the caterpillar spread to the Midwest, with outbreaks in Wisconsin, Illinois, Michigan and Iowa. It defoliates euonymus (Euonymus spp.) bushes and covers them with webbing.
Euonymus caterpillars are cream-colored with black spots along their sides, and about 3/4-inch long when mature. They go through several molts after hatching out of the egg before they reach full size. What you notice first is not the caterpillars themselves, which are quite small when young, but the white webbing they spin on the euonymus branches that can envelop whole bushes as the caterpillars grow and feed.
The female euonymus moth is called an ermine moth. It has a 1-inch wingspan with white, black-spotted wings. She lays eggs on branch tips in mid- to late July, covering the eggs with a gummy substance that hardens into a shell-like covering. The caterpillars hatch in mid-August. After feeding briefly, they go beneath the eggshell remnants to overwinter. They emerge in spring, usually April to June, and construct webbing, feeding until full grown, usually June. They pupate in a cocoon and adult moths emerge in July.
There is only one generation of euonymus caterpillars a year. They can defoliate an entire bush; this usually doesn't kill the plant, but successive feeding in subsequent years can kill the euonymus. For best control, treat early in the year, usually early to mid-May, when you first notice the webbings. Prune out and destroy the webbings and enclosed caterpillars while they are still young. The most effective treatment for larger infestations is Bacillus thuringiensis, Bt for short, a bacterial biological control agent that kills the caterpillars after they eat it. For ornamental plants, mix 4 teaspoons of Bacillus thuringiensis concentrate in a gallon of water and spray it on the foliage the caterpillars are eating. Reapplication might be necessary.
The most commonly attacked plants are European spindle tree, eastern wahoo (Euonymus atropurpureus), winter creeper euonymus (Euonymus fortunei), burning bush (Euonymus alatus) and spreading euonymus (Euonymus kiautschovicus). The European spindle tree is a small deciduous tree hardy in U.S. Department of Agriculture plant hardiness zones 4 through 7, and can be invasive due to reseeding. Eastern wahoo has dark purple flowers in early summer followed by red berries and grows 12 to 20 feet tall in USDA zones 3 through 7. The evergreen winter creeper euonymus grows 1 to 3 feet tall and is used as a groundcover in USDA zones 5 through 9. Burning bush grows slowly to 15 feet tall and wide in USDA zones 4 to 8 and gives reliable red fall color. Evergreen to semi-evergreen spreading euonymus grows 8 to 10 feet tall in USDA zones 5 through 8.