Scale insects

Scale insects are small, piercing-sucking insects that attach to stems and the undersides of leaves to drink sap. They take two forms: a white, cottony cluster or a small, immobile brown shell, often accompanied by a sticky leaf. Treatment depends entirely on which form you find; the scale insect treatment explains the protocol for each type.

This page helps you identify them: whether it really is scale, which type, why it returns, and which plants it attacks. A wrong diagnosis leads to the wrong action: an alcohol-soaked cotton swab that removes a mealybug¹ will not solve an armored scale problem, because its covering provides better protection from products².

What is a scale insect?

A scale insect is a hemipteran insect in the suborder Sternorrhyncha, the same group as aphids and whiteflies. Wikipedia lists nearly 8,500 species in 21 to 24 families³. It feeds through a rostrumA slender, stylet-like mouthpart that the insect inserts into the plant to suck sap. inserted into bark, a leaf, or fruit, and eats nothing but sap².

The group has marked differences between the sexes. The adult female resembles a larva, shaped like a scale or covered in wax, without wings. The male has a single pair of wings and does not feed⁴: lacking mouthparts, it lives for only one to two days, long enough to mate³. What you see on the plant is therefore almost always females and their young.

From honeydew to sooty mold

Many species excrete excess sugar as honeydewA sweet, sticky liquid excreted by sap-feeding insects: scales, mealybugs, aphids, and whiteflies.. Sooty moldA black coating formed by fungi that live on honeydew without penetrating the leaf. grows on this honeydew: a black fungal coating that does not penetrate the leaf but covers it like soot⁵. The plant then loses light as well as sap. The pages on sooty mold and cleaning off honeydew explain how to wash this coating away.

Why there are ants on the plant

A line of ants climbing a stem often signals a colony higher up. Ants collect the honeydew and, in return, drive away the scales' predators and parasites². As long as ants stand guard, the colony grows faster.

How do you recognize scale insects?

Three signs together are almost always enough. Immobile bumps or white clusters on stems, in leaf axils, and beneath veins; a sticky leaf or one that looks shiny below the colony; then a black coating of sooty mold. Yellowing leaves, premature leaf drop, and drying shoots follow when infestation is severe². The RHS gives a size of 1 to 6 mm (about 0.04 to 0.24 in) for most species⁶.

The sticky-finger test

Honeydew falls downward. Run a finger over a lower leaf: if it feels sticky, the colony is directly above. On an oleander or lemon tree, also check the ground or windowsill, which become sticky before the insect is visible. Possible causes of sticky foliage are covered on the page about sticky leaves and black deposits.

The fingernail test

Lift a covering with your fingernail. If the cover comes off by itself, leaving a small, soft body on the plant, it is an armored scale. If the entire shell comes away with the insect, it is a soft scale². This two-second check settles the treatment question.

Look-alikes

Several things resemble scale insects without being scales: settled whitefly nymphs, some psyllids, or the woolly aphidAn aphid covered in fluffy white wax, often found on apple trees and pyracanthas.². The RHS adds that the white egg masses of some scales can be mistaken for mealybugs⁶. White fluff without an insect underneath is more likely a white, cottony coating, while a white film on potting mix is simply surface mold.

Types of scale insects

Four types cover almost everything found indoors, on balconies, and in gardens. They can be distinguished by sight and touch, and this classification matters more than the exact species name.

Sizes: Wikipedia for mealybugs and cottony cushion scale⁷⁸, UC IPM for armored and soft scales². Honeydew: UC IPM for mealybugs, which often produce it¹, and armored scales, which do not²; the RHS for soft scales, which excrete large quantities⁹, and cottony cushion scale¹⁰.

Mealybugs

Mealybugs, often described as white scale insects, form small cotton-like tufts in leaf axils, between two fruits, or at the crown. Their bodies remain soft and can still move a little, slowly¹. Some species live on roots and are visible only when the plant is removed from its pot. They have their own page: mealybugs, with their signs and preferred plants.

Armored scales

An armored scaleA scale insect in the family Diaspididae, protected by a covering of wax and old shed skins that separates from its body., or diaspid scale, lives beneath a small, flat plate less than 3 mm (about 0.12 in) across, often gray, white, or brown. It produces no honeydew: no sticky leaf, so one fewer warning sign². It is the hardest type to reach because contact products reach it less effectively than other types².

Soft scales

Soft scales, or coccid scales, are larger and domed like half a lentil. Their shell is the body wall itself. Brown soft scale (Coccus hesperidum) reaches 4 mm (about 0.16 in), with a flat, oval, pale yellow to brown body attached along leaf veins⁹. It excretes a great deal of honeydew, explaining the sooty mold that almost always accompanies it.

