Spider mites: how to spot and control them on houseplants

Mites, not insects

Spider mites are not insects: they are arachnids, more closely related to spiders. Adults have eight legs, an oval body, and two red eyespots near the head; the larvae that hatch from the eggs have only six. They are barely a millimeter long, so a ten-power hand lens is almost essential to see them clearly.

The best-known species is the two-spotted spider mite (Tetranychus urticae), also called the red spider mite. In spring and summer it is yellowish green with two dark blotches on its sides; in fall and winter it often turns orange-red, which explains its other name. Its eggs are spherical, translucent at first, then cream or amber. Spider mites live in colonies, mostly on the undersides of leaves, and a single colony can hold hundreds of individuals. They spin fine silk, which sets them apart from other small pests such as aphids or thrips.

Few plants are completely immune. Among those described on Verdepedia, palms such as the parlor palm, kentia palm, or areca palm, as well as English ivy, heartleaf philodendron, and indoor figs (weeping fig, rubber plant, fiddle-leaf fig) are known to be especially susceptible. The poinsettia is also among their hosts. By contrast, succulents and cacti, golden pothos, snake plant, wax plant, and ferns such as the Boston fern tend to have fewer problems, though they are not entirely safe.

Why they thrive in the cold season

Spider mites love heat and dryness. When the heating is on, the air in a home gives them exactly what they want: that is why they so often turn up during the cold season, whether you live in the Northern or the Southern Hemisphere.

They need about fifty-five days to complete a life cycle at 50 °F, but only about twelve days at 70 °F, and less than a week under the most favorable conditions. A few mites that go unnoticed quickly become a colony.

A plant that is short of water suffers more from their feeding, and dust on the leaves favors them. A plant that is watered according to its needs and fertilized copes better with their damage.

Recognizing the damage

You almost always notice the damage before the mites themselves. They pierce leaf cells and suck out the contents, which leaves characteristic marks:

• fine stippling, small pale yellowish-green dots on the upper side of the leaves, giving the plant a dull, washed-out look;
• leaves that turn yellow, reddish, or bronze, then tan or brown;
• leaves that dry up and drop as the attack goes on;
• very fine webbing on the undersides of leaves and between leaves and stems, which can end up wrapping leaves and twigs in severe cases;
• tiny whitish cast skins, egg shells, and minute black specks (their droppings) on leaf undersides.

Left untreated, a heavily infested plant becomes badly weakened and can die.

Keep in mind, though, that yellowing or leaf drop can have many other causes: poorly suited watering, too little light, damaged roots, a change of location… Stippling alone is not proof. Before any treatment, confirm that mites are actually present; on the other hand, a plant the mites have left often recovers on its own.

The white paper test

The simplest way to confirm a suspicion:

1. Hold a sheet of white paper under a suspect branch.
2. Tap or gently shake the leaves over the paper.
3. Watch for a few moments: if tiny specks start to move, or if you see orange specks, they are most likely mites.
4. Then examine leaf undersides with a hand lens, following the veins, looking for mites, eggs, and webbing.

Run this check regularly, for example when you water, paying special attention to leaves that are losing color.

First steps: isolate and wash

As soon as you confirm they are there:

1. Isolate the plant from the others. If you can, move it to a cooler room during treatment, since spider mites prefer heat and dryness.
2. Wash your hands and clean your tools after handling it: mites travel easily on hands, clothing, or a watering can.
3. Shower the plant: a firm spray of water, aimed mainly at leaf undersides, will dislodge the mites and break up their webs. Rinse small plants under the kitchen faucet and large ones in the shower; this only suits plants sturdy enough to take it.
4. Wipe the leaves with a damp paper towel, changing it often so you don’t spread mites from one leaf to the next.
5. Repeat: one wash is not enough. Regular showers, close together for as long as the infestation lasts, often clear up a light population.

Keeping the air around plants more humid lowers the risk of an attack, but it will not get rid of spider mites on its own.

Soaps and oils: using them well

If washing is not enough, insecticidal soap and horticultural oils are selective options that are generally fine to use indoors. They work only on contact: they kill only the mites they touch and have no effect once dry. A few precautions apply:

• Choose a product sold as an insecticide and check that your plant is listed on the label; do not use homemade soap mixes, which can burn leaves.
• Water the plant a day or two beforehand: never treat a thirsty plant.
• Test on a small part of the plant and wait at least two days before treating the rest. Some plants are sensitive to insecticidal soap, notably ivy and certain palms.
• Thoroughly wet leaf undersides, where the colonies live.
• Treat outdoors when possible, in the shade and in mild weather, never in direct sun, and bring the plant back in once the foliage is completely dry.
• Reapply: it often takes one treatment a week for several weeks.

Miticides for houseplants are also available; whatever the product, follow the label directions exactly. Avoid broad-spectrum insecticides, however: they kill the mites’ natural predators and can actually trigger an outbreak within days.

Predatory mites

In a greenhouse, sunroom, or large collection, biological control is an option. One predatory mite, Phytoseiulus persimilis, is the most widely used: it eats spider mites at every life stage. Another, Amblyseius californicus, is less effective against heavy infestations but can be used preventively. These helpers need prey to persist: with no mites to eat, they starve or move on.

Bonsai: keep a close eye

Bonsai are not spared. On their small foliage, spider mites cause fine speckling and dull color, then leaf drop; webbing appears between branches once the attack is advanced. Junipers, such as Chinese juniper bonsai or Japanese garden juniper bonsai, are prone to them. Overly dense foliage makes it easier for pests to settle in: when a tree is attacked, also look for the reason it was vulnerable.

Check the tops and undersides of leaves, the branches, and new growth every few weeks during the growing season, rinse the foliage gently, provide good air circulation, and avoid crowding trees together, which helps pests spread.

When to let the plant go

Spider mites are hard to eradicate and spread easily. For a small plant that is heavily infested and badly damaged, discarding it is often simpler and safer than trying to save it: above all, it protects the rest of your collection.

Keeping them from coming back

• Inspect every plant before buying it, leaf undersides included.
• Isolate newcomers for one to two weeks, long enough for any problem to show up.
• Rinse your plants regularly during the cold season, and wash the foliage of smooth-leaved plants every two to three weeks to discourage mites.
• Water according to each plant’s real needs, as explained in Water houseplants by observation, not by the calendar: a plant stressed by lack of water is more vulnerable.
• Clean up: females spend the winter in crevices, stakes, plant debris, and cracks. Remove dead leaves and debris, and clean your stakes.

Other pests can turn up too: see Mealybugs and scale and Fungus gnats.

At a glance

Situation	What to do	Good to know
Speckled, dull leaves	White paper test, hand lens on undersides	Yellowing has many other possible causes
Mites confirmed, few of them	Isolate, shower leaf undersides, repeat	Wash hands and clean tools
Infestation that persists	Insecticidal soap or horticultural oil, weekly	Test first, water the day before, wet undersides well
Greenhouse or large collection	Predatory mites	Avoid broad-spectrum insecticides
Small, heavily infested plant	Discard it	Protects your other plants
Heating season	Rinse regularly, keep watching	Warm, dry air favors them