Scale

¹scale [skayl ] noun: in botany, any small or thin scarious (dry and membranous) structure; also, a single bract of the involucre (whorls) in the Asteraceae family; glume [Note: See bract.] 

²scale [skayl ] noun: in entomology, any of more than 8,000 described species of sap-sucking scale-covered insects of the order Hemiptera, super-family Coccoidea

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Scale insects are named for the scale- or shell-like waxy covering that conceals the bodies of many of these small homopterous insects that have winged males, often so tiny as to be hard to see; wingless scale-covered females attached to the host plant, and which often die after laying eggs; and nymphs (crawlers) that suck the juice of plants and some of which are serious pests. The crawlers move about, and are the life stage often targeted by chemical controls at specified calendar intervals. Once the crawlers grow and choose a spot to stay in, they are called settlers, develop their scale shell, and in some cases lose their legs.

Scale insects have long existed in the Mid-Atlantic climate area, being prevalent on a wide variety of plants. The scales come in two basic types: hard (armored) and soft. The armored scales have a hard, shield-like, often mounded form made of shed skins and wax that hides the body of the insect but is not attached to it.  The hard scales are often brown or brownish yellow, or dark in color, though this varies. The soft scales produce a self-covering layer that is usually soft, thin, powdery or waxy, and that cannot be separated from their bodies. The soft scales also often produce copious amounts of honeydew, a sweet, sticky form of excrement that can attract other insects that may further harm the plant, as well as promote growth of a fungus called sooty mold, which is extremely unsightly. Although sooty mold feeds on the honeydew and not the plant itself, it inhibits photosynthesis by blocking light from the leaves placing low-growing plants in shady areas most at risk of harm.

Examples of Armored and Soft Scales

Left to right: 1. Lady beetle devouring armored Unaspis euonymi (euonymus scale). 2. Mature females and crawlers of armored Lepidosaphes ulmi (oystershell scale). 3. Domed shells of deceased, soft scale Parthenolecanium (lecanium) females. 4. Dead females in cottony egg sacs (maybe soft scale Pulvinaria innumerabilis—cottony maple leaf scale).

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Light scale infestations seldom kill plants, and are often ignored or even undiagnosed, as the crawlers can be hard to see. Though there may be some yellowing and wilting of leaves due to loss of nutrients from the sap-sucking creatures, the plants may not be seriously harmed. But heavy infestations can be a major problem, leading to death or deformation of parts of the plant or to attacks by other insect pests attracted by the sweet honeydew – causing further damage. Conversely, sometimes the honeydew attracts beneficial insects that eat or suck dry the scale, benefiting the plant.  

For explanations of plant terms, see bark/phloem, cambium, xylem.
Graphic by Joe Boggs, OSU Extension

Here in Northern Virginia, scales commonly attack holly, euonymus, prunicola, many evergreens and conifers, maples, beeches, birches, oaks, and countless other trees, shrubs, perennials, and even houseplants. In the past few years local crape myrtle (Lagerstroemia) trees and shrubs have come under attack by a scale species called crape-myrtle bark scale (Acanthococcus lagerstroemiae), which has led to unsightly plants covered in heavy infestations of white scale often followed by a very sticky honeydew, and then the unsightly and unpleasant sooty mold fungus mentioned above.

Scale insects generally go dormant in winter’s cold, providing a window of opportunity for treatment of surviving scale insects by spraying or wiping with dormant oils that smother or poison the insects. The downside of this is that the oils are broad spectrum insecticides that will also kill beneficial insects. This harm can be minimized by treating at the correct calendar and temperature time to avoid the active stages of many beneficial insects.  Other means of combatting scale are mechanical removal, useful in small, limited areas of infestation. Pruning to let light and air into shrubs and trees also gives plants a better chance to fight off scale infestations. Be careful not to put any removed materials in compost – it must be disposed of properly lest it infest other soils, plants or areas. Alternatively, soil drenches containing systemic poisons that will be absorbed by the roots and carried into the plant’s sap systems to be sucked on by the scale can be applied to the drip line of trees and shrubs.

There are also chemical sprays that can be used during times when the crawlers are active, though reaching the tops of tall trees is a challenge.  For any of the treatment methods to be effective, they must be applied at just the right calendar time and temperature to tie in with crawler hatching. Some scale insects hatch only one generation of crawlers per growing season; others have two or more generations. Virginia Tech’s Home Grounds & Animals 2023 Pest Management Guide (Publication 456-018) has useful tentative calendars for this purpose, which can be augmented with the Pest-Predictive Calendar, based on degree days and put out by the University of Maryland’s IPM publication. Note that seasonal weather variations and climate change can both affect the “normal” timing of the crawler hatches, hence paying attention to the current season’s degree days and pest predictive calendar is very important. 

For a more detailed discussion of the diagnosis and treatment of scales in general and most particularly in the Northern Virginia/Mid-Atlantic area, please consult a 2023 MGNV public education class video— Scale Control: Crape Myrtle, Holly and More—conducted by Kirsten Conrad, Agriculture and Natural Resources Extension Agent for Arlington County and the City of Alexandria.

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References

Boggs J. 2021. Look Up for Cottony Maple Leaf. Buckeye Yard & Garden onLine. The Ohio State University.

Boggs J. 2018. Oystershell Scale Eggs Are Hatching! Buckeye Yard & Garden onLine. The Ohio State University.

Boggs J. 2023. Sticky, Dripping Maples. Buckeye Yard & Garden onLine. The Ohio State University.

Crape-myrtle Bark Scale. Henrico County. Virginia Cooperative Extension.

Henn A. 2018. The Plant Doctor – Sooty Mold. Publication Number: IS1938. Mississippi State University Extension.

Scale Insects. Last Reviewed: October, 2022. Horticulture and Home Pest News. Iowa State University Extension and Outreach.

Talabac M. 2021. Euonymus Scale. University of Maryland.

Talabac M. 2022. European Fruit Lecanium Scale. University of Maryland.

Talabac M. 2021. Introduction to Scale Insects. University of Maryland.