Your plant isn't growing well anymore, and maybe the leaves are looking sort of yellow. You can't see any insects that you're familiar with, such as aphids or mealybugs, on the plant, but there are some funny bumps on the leaves and stems. But what are those bumps: part of the plant or actually insects masquerading as plant parts?  Scale insects look quite different from your typical insect. Female scales are wingless and often legless and don't even look like an animal. Small, immobile, with no visible legs or antennae, they resemble individual fish scales pressed tightly against the plant on which they are feeding. (The seldom seen male scale looks somewhat like a tiny gnat, but doesn't have mouthparts and cannot feed.) Scale insects are common pests of many indoor, greenhouse and ornamental plants as well as many trees and shrubs. They are a large and diverse group, closely related to aphids and whiteflies. They feed in a similar manner by sucking plant sap through their long, needle-like mouthparts (six to eight times longer than the insect itself!). Many also excrete sticky honeydew which supports the growth of sooty mold. Sooty mold is a black-colored fungus; when it coats the top side of leaves, that interferes with photosynthesis and makes the plants unattractive and yellow. 

There are two types of scales: the soft scales and the armored scales.

The soft scales (Family Coccidae) are the more important of the two groups of scales found on indoor plants. They have a wide host range within the flowering and decorative leaf ornamentals, from orchids to ferns.  Brown soft scale and hemispherical scale are the most common species on houseplants. Some species on indoor plants include:

Black Scale (Saissetia oleae)	Dark, oval, globular. Has ridges on the scale that form an "H" pattern.	Prefers woody plant hosts.
Brown Soft Scale (Coccus hesperidum)	Oval, soft, rather flat, and l/8 inch long. Mottled shiny pale brown, yellow, or grey with dark brown grid-like mottling.	Attacks a wide variety of hosts, but seems to prefer perennials over annuals. Common on gardenia, fern, camellia, oleander, and fig.
Hemispherical   Scale (Saissetia coffeae)	Mature scales are brown, smooth, glossy, and very convex (thus the name), with a slightly flared margin, and about l/8 inch long.	Favors ferns, asparagus fern, Schefflera, palms, begonia, citrus, chrysanthemum, fig, lily, orchids, and many non-woody evergreen plants. Plants in the family Acanthaceae are especially susceptible.
Nigra Scale (Parasaissetra nigra)	Size, form, and color of this pest varies with different hosts. Generally dark brown to shiny black, up to 1/5 inch in length. On leaves broadly oval and only moderately convex, but on twigs narrow, elongate, and relatively more convex.	Anthurium, croton, and many other ornamentals.

The armored scales (Family Diaspididae) get their name from the hard, waxy covering over the body. Although common, armored scales are not as much of a problem as soft scales because they are limited in the types of plants they infest (mainly trees and shrubs) and don't produce honeydew.  Species common on houseplants include: 

Boisduval Scale (Diaspis boisduvalii)	Circular or oval, thin, flat, white to light yellow, semitransparent, 1.2 to 2.25 mm in diameter.	Cattleya and Cymbidium orchids are the most common hosts, but also on the leaves and bark of other orchids, palms, cycads, dracena, banana, and cacti.
Cactus Scale (Diaspis echinocacti)	Circular, flat, dirty white with central dark spot, 1-2 mm.	On various genera of Cactaceae, and occasionally on Portulaca and jade plants.
Dictyospermum Scale (Chrysomphalus dictyospermi)	Circular, rather flat, dirty gray to yellow brown, 1.5 to 2mm.	Citrus and palms.
Fern Scale (Pinnaspis aspidistrae)	Males white and narrow; females brown and pear- shaped, 1.2.5 mm.	On ferns, African violet, begonia, camellia, citrus, fig, geranium, hibiscus, liriope, orchids, palms, violet, and many others.
Florida Red Scale (Chrysomphalus aonidum)	Circular, moderately convex, dark reddish brown to almost black, with an ash gray margin, about 2 mm.	Palms, citrus, and holly.
Greedy Scale (Hemiberlesia rapax)	Round and convex (irregular when crowded), white, dirty white, or grey, 1 - 2 mm.	Cactus, camellia, English ivy, Ficus species, palm, and many others.
Ivy or Oleander Scale (Aspidiotus nerii)	Flat, circular, and white or yellowish.	On oleander, orchid vine, palms, cycads, and ivy.
Latania Scale (Hemiberlesia lataniae)	Round and convex (irregular when crowded), white, dirty white, or grey, 2 mm.	A very wide host range including palms, orchids, croton, cycads, philodendron, plumeria, rose, Schefflera.

Soft scales are round to oval, dome shaped, and 2 to 6 mm long when mature. Immature scales start out light in color and darken at maturity. Many resemble miniature tortoise shells. Fifty to 2000 eggs or live young, depending on the species, are produced in or beneath the female's body. The eggs hatch in 1 to 3 weeks. The pale, newly hatched nymphs are the mobile "crawler" stage, but after finding a suitable part of the plant they settle down for the remainder of their lives. The nymphs go through three instars. A waxy scale covering is produced over the female after she becomes an adult. The waxy scale covering adheres tightly to the body of the female and cannot be separated from it. Female soft scales don't lose their antennae and legs completely, but are greatly reduced so the adults seldom move. 

