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Euonymus Scale

May 25, 2005

Beautybush, Kolkwitzia amabilis, is blooming throughout portions of Illinois, which means that euonymus scale, Unaspis euonymi, crawlers are out and about on plants looking for a place to settle down and initiate the feeding process. Evergreen euonymus and pachysandra are their primary hosts in Illinois.

The crawlers resemble small yellow spots that move around on leaves and stems. Plants that are stressed (as with other insect and mite pests) are more susceptible to attack than plants that are receiving adequate (but not too much!!) water and fertilizer.

Euonymus scale overwinters as a mated female on plant stems. Eggs develop underneath the scale and then hatch over a 2 to 3 week period. The newly hatched crawlers migrate on the stem and start feeding near the base of plants. Crawlers may also infect nearby plants by being blown around on air currents, which results in infestations oftentimes not detected until populations are excessive and damage is noticeable. Leaves become spotted with yellow or white areas. Plants that are growing near structures such as along foundations tend to be more susceptible than those growing in open areas receiving sufficient air movement. The variegated forms of euonymus are more susceptible to attack than the green forms.

Heavy infestations of euonymus scale can cause complete defoliation or even plant death. Euonymus scale females are dark brown, flattened, and shaped like an oystershell. The males, however, are elongated, ridged, and white in color. Males are commonly found on leaves, whereas females are located on stems and along leaf veins. There are two generations per year in Illinois.

Pruning out heavily infested branches, without ruining the aesthetic quality of the plant, is an extremely effective means of quickly reducing the scale population. Be sure to discard pruned branches away from the area as soon as possible. Another recommendation is to avoid planting Euonymus japonica in landscapes, as this species is highly susceptible to euonymus scale. Euonymus alata is less susceptible (or resistant) to euonymus scale even when nearby plants are infested. Insecticide applications performed in late May through early June—when the crawlers are active—can prevent problems from occurring later in the season. Insecticides recommended for controlling euonymus scale including acephate (Orthene), bifenthrin (Talstar), cyfluthrin (Tempo), insecticidal soap, and horticultural (= summer) oil. Be sure to check plants regularly for crawlers, which can help time insecticide applications. Depending on the infestation, three or four applications made at 10-to-12-day intervals may be needed. Remember that euonymus scale is a hard (armored) scale, so soil applications of imidacloprid (Merit) will not be effective.

Euonymus scale is susceptible to a variety of natural enemies, including parasitoids and predators. These include braconid and ichneumon wasps, lady beetles, green lacewings, and minute pirate bugs. However, natural enemies generally don’t cause enough mortality to impact high populations of euonymus scale. In addition, the insecticides acephate (Orthene), bifenthrin (Talstar), and cyfluthrin (Tempo) are extremely harmful to natural enemies, so applications of these materials will disrupt any natural control.