Don't Bring Scale Insects Indoors This Fall – How to Stop Them Before They Infest Your Houseplants

Almost every year in my house, when the porch plants come inside in October, scale insects often come in with them. Adults don't move. Each one sits on a stem or the back of a leaf under a hard or waxy cover, and most people mistake them for rough spots on the bark until the leaves start to yellow.

The best time for controlling scale insects is before the pots come inside. Outdoors, cold nights slow scale down, and lady beetles and tiny wasps eat a share of it. Indoors, the pests have nothing to stop them. They just keep breeding through the winter on whatever plant they came in on.

An adult's waxy shell keeps most sprays from reaching it, making scale harder to get rid of than aphids or spider mites. Newly hatched crawlers have no shell yet. They're the stage sprays work best on, and new ones keep hatching, so treatments have to be repeated.

What Are Scale Insects?

All scale insects feed through a thin, straw-like mouthpart sunk into a stem or leaf. Brown soft scale (Coccus hesperidum) is rounded or slightly domed, and its covering is part of its body. Armored types such as fern scale (Pinnaspis aspidistrae) are smaller and flatter. Pry one up with a fingernail and the cover usually comes away by itself, leaving the insect behind.

Only the young get around. Crawlers are about the size of a speck of dust, usually yellow or orange, and they spend a day or two wandering before they settle down to feed. Indoors, they cross from pot to pot wherever leaves touch and ride along on hands and pruners. There isn't one hatch to wait out – new ones keep turning up all winter.

How to Identify Scale

Look for small bumps on stems, along leaf veins, and on the undersides of leaves. They can be brown, tan, white, or black. Black scale (Saissetia oleae) is dark, while oleander scale (Aspidiotus nerii) has a pale, nearly round cover. Use a magnifying glass to check plants properly; Carson's TriView folding loupe, available on Amazon, magnifies up to 15x.

Soft scale leaves sticky honeydew on the leaves and on the table under the pot. Black sooty mold, a group of fungi that includes Capnodium species, often grows on it. Badly infested plants show yellowing leaves, leaf drop, and stunted growth. Dead scale can stay stuck for months – press a bump with a toothpick; if it leaves a wet smear, it's alive.

Which Plants Are Most at Risk?

Citrus, jade plants, hibiscus, ficus, bay laurel, oleander, and orchids are some of the most common hosts. Plenty of other tropical houseplants get scale as well, from peace lilies and pothos to palms and snake plants.

Plants that spent the summer outside need the closest look, especially ones that sat close together on a deck or patio. Edible plants like citrus and bay laurel need extra care during treatment, since some products aren't labeled for anything you plan to eat.

Isolate Infested Plants Immediately

An infested plant needs its own spot, starting the day you find scale on it. A spare room is ideal. Otherwise, a windowsill on the other side of the house will do. Wash your hands and tools after touching it.

Leave the plant on its own until several weekly checks in a row turn up nothing new. One crusted with scale from top to bottom is often better off in the trash.

Remove Scale by Hand

If you only find a handful of scales, there's no need to spray. A cotton swab dipped in isopropyl alcohol loosens them as you rub, and cheap Equate 70% isopropyl alcohol from Walmart is fine for it. Stick with isopropyl, but test it on one leaf first and wait a day or two to see if it leaves marks.

Scale that won't budge usually gives way to a fingernail, a toothpick, or an old soft toothbrush. You can wipe tougher leaves with a soft cloth and a weak dish soap solution, which also removes the honeydew. The shells need to come off, not just get wet. Alcohol doesn't reach the eggs a female keeps under her cover.

Treat Heavy Infestations

Heavier infestations call for a spray. Horticultural oil smothers scale, and insecticidal soap kills the young on contact. Bonide's All Seasons horticultural oil, available on Amazon, comes ready to spray, and for soap, there's Garden Safe insecticidal soap. Coat every stem and both sides of the leaves.

Adults under a thick shell often survive a spray; crawlers don't. More eggs hatch after the first round, so one spray won't finish the job. A common schedule is two or three rounds spaced 10 to 16 days apart, though the label has the final word. Test any spray on a few leaves first, and skip soap on jade plants and ferns, which it can damage.

Broad-spectrum insecticides rarely need to come indoors. Hand removal followed by oil or soap is usually enough.

Monitor Throughout Winter

Keep checking all winter, once a week, and include plants that never had scale. Go slowly over new growth, the leaf joints, and the undersides of leaves. A strip of double-sided tape wrapped around a stem catches crawlers on the move. Under the loupe, they're easy to tell from dust.

Sticky traps won't tell you much about scale. The yellow cards target flying pests, whiteflies (Trialeurodes vaporariorum) and fungus gnats (Bradysia species), both of which turn up indoors in winter. These Safer Brand houseplant sticky stakes from Walmart push straight into the potting mix.

Protection Plan

Start outside, before nights turn cold. Go over each plant with the loupe, and treat anything with scale while it's still outdoors. Mix Garden Safe neem oil extract concentrate from Home Depot at the label rate in a Chapin trigger sprayer.

Once plants are inside, keep the ones that summered outdoors apart from the rest for a few weeks. Leave enough space that leaves from different pots don't touch. Check any new plant before it joins the others, too.

Plants that went in clean in October should come out clean in spring. Any that needed treatment can go back outside once nights stay warm. Lady beetles and tiny parasitic wasps live out there, and both attack scale.

Scale Treatment Essentials