Many new gardeners are amazed to discover that pests can seek out and attack plants growing indoors, or even out on a balcony of a high-rise building. One way or another, pests find plants - even when they are inside and away from the natural environment. They may already have been present when you purchased the plant, or hitched a ride on another plant. Some indoor plant pests can be brought in on clothes or shoes, and the odd one may even find its way in through an open window.


Prevention: The best cure

There are several ways to guard against introducing pests, as well as methods of treating them. The most important trick is to keep plants growing strongly. Pests are most likely to get a foothold if the plant is stressed, for example by growing in too little light, being in poorly-drained potting mix or even being too dry.

It is also important to take care when introducing new specimens to your plant menagerie. Plants bought from a garden centre should be free of pests, but plants bought at markets or taken from a friend’s collection could have hitchhikers on board. If possible, always re-pot new plants into a clean pot with fresh potting mix, and keep new plants away from the rest of your collection for a few weeks to quarantine them while you assess them for problems such as snails, slugs, caterpillars and other pests. Always check the pot thoroughly (for example, snails and slugs can hide under the rim or in the base of pots) and look for signs of chewed leaves, which may indicate that a caterpillar or snail is hiding on the plant. Try to locate the culprit and squash it!


At left, a close-up of a fungus gnat (Sciaridae) feasting upon a plant, alongside some mealy bugs on a leaf.

From left: Fungus gnats and mealy bugs are prolific pests of indoor plants.

Types of indoor plant pests

One of the most commonly-encountered indoor plant pests is a tiny insect called a fungus gnat or fungal gnat. It is a very small black fly that may be seen buzzing around indoor plants. The damaging part of its life cycle is its tiny maggot, which feeds on plant roots in the soil. It is most likely to be found in potting mix that’s overly wet with high levels of organic matter. The adults can be treated with fly spray or an all-purpose insecticide, but the maggots are harder to remove. The best option is to re-pot the plant into fresh indoor plant potting mix. This removes unhatched eggs and maggots. Clean the pot and gently wash the roots before replanting. Eco-neem is also registered against this pest and can be used as a soil drench. For more info on fungus gnat elimination, read this article.

Another pest commonly seen on indoor plants is scale. This insect may cluster under a small, often round, brown or black protective cover on the stem or leaves. The scale insects are feeding on plant sap. Their presence can lead to a secondary problem called sooty mould, a black covering on the leaves. Scale can be controlled by pruning off affected parts of the plant (such as fern fronds) and improving growing conditions by moving the plant into more light and increasing watering. Oil- and soap-based pesticides and a range of insecticides are recommended to control scale, but be cautious when treating ferns - these plants can be damaged by high concentrations of oil-based sprays. Once the scale has gone, the secondary sooty mould will clear up too.

Also watch out for another sap-sucking pest called mealy bug. This insect hides under a white, mealy cover and looks like small pieces of fluffy cotton wool. It is often quite severe, as it can attack the roots (where it is not noticed) as well as leaves and stems. Like scale, it is an indication that the plant is stressed by lack of light and overly dry soil. Treat as for scale (above), then apply a registered insecticide. If the mealy bug persists, dispose of the plant and buy a new one.


Need more indoor plant help? Click here for some common problems and fixes!