Treating common houseplant pests

Almost every plant picks up a little critter eventually. It happens to everyone, including us, and it is very rarely the end of the plant. During the winter months our houseplants can become vulnerable to pests because the environment changes to different temperature and humidity levels, which they absolutely love.

So although we would rather avoid pests entirely, it can at times be tricky to prevent them from materialising, so what matters most is identifying the pest correctly and knowing what to do to kill the infestation. The five common pests listed below want completely different treatments. Here’s how to tell them apart, what to do about each, and how to make your home a less appealing place for the next one!

Fungus gnats – small black flies on your soil

Ah, the fungus gnat. One of the most common houseplant pests, and one that loves the damp, cool winter months.

What you'll see

Small black flies, a couple of millimetres long, typically scuttling along the soil's surface. They're erratic fliers and not very strong ones at that, so they like to stick to the soil and move across the leaves. The larvae live in the top couple of centimetres of soil, loosely resembling a maggot; they are tiny translucent grubs with black heads and only really visible if you scrape the surface of the soil back.

What it means

Fungus gnats enjoy moist, damp soil. The larvae feed on fungi and decaying organic matter in permanently damp soil, so a plant with gnats is telling you that the soil is staying too wet for too long. This happens easily in winter, when plants are drinking far less than they were in summer. Treat the flies without changing your watering frequency and they'll simply come back.

What to do

Following a fungus gnat infestation you will want to dry the soil out more so than usual. Let at least half of the soil dry out fully before you water again and empty any water standing in saucers or cover pots. Once the surface is dry, the life cycle breaks on its own, usually within two to three weeks.

To clear them properly you need to deal with the adults and the larvae together. Spreading a centimetre of horticultural grit or coarse sand over the surface stops the adults reaching the soil to lay. Yellow sticky traps laid flat on the soil will catch the adults and give an indication as to how bad the infestation is.

For a heavy infestation, or with seedlings and cuttings where the larvae do real damage to fine roots, you can try using nematodes in your watering solution. Nematodes come in packs that can be mixed into water. You can also try a hydrogen peroxide solution to drench the soil and kill the larvae. Repeat treatments are recommended.

Spider mites – fine webbing and speckled leaves

What you'll see

Very fine, delicate webbing across the leaves and stems, and speckling across the leaf surface which can sometimes look like sandpaper residue. The mites themselves are barely visible so hold a leaf up to the light to make it easier to spot them or tap a leaf over a sheet of white paper and look for specks that move.

What it means

Spider mites love warm, dry air, which is why they often arrive in winter when the heating goes on. Anything sitting close to a heat source is a tempting home for spider mites. They breed very quickly in those conditions and will move onto neighbouring plants so vigilance and fast action are key to killing them off.

What to do

Isolate the plant straight away, in a different room if you can. Take it to the shower or the sink and rinse the whole plant thoroughly, undersides especially because that is where they like to live. Wipe the leaves down, then treat with insecticidal soap or a pest spray, covering the plant thoroughly.

Repeat every five to seven days for at least three weeks to prevent more eggs from hatching. 

Mealybug - tiny white fluffs that resemble cotton

What you'll see

Small tufts of what looks like white cotton wool, tucked into leaf joins, along stems, and on the undersides of leaves. You may also notice a sticky residue on the leaves below, or on the surface the plant is standing on. Underneath the fluff are soft, pale, oval insects. These are the females, and they tend to stay in one spot, sucking sap from the plant. The males mature into small gnat-like flies you may see around the plant and are easily mistaken for fungus gnats, though they don't feed and are only looking for mates.

What it means

Mealybugs feed on plant sap and hide in the spots we don't routinely check. They spread slowly, which means you can usually catch them early if you inspect your plants properly through autumn and winter.

What to do

Dip a cotton bud into 70% isopropyl (rubbing) alcohol and touch it directly to each cluster which dissolves the waxy coating and kills them. Then wipe the whole plant down with soapy water, paying attention to the joins, the undersides and the base of the stems. You can also use a 50/50 alcohol and water spray solution to cover larger areas.

Check again every week for a month or so because Mealybugs can be quite persistent and repeat treatments are usually needed.

Scale insects – the brown bumps that don't move

What you'll see

Small brown bumps, about three or four millimetres in size, sitting flat against stems and the undersides of leaves like tiny limpets. They don't move, which is why they're so often mistaken for part of the plant. The giveaway is stickiness – scale excretes a sweet, sticky liquid called honeydew, so the leaves feel tacky, and you may spot black sooty mould growing on it, since the fungus feeds on the honeydew.

What it means

They are very similar to Mealybugs. They like warm, stagnant air often created by central heating systems and closed windows. They will thrive in dry air, particularly in areas prone to dust.

What to do

First, isolate the plant to stop them infesting neighbouring plants. Then use rubbing alcohol and a cotton wool bud, placing the solution directly on the scale to dissolve their shells. Follow with a neem or 50/50 alcohol solution and spray the whole plant down

Repeat every seven to ten days for at least four to six weeks.

You can also remove them physically, using an old toothbrush or a cloth will lift them off and you can follow up with an insecticidal spray.

Plants with waxy leaves are particularly prone to scale infestations.

Thrips – silvery streaks and distorted new growth

What you'll see

Silvery or bronze streaking on the leaves, distorted or stunted new growth, and small dark flecks of debris. The insects themselves are slivers about a millimetre long that scurry when disturbed. Monstera, alocasia and anything with soft new leaves are frequent targets.

What it means

Thrips are hard to shift, mainly because part of their life cycle happens in the soil and they lay their eggs inside the leaf tissue itself, where sprays can't reach them.

What to do

Isolate the plant immediately as thrips move readily between neighbouring plants. Remove the worst-affected leaves. Treat with insecticidal soap or a pesticide suited to thrips five to seven days for at least a month, covering everything including stems and leaf undersides.

Replacing the top layer of soil with fresh material helps interrupt the soil-dwelling stage and you can combine this with a thrip nematodes watering solution to sterilise the soil. Use sticky traps to catch the adults and help indicate the stage of infestation. Once the infestation is under control, use insecticidal sprays to protect the plant and keep thrips  at bay.

 It is worth noting that thrips are persistent and can be very hard to get rid of. Often, biological control will be needed to manage the outbreak but any badly infected plants should be thrown away to avoid spread. 

Make it less likely next time by following a few routine steps:

Quarantine anything new. Keep a new plant away from the others for two weeks and observe it closely during that time.

Look at the undersides. Turn a few leaves over whenever you water to catch an infestation at the early stage.

Wipe the leaves. A damp cloth once a month removes eggs and early colonies before they establish, and the plant gets more light out of clean leaves too.

Don't overwater in winter. Consistently wet soil invites gnats, and a stressed plant is a more appealing target for everything else.

Deal with it early. Breaking the cycle at the start makes treatment far easier than catching it weeks in.

If you find something you can't identify, send a few photos to hello@forest.london and we'll help you work out what it is and suggest a treatment.