Armored scales are generally smaller (1 to 4 mm) and not raised and bumpy like soft scales - and therefore are more easily overlooked than soft scales. The scales vary in shape from circular to elongate (elliptical or oystershell-like) and in texture from smooth to rough, and are variously colored. Underneath a hard cover made of wax and protein is a yellow or orange soft body. Eggs are laid beneath the female's scale, or in some cases, live young are produced under the scale. The crawlers move to another part of  the plant and settle for the remainder of their life. A waxy cover that incorporates the shed skin of the crawler and subsequent stages is produced over the immature insect. Female scales lose their legs and antennae during the first molt. The legless adult female remains under the scale.

Soft scales are usually found on the undersides of leaves and stems, although some species may occur on upper leaf surfaces. A heavy infestation will cause yellowed leaves, distorted foliage especially at the growing tips, twig dieback, or defoliation. However, soft scales can be a nuisance long before there are any visible symptoms. Copious amounts of honeydew excreted by the scales will make the plants and everything around or under them sticky. Sooty mold on leaves often indicates the presence of scales, too. 

Armored scales are generally found on lower leaf surfaces and stems, often forming thick crusts. Plant injury caused by armored scale feeding is similar to that of soft scales. Toxins injected while feeding on leaf tissue kills cells around the feeding site, causing a yellow or brown halo. Heavy feeding results in premature leaf drop. No honeydew is produced, however, so sooty mold is not a problem. 

Both types of scales generally do well at household temperatures. Indoors there may be as many as six generations per year with a new generation produced every 40 to 80 days. All stages may be present simultaneously throughout the year.

Controlling scales can be a difficult challenge. Of course it's best to prevent infestations in the first place. Don't buy plants that are not healthy, and carefully inspect even nice-looking plants before purchasing. Since scales can occur on all plant parts, check every part of the plant. Examine both upper and lower leaf surfaces, and particularly along the midrib of the underside. You may wish to use a 10X hand lens or magnifying glass to aid in their detection. Check stems around buds, leaf petioles and lenticles or other depressions which may afford a place for the scales to hide and feed. Pay particular attention to old wound scars where the bark is thin. Many times the scale wax covering blends in with the bark and is difficult to see. It's probably a good idea to isolate new plants for a few weeks, if possible, to prevent undetected scales (or other pests) from moving onto your clean plants.

Don't overlook the fact that house plants taken outside during warm weather can become infested with insects. Before bringing any plant back into the house, examine it closely for insects.

If the infestation is not too severe, you may be able to remove most of the scales with a soft-bristled toothbrush or cotton swab dipped in either soapy water or 70% isopropyl alcohol. Thoroughly washing the leaves with a mild detergent solution (being careful to avoid wetting the soil) may also work. This will also wash off honeydew and dislodge crawlers. Inspect the plants closely at weekly intervals, and repeat the procedure as necessary (probably several times). If a plant is heavily infested, disposing of it may be the best solution, although that may not be an option for a valuable or cherished plant. 

Biological control is generally not a good option for the homeowner. There are some commercially available parasitic wasps that attack scales, such as Metaphycus helvolus for brown soft scale, but most are useful only for certain scale species on certain plants under optimum conditions - which cannot be met in a home or small greenhouse. The same goes for the few predatory insects that will attack scales. The mealybug destroyer, Cryptolaemus montrouzieri, will attack scales if mealybugs aren't available, but scales are not their favorite food.

Chemical control is often the most effective way to combat scale problems. Adult scales, however, are protected from insecticides by waxy coverings, so chemical control measures should be aimed at unprotected immatures (crawlers). Spot treatments should be applied when scales are present. Applications of insecticidal soap or horticultural oil will kill scales, but at least three treatments are needed to control an infestation. Repeat every six to seven days until scales have been eradicated. Dead scales do not fall from plants, so you'll have to examine plants to determine whether the scales are dead or alive. Crush the scale - if it's dry, the scale is dead; if the body is juicy or leaves a streak when smeared on a piece of paper it was alive.

Systemic insecticides (Di-Syston) applied to the soil help suppress populations, but do not eliminate the problem. Applications on larger woody plants can even give unsatisfactory control due to unequal movement of the systemic material in the plant. Sprays containing dursban, malathion, and pyrethrin are labeled for scale control, but these treatments can often cause plant damage and can have a strong chemical odor. Insecticidal soap treatments are usually preferred, but require thorough coverage and consistent applications for good control.

- Susan Mahr and Phil Pellitteri, Department of Entomology, University of Wisconsin - Madison

Additional Information:

• Scale Insects on Houseplants

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