Mealybugs and scale: how to spot and get rid of them on houseplants

Two faces of the same problem

Mealybugs and scale insects are closely related piercing-sucking insects that feed on plant sap. Indoors, you will mostly meet them in two forms.

Mealybugs are small, soft-bodied insects with an oval, segmented body covered in a white, powdery, waxy secretion. They have fine white filaments along their sides and often “tails” at the rear. They are only a few millimeters long and move sluggishly: most species keep their legs throughout their life. Females wrap their eggs in a cottony mass, which is why colonies look like little tufts of cotton.

Scale insects hardly look like insects at all. Adult females are immobile and legless, hidden under a waxy covering that is round, oval or oyster-shell shaped, and brown, gray or white, like a fish scale stuck to the plant. Depending on the species, they range from less than a millimeter to over a centimeter across, though most are just a few millimeters. There are two groups:

• armored scales, whose flat cover can usually be lifted off the body and which produce no honeydew;
• soft scales, larger and rounder, whose cover is part of the body and which excrete honeydew.

Warning signs

These pests often give themselves away before you actually see them:

• honeydew, a shiny, sticky substance on the upper side of leaves and also on the furniture or floor under the plant;
• sooty mold, a black fungal growth on top of that honeydew;
• ants traveling up and down the plant to collect the honeydew;
• white cottony masses, or a plant that looks dusted with wax;
• leaves that yellow and drop, slow growth, a stunted or wilting plant. On cacti and succulents, leaves can wrinkle and shrivel.

A heavy infestation clearly weakens a plant and, in severe cases, can kill it. Some mealybugs also inject a toxin while feeding that distorts growth.

Keep in mind, though, that yellowing, leaf drop or wilting can have many other causes: too much or too little water, poor light, suffocated roots… Sticky leaves can also come from other sap-sucking insects such as aphids. Before treating, confirm that the insects themselves are there.

Where to look

Mealybugs and scale settle in colonies in sheltered spots, which makes them hard to find and to reach. Grab a magnifying glass and check:

1. the undersides of leaves, following the veins, especially the midrib;
2. leaf axils, where leaves join the stem, and branch crotches;
3. the crown of the plant and the base of stems near the potting mix;
4. the stem just below the soil surface, where some mealybugs hide;
5. the upper sides of leaves, where scale insects sometimes settle too.

Get into the habit of checking leaf undersides every week, for example when you water: the sooner you find pests, the easier they are to get rid of.

Understand their life cycle so you don’t give up too soon

A female mealybug lays a hundred to two hundred eggs or more in her cottony sac over roughly ten to twenty days. The young that hatch, called crawlers, are yellow, orange or pink, have no wax yet, and are very mobile: they are the ones that spread to new parts of the plant and to neighboring plants. They then settle down and cover themselves with wax in turn.

In scale insects, crawlers are the only stage that can move. Once settled under their cover, adults are well protected. That is why a single treatment almost never works: eggs sheltered by wax and adults under their shells escape the first attempts. You have to repeat the work and keep watching the plant for several weeks in a row.

Removing them by hand: the first step

For a light infestation, removing the insects by hand is often the most effective approach:

1. Isolate the affected plant to protect the others.
2. Pick off mealybugs with tweezers or a cotton swab dipped in rubbing alcohol. A solution of 70% or less isopropyl alcohol, dabbed directly on the insects and egg masses, works. Test it on a small part of the plant one or two days beforehand to make sure it does not burn the leaves.
3. Scrape off scale insects with a fingernail, a nail file or the edge of a card.
4. Wash the plant: a forceful spray of water dislodges loosely attached insects on sturdy plants, and wiping the leaves is one of the first things to do.
5. Prune out heavily infested parts if the plant can handle it.
6. Repeat every time you spot a new insect, until none come back.

Soaps, oils and other products

When cleaning is not enough, other options exist, each with its limits:

• Insecticidal soap, horticultural oils and neem oil reduce numbers when they reach the young stages directly. The waxy coating of mealybugs repels most contact products, and their habit of hiding makes them hard to reach. Against scale, these products work on crawlers; adults resist insecticides thanks to their wax, but oils can smother them. Several applications are needed.
• Sensitive plants, especially cacti and succulents protected by natural oils and waxes, can be damaged: always test a small area before treating the whole plant.
• Systemic insecticides move into the plant, but they are less reliable against mealybugs than against other sap-sucking insects. Some are toxic to bees: do not use them on a bee-attractive plant that spends the warm season outdoors.

Whatever the product, follow the label instructions to the letter.

Root mealybugs

Some mealybugs live below ground and feed on roots. They especially affect African violets, cacti and succulents. The symptoms look like a watering problem: yellowed leaves, wilting, stunting, fewer blooms. They also leave the plant more vulnerable to rot.

To find them, gently slide the plant out of its pot: they show up as white deposits on the roots and on the outside of the root ball. Scrape off what you see, then repot into fresh potting mix if the roots are infested, following the steps in How to repot a houseplant.

The most exposed plants

These pests attack a wide range of houseplants. Among the plants covered on Verdepedia, tropical hibiscus and bird of paradise are among those commonly infested by mealybugs. The jade plant, like other succulents, can host mealybugs, scale and root mealybugs alike. Check them with particular care, without neglecting the rest.

When to give up on a plant

A heavily infested plant is hard to clean up, and treating it over and over often spreads the problem to neighboring plants. For a badly infested houseplant, the wisest choice is to discard it. On the other hand, a well-tended, vigorous plant can tolerate a light population without harm, as long as you keep an eye on it.

Prevention beats cure

Most infestations arrive with a new plant. A few habits greatly reduce the risk:

• Inspect every plant before buying it and before bringing it home, leaf undersides included.
• Quarantine newcomers away from your other plants for at least one to two weeks, or even several weeks, so any pests have time to show up.
• Repot with new, sterile potting mix, never with garden soil.
• Keep your plants healthy by choosing species suited to your home and watering according to their needs, as explained in Water houseplants by observation.

In a greenhouse or sunroom, natural enemies can help too: tiny parasitic wasps and predators such as lady beetles and lacewings. One specialized lady beetle, Cryptolaemus montrouzieri, known as the mealybug destroyer, is sold for mealybug control. Avoid broad-spectrum insecticides, which would wipe out these allies.

At a glance

Situation	What to do	Good to know
A few white tufts	Alcohol-dipped cotton swab, tweezers	Test the alcohol on one leaf first
Small shells on stems	Scrape with a fingernail or file, wash	Crawlers are the most vulnerable stage
Infestation that persists	Insecticidal soap or oil, several applications	Wet every nook; test on succulents
Plant declining with no visible insects	Unpot it and check the roots	Repot into fresh mix if they are infested
Heavily infested plant	Discard it	Keeps the pests from reaching other plants
New plant	Inspect, then isolate for one or more weeks	Prevention is the surest method