How to get rid of thrips

Most infestations come in on a new plant or a bunch of cut flowers. Check the flowers when you buy a plant, and leave behind any with thrips or pale flecked petals.

Warm, dry indoor air speeds them up, and females can reproduce without mating, so a few insects become a colony quickly. Potting mixes high in compost may favour them.

The species most often found indoors is the western flower thrips, which feeds on a very wide range of plants, can spread plant viruses and resists many insecticides.

Plants that get them most: Monstera, philodendron, anthurium and other aroids. Ficus. Calathea. Palms. Orchids. African violets and other gesneriads. Pothos. And flowering pot plants such as chrysanthemum and gerbera.

On Monstera and other aroids the silvery patches often start on the undersides and along the veins of the newest leaves. Look under the leaf and inside the unfurling leaf, rinse there, and treat the potting mix as well.

Treat the leaves and the potting mix at the same time. Eggs sit inside the leaf tissue, larvae and adults on the leaves, and pupae in the top of the mix, and no single treatment reaches all of them.

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   Step 1. Move the plant to another room. Thrips fly short distances and walk across touching leaves. Check its neighbours the same day.

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   Step 2. Rinse it. A strong jet of lukewarm water over the tops and undersides of the leaves, with the pot tilted or the soil bagged, knocks off many larvae and some adults. It is the cheapest step, but not enough on its own.

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   Step 3. Cut off the worst leaves. A leaf that is mostly silver barely works any more and still carries eggs. Remove it if enough healthy foliage stays.

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   Step 4. Hang blue sticky cards. Thrips are drawn to blue. Cards among the shoots catch flying adults and show you week by week whether numbers fall.

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   Step 5. Spray insecticidal soap or oil, in rounds. Insecticidal soap and horticultural oil kill thrips on contact, so they must wet the shoot tips, buds and leaf undersides where thrips hide. They do not reach eggs inside the leaf, so repeat as the label says, typically every 5 to 7 days for several weeks. Test on one leaf first and never spray in full sun.

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   Step 6. Treat the potting mix. Water nematodes (Steinernema feltiae) into the mix and keep the top layer evenly damp, or add soil-hunting predatory mites (Stratiolaelaps, sold as Hypoaspis), which work above about 12 °C (54 °F). Both target the pupae in the soil. A layer of sand or fine grit slows the larvae on their way down. Repotting into fresh mix removes pupae in the old soil but not the thrips on the leaves.

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   Step 7. Predatory mites for the leaves. Amblyseius (Neoseiulus) cucumeris and related species eat young thrips on the leaves and need above about 16 °C (61 °F) and 65 % humidity. Predatory bugs (Orius) and lacewing larvae hunt thrips too. In dry room air these helpers struggle, so they work best among grouped plants, in a bright bathroom or a conservatory, and need several weeks without spraying.

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   Step 8. Spinosad, if soap and oil fail. Spinosad is generally more effective against thrips than soaps and oils. It moves a little into the leaf and lasts about a week. Use a product whose label allows use on houseplants indoors, and keep it off plants in flower because it can harm bees for a day after spraying. Some common systemic insecticides, imidacloprid among them, often fail against thrips.

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   Step 9. Keep a diary and check weekly. Write down the date of each treatment, keep the leaf and soil treatments running in parallel and check weekly even when you see nothing, because eggs in the leaves hatch at different times.

Lukewarm water removes many larvae and adults and costs nothing. Repeat it between treatments.

Dawn and other dish soaps are detergents. Extension services advise against household soap and detergent on plants. Use an insecticidal soap.

Rubbing alcohol kills the thrips it wets and burns many leaves, while the insects in the leaf joints and the soil survive.

Neem works over several applications, not at once, and does not replace treating the mix.

Higher humidity discourages thrips but does not end an infestation.

Smells such as garlic, peppermint or essential oils have no good evidence against thrips indoors.

A thrips develops from egg to adult in about a month at 16 °C (61 °F) and in two weeks or less at 29 °C (84 °F). Under good conditions the whole cycle takes 24 to 35 days. Each female can lay up to 100 eggs, inside young leaves.

Plan on four to eight weeks of parallel leaf and soil treatments. The proof is the new growth: once the next leaves come out clean and undistorted, the infestation is over. Silvery marks on old leaves stay even when nothing on them is alive.

To people and pets, no. Thrips live on plant sap. Now and then one lands on skin and gives a brief prickling bite that passes without consequence. They do not live on or in skin.

To plants, it depends on numbers. A few cause cosmetic marks. A heavy infestation stunts and distorts new growth, kills flowers and can spread plant viruses. Young and soft-leaved plants suffer most. Plants do recover once the thrips are gone, through new growth.

Check flowers and young leaves before you buy, and keep new plants and cut flowers away from your collection for two to three weeks.

Keep a blue card among plants that had thrips. It warns you early.

Avoid compost-heavy potting mixes for prone plants.

Group humidity lovers away from radiators in winter.

Look at the youngest leaves of your aroids and ficus every week in the heating season.

A plant that is heavily infested is often best thrown away. Extension services note that this is usually simpler and cheaper than treating it, especially for an easily replaced plant or one that keeps reinfesting others. Bag it before you carry it through the house.

A valuable plant is worth the weeks: cut away the worst leaves, repot into fresh mix, and run the leaf and soil treatments in parallel.