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Euonymus Scale

Old male shells (white dash marks) and recently settled crawlers, an ideal time to treat!

Female euonymus scales are oystershell-shaped.

Species: Unaspis euonymi (Comstock) [Homoptera:Diaspididae]

Distribution:

This pest is suspected to be of Asian origin and is now found across North America and many foreign countries where euonymus is grown.

Hosts:

Most fact sheets simply list "euonymus" as being the host, but it appears that Euonymus fortunei (wintercreeper), E. kiautchovica (spreading euonymus), E. hamiltonianus (= E. sieboldianus & E. yedoensis) (Yeddo euonymus), E. atropurpureus (eastern wahoo), and E. europaeus (European euonymus) are the most susceptible species. E. alatus (winged euonymus is quite resistant to this scale, but winged euonymus gets a smaller, look-alike scale, the winged euonymus scale.

Euonymus scale occasionally infests Pachysandra, bittersweet (Celastrus), as well as a variety of other landscape trees where it is rarely a damaging pest.

Variegated varieties of wintercreeper and other small shrub-type Euonymus can be completely killed or severely damaged by this pest.

Damage:

Scales that settle on leaf undersurfaces cause yellow spots to appear on the upper surface. Scales, especially the females, can completely cover stems and branches, thereby girdling and killing them. Early signs of euonymus scale attack are usually spots appearing on leaves, general thinning of the foliage and branch death.

In some parts of North America, this pest makes it very difficult to use any of the creeping or small variegate leaf forms of Euonymus in landscapes.

Description and Life Cycle:

Male scales are about 1/16-inch long and appear to be white "exclamation marks" on the undersides of leaves. The female scales appear as mottled brown, oystershell-shaped incrustations attached mainly to stems and petioles of host plants. The female covers are slightly longer than the male covers and much wider.

Male and female euonymus scales undergo very different life cycles. Both sexes hatch into a mobile nymph called a "crawler." These crawlers settle down on their preferred sites (males on leaves and females on stems), feed for a few days and molt into a second instar nymph. The settled second instar begins to form a waxy cover over the body.

If the scale is a female, it molts one more time (third instar) into the sexually mature adult. If the scale is a male, the next molt (third instar) results in a "prepupa," which molts again into a "pupa." The "pupa" then molts into the adult male (fifth instar) which has functional wings and legs. The males creep over the plant to fertilize the females.

In Ohio and states of similar latitude, two generations of this scale are the norm. The mated females overwinter under their covers and begin to lay eggs in late April through May. The first generation of crawlers can emerge from mid-May into July. New adult females can be present by early July and these lay eggs for several months, often July through September. By November, most of the scales that remain are new females that have been mated.

Unfortunately, this scale has a considerable overlap of stages by early summer and this continues until October. Therefore, it is very difficult to find a time that all the insects are at a stage vulnerable to control.

In more southern states, four to five overlapping generations are common.

Control Hints:

Once Euonymus plants are infested, this pest is very difficult to control because of its overlapping stages.

Option 1: Cultural Control - Use Alternatives to Euonymus - Use other ground covers as a substitute to wintercreeper. Some variegated forms of privet hedges can be used where small, variegated shrubs are desired. Winged euonymus, E. alatus, does not get this scale.

Option 2: Biological Control - Predators and Parasites - Several lady beetles (the ones that are commonly called "twice-stabbed" lady beetles - black with two red spots) are known to feed on this scale and a few parasites are known. None of these are effective on highly susceptible euonymus species.

Option 3: Chemical Control - Insecticidal Soaps or Oils - Insecticidal soaps and oils are fairly effective against the crawlers and recently settled crawlers. Several applications and thorough coverage is essential for success.

Option 4: Chemical Control - Standard Insecticides - Most insecticides labeled for "crawler control," can reduce euonymus scale populations if the application is made when the first generation crawlers appear - mid May.

Option 5: Chemical Control - Systemic Insecticides - A few insecticides that claim scale control and systemic action can be very effective for control of this pest. Best control is obtained against the developing first summer generation.