Spider mites on bonsai: how to spot and get rid of them

Spider mites on bonsai aren't spiders but tiny mites, barely half a millimetre long. With the naked eye you'll at most see moving dots. They usually sit on the underside of the leaves, pierce the cells and suck them dry. Every bite leaves a pale dot.

How to recognise spider mites

• Finely speckled leaves: lots of tiny pale dots, the leaf looks dull, grey or yellowish.
• Fine webbing: delicate threads between leaves, needles and twigs, easiest to see when you mist the tree with water.
• On conifers such as juniper: the needles turn dull, greyish or bronze. Conifers often show damage late, so it's worth testing before you treat.
• The tapping test: Hold a sheet of white paper under a branch and tap it firmly. If tiny dots start crawling, they are mites. If you squash a dot and it leaves a greenish or reddish smear, it's definitely not dust. Test in several places.

Why now?

Spider mites love dry, warm air. The warmer it is, the faster they multiply; at room temperature a new generation takes only a few weeks.

• Indoors in winter: Heated air is often extremely dry. That's why spider mites show up especially on indoor bonsai like the ficus during the heating season.
• Outdoors in high summer: Heat and drought favour them in the garden or on the balcony too.
• Weak or crowded plants are attacked more easily.
• Brought in: New plants often carry the mites already.

In Japan they are called ハダニ (hadani), roughly "leaf mite". Japanese plant lovers like to mist their leaves regularly, which they call 葉水 (hamizu, "leaf water"). It is considered a simple way to prevent exactly these pests.

Step by step

1. Isolate. Move the infested tree away from other plants straight away. Spider mites spread across leaves and on air currents.
2. Shower it. Rinse the tree firmly with lukewarm water, especially the undersides of the leaves. Cover the soil with plastic first or tilt the pot so the soil doesn't wash out or get soaked. This washes away a large part of the mites and webbing.
3. Repeat. Once isn't enough, because new mites keep hatching from the eggs. Shower the tree every 2–3 days for about two weeks.
4. Raise the humidity. Mist the leaves regularly, stand the tree on a tray of pebbles and water (the pot must not stand in the water) or use a humidifier. Humid air slows the mites down but won't beat them on its own.
5. Use beneficial insects. Predatory mites eat spider mites and their eggs. You can order them from garden suppliers and simply place them on the tree. They prefer warm, humid conditions. Important: stop spraying products then, or you'll hit the helpers too.
6. Products only if nothing else works. For heavy infestations, products based on insecticidal soap or plant oil (such as rapeseed oil) help. Pick one that is specifically approved against spider mites, test it on one branch first and follow the label exactly. Here too, repeat after a few days.
7. Follow up. Do the tapping test once a week for a few more weeks.

What NOT to do

• No ordinary insect spray. Many insecticides hardly work against mites but kill their natural enemies. The infestation can even get worse afterwards.
• Don't stop after one treatment. The eggs survive and the next generation follows.
• Don't spray oil or soap in full sun. It can scorch the leaves.
• Don't place the tree right above a radiator. That's where the air is driest.

Prevention

• Check the undersides of the leaves regularly and do the tapping test now and then.
• In winter, keep the air more humid and don't put the tree right above a radiator. More in our guide to indoor bonsai.
• Keep the tree healthy: correct watering and enough light, see bonsai care.
• Keep new plants separate for a few weeks and check them.

When to see a bonsai nursery

If the infestation doesn't ease after two to three weeks of showering, many leaves or needles die, or you're unsure which product is right, ask at a specialist nursery. Ideally bring a twig in a sealed bag. If you also see yellow leaves, take a look at yellow leaves.