There are many species of scale insects that feed on a wide range of host plants. Scale insects are a unique group, that look quite different from other insects. In their juvenile growth stage, they are referred to as "crawlers". As crawlers, they are highly mobile, six-legged, have no protective cover, and are usually smaller than a pinhead. However, at maturity, scale insects are immobile, have no visible legs or antennae, and in the case of armored scales, are covered with a protective shell that ranges from about 1/16-to-3/8-inches in size.
Scale insects can be divided into two groups: armored scales and soft scales. Armored scales secrete a protective cover over their bodies and usually overwinter as eggs beneath the female cover. Soft scales are usually larger, lack the protective cover, but protect themselves with waxy secretions. Most soft scales overwinter as immature, fertilized females.
Scale insects feed on plant sap. Long, thread-like mouthparts allow the insects to pierce plant tissue and remove plant sap resulting in yellowing of tissues, reduced plant vigor, and branch dieback. During feeding by soft scale species, excess plant sap is excreted as a sweet, sticky material called honeydew. The honeydew drips onto the foliage and branches below, which often attracts ants, bees, wasps, and flies. Also, a dark fungus called black sooty mold can be found growing on the sweet honeydew. This fungus blackens roofs, porches, and any plant foliage where the honeydew is deposited.
Armored Scale Species
Euonymus scale is usually found on lower branches or on the new leaves of several species of euonymus, bittersweet, and pachysandra. The male scale produces a small, thin, white covering and can be quite numerous on the undersides of leaves. The female scale lives under a gray or brown shell and is usually found on the branches. The female scale overwinters under its protective shell and deposits eggs in early spring under the scale covering. The eggs hatch into yellow-orange colored crawlers over a two-to-three week period in late May or early June. As the nymphs develop, they crawl to other parts of the host plant to feed, although they can be blown to other susceptible hosts. Once they start feeding, they quickly begin to produce the hard protective covering as they grow. Two generations are commonly produced per year.
Oystershell scale found in our area is either the gray race or the brown race. Host plants include ash, dogwood, lilac, maple, and willow. The oystershell scale adult armor is light to dark brown and shaped like a tiny oystershell. The scale overwinters in the egg stage under the females' protective cover. Two generations are produced per year. Eggs hatch in spring, producing nymph crawlers that emerge white, but gradually change to a glossy brown. By mid July, the scale is fully grown. A second generation follows in late August or early September.
Pine needle scale
Pine Needle Scale is probably the most common armored scale found on conifers in the United States and Canada. The white, oystershell-shaped scale can completely cover needles causing plant discoloration, needle yellowing, and even branch death. This scale settles on the needles of its host and forms a 1/8-inch white, oystershell-shaped cover. Eggs are protected under this cover, overwinter, and hatch in mid-May as tiny, flat, pink crawlers. These crawlers search for suitable needles on which to feed and once settled, begin to form their protective armor. Males molt into a prepupa for a week and then emerge as winged adults. Females, however, molt into wingless nymph-like adults. After mating, the females lay eggs under their protective shell. There are two generations each year.
Soft Scale Species
European elm scale
European elm scale males and females differ considerably in appearance and life cycle. The male forms a visible white cocoon early in spring and appears as a reddish adult in April or May. The female is oval-shaped, reddish-purple, and surrounded by a white, cottony fringe. The female deposits her eggs beneath herself on a twig of the host plant. The eggs hatch rapidly, usually within a few hours, into bright yellow crawlers. The crawlers then migrate to feeding sites along the midrib and other prominent veins on the underside of leaves. Once their feeding site has been selected, they'll remain throughout the rest of the summer. In the fall, the crawlers return to a limb or trunk crevice where they hibernate as nymphs (immature females). Hibernating females often resemble small mealy bugs: oval-shaped and covered in short, white, waxy filaments. All native elms are susceptible to this scale. There is one generation per year.
Fletcher scale is common in the northern parts of the Midwest and Canada and is most frequently found on arborvitae (Thuja sp.) and yew (Taxus sp.). Pachysandra and Eastern red cedar are also susceptible. Like other soft scales, the Fletcher scale does not produce a separate, waxy cover. Instead it secretes a thin, transparent film, which does not totally cover the insect. The amber to reddish-brown nymph overwinters on a branch. The following spring, it feeds heavily as it grows into an adult. At maturity, a single female can produce 500-600 eggs in May, which hatch in late June or early July. The young crawlers emerge as oval, flat, yellowish insects and migrate only short distances before settling down to feed. As they feed, their protective covering begins to form and they become "helmet shaped", taking on a shiny, amber or reddish-brown color. One generation per year is produced.
Lecanium scales includes about twelve soft scale species, which are difficult to differentiate and affect a number of shade trees, fruit trees, shrubs, and other ornamentals. The scales can vary in length from 1/8-to-1/2-inch, depending on species. Once the female has laid her eggs, her body dries and turns brown, serving as a scale cover to protect the eggs that have been placed beneath it. Eggs hatch beneath the females in late spring or early summer, and crawlers then migrate to leaves of the host plant to feed. Excessive amounts of honeydew can attract black sooty mold fungus. In late summer, immature females return to twigs to overwinter. There is one generation per year.
Magnolia scale is our largest soft scale insect, reaching ½ inch in length. This scale spends the winter on one-to-two-year-old twigs as tiny, dark-colored nymphs. In the spring, the scales begin to feed, mature, and change color. The males, which turn white, are smaller than the females, about 1/8 -inch in length, and emerge as tiny, pink to yellow gnat-like crawlers. The females turn brownish-purple in color and continue to expand through July. Magnolia scale eggs hatch internally, and the crawlers are born alive. Crawler emergence occurs in the fall. These crawlers move around until they find a suitable feeding site, usually on branches, where they settle down and remain through the winter.
Management of scale insects varies with the species. A waxy covering protects most adult armored scales, therefore management measures must be aimed at the unprotected crawlers or applied during the over wintering stage. Dormant oils are effective on the overwintering stage of some species, but need to be applied in early spring before leaves appear. They are less effective on armored scale species. Insecticidal soaps can be effective against the crawler stage but usually have no effect on the adult scale. Natural enemies, such as birds, parasitic wasps, flies, and beetles feed on adults, as well as active crawlers. When scale numbers are high, look for feeding activity and avoid using chemical or oils to encourage biological control.
Scale insects are very vulnerable in the crawler stage when the young are looking for a place to feed. Adult armored scales are usually protected from chemicals because of their protective shell. Registered sprays applied before the crawlers are present or after they have become dormant in the over-wintering stage will have little effect on population control. Timing of application is critical and will vary with species. Contact the Plant Clinic (630-719-2424 or email@example.com) for current recommendations. Use pesticides safely and wisely; read and follow label directions.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
SCALE INSECT (CRAWLER)/ TREATMENT TIME
Cottony maple scale / Mid- July,
Euonymus scale / Early June,
European elm scale / Early April
Fletcher scale / Early April and early July
Juniper scale / Early July
Lecanium scale / Mid-June
Magnolia scale / Late September or early spring when buds are opening
Oystershell scale / Early June and late August
Pine needle scale / Late May
Spruce bud scale / Mid-to-late July