Cottony cushion scale

Cottony cushion scale (Icerya purchasi) is visible from a distance: a 5 mm (about 0.2 in) reddish-brown female resting on a large, white, ribbed egg sac⁸. Long confined to British greenhouses, it now lives outdoors in sheltered locations there, and the RHS recommends checking citrus, mimosas, and choisyas before buying¹⁰.

Scale insect, aphid, or whitefly?

Touch the insect lightly: if it scurries or flies away, it is not an adult scale insect. The damage looks similar because aphids and whiteflies also drink sap and also excrete honeydew². Movement and location make the difference.

The most common confusion is between mealybugs and woolly aphids, both covered in white wax. UC IPM recommends looking beneath the wax before deciding¹: a mealybug has an oval, segmented body, while an aphid has a pear-shaped body with long legs.

When scale insects multiply

The female lays eggs under cover, and only the newly hatched crawler is mobile. Eggs remain beneath her body or armor; some species lay them outside beneath a coating of white wax⁶. The young, yellow to orange in most species, walk over the plant or travel on the wind, then settle to feed within one to two days². This crawlerThe first stage of a scale insect, equipped with legs, which moves over the plant before settling. It is the stage most sensitive to treatments. is the only stage when the insect has neither thick wax nor armor.

Some species do without males, and females lay eggs without mating². Among mealybugs, a female lays 100 to 200 eggs or more in 10 to 20 days, with two to six generations a year depending on species and temperature¹.

Indoors: year-round

In greenhouses and homes, scale insects reproduce without a break: the RHS notes this for orchid diaspid scales and oleander scale¹¹¹², and UC IPM for mealybugs, whose life stages coexist indoors all year¹. In a heated apartment, there is no right season for treatment; there is a right interval.

Outdoors: a summer window

Outdoors, most species have only one generation a year⁶, and crawlers emerge over a short period. The published dates for four common species all fall between late June and September in the source regions described below.

The RHS dates apply to the British climate, and the France Olive dates to French olive groves. On a warmer balcony, emergence may start a few weeks earlier: that is why UC IPM recommends watching for crawlers rather than following a fixed calendar².

The plants most often affected

Each scale insect has preferred plants, and UC IPM notes that most species are highly host-specific². The table brings together hosts for which a source names the species responsible.

The sources name these hosts without ranking their susceptibility: the table order is not a ranking. For monstera, the pest table on the sick monstera page shows where to look for mealybugs and brown shells. For olive and oleander, the care guides for olive and oleander detail each shrub's own calendar. The page on oleander scale insects distinguishes armor, soft shells, and white cottony clusters.

How to get rid of them by type

The right action depends on the type identified above, and the full scale insect protocol provides doses, intervals, and mistakes to avoid. By type:

• Mealybugs: remove each cluster with an alcohol-soaked cotton swab, the method UC IPM recommends for houseplants¹. The wax repels most sprayed products.
• Armored scales: scrape off the plates, then target crawlers as they emerge, because adults beneath their covers resist contact products².
• Soft scales: young crawlers are susceptible to insecticidal soap and horticultural oil¹⁸², while domed adults are much less so. Scrape off the large ones and treat the small ones.
• Cottony cushion scale: on an outdoor plant, the predatory lady beetle (Rodolia cardinalis) has controlled this scale in California orchards⁸.

Whatever the type, one treatment is not enough: hidden eggs hatch after treatment. Judge the course of treatment over several weeks, not by the first cleanup. A heavily infested plant also calls for a decision: UC IPM suggests discarding a severely affected houseplant rather than treating it endlessly¹.

Preventing a return

Scale insects almost always enter your home on a plant. Adult mealybugs do not fly and move slowly: they arrive on a new plant, a tool, or a pot¹.

• Inspect every new plant on the undersides of leaves and in leaf axils before placing it near the others, and return it to the seller if it is infested¹.
• Check reused pots, stakes, and saucers: egg sacs can lodge there¹.
• Measure nitrogen fertilizer carefully: excess promotes tender growth and stimulates mealybug egg laying¹. The guide to feeding a houseplant provides guidance.
• Block ants' access with a sticky band placed on a sleeve around the trunk, never directly on the bark². See also installing an insect barrier.
• Prune heavily infested shoots when only a few are involved², following the rules for sanitation pruning.
• Rinse dusty foliage: dust interferes with scales' natural predators². The method is explained in rinsing plant leaves.

Frequently asked questions