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Bonsai Trouble: Bonsai Pests and Diseases

Updated on November 26, 2010

Neither pests nor diseases need loom very large in mind of a bonsai grower. If the soil mixture is correct, the watering and feeding carefully administered, and the trees are not kept in too warm or too dry an atmosphere, all should be well.

However, both pests and diseases have the habit of appearing with no explanation, except that they have spread from other plants in the garden or neighborhood, which of course cannot be avoided. It is therefore wise to know what to do should anything adverse appear, and the most common troubles are listed below, together with a recommended treatment.




Little piles of powdery soil appear on the soil surface and under drainage holes. Use proprietary ant killer or water with pyrethrum liquid.



These small ‘flies’ can be black, green,  or a brownish red; they suck sap, thereby damaging the leaves, and also leave on the foliage a sticky substance which encourages the growth of fungal diseases. Spray or dust with malathion or pyrethrum.

Damage caused to bonsai tree by wood boring insects
Damage caused to bonsai tree by wood boring insects

Boring insects

These leave a shiny trail on the surface of bark and bore right into the wood, in extreme cases killing a branch of tree. Fill the holes with DDT dust and seal them with wax and clay.


Earth worms

These can be tiresome and cause disturbance in the roots, especially if the containers have been plunged in the garden. Water with liquid pyrethrum or a proprietary worm killer.

Red spider
Red spider

Red spider

These minute red or yellowish mites appear on the underside of leaves of deciduous trees, causing them to become mottled and later yellow; they are particularly a nuisance in dry weather. Frequent syringing (but not in the sun) can do so much to prevent attack; spray or dust with malathion.

Scale insects
Scale insects

Scale insects

These look like little brown or whitish shells or bumps mainly on the bark but sometimes on the foliage, they can be seen at any time of the year. Spray or dust with malathion.

Slugs and snails
Slugs and snails

Slugs and snails

Occasional use of slug bait in pellet form placed round the base of containers and on soil surface will control these pests.




This fungus appears as powdery patches on the leaves and is encouraged by the ‘honeydew’ left by the aphis attack; the leaves curl and become useless and young shoots gradually wither. Spray with Karathane or another proprietary mildew spray.

Other Damage

There are a few other injuries that may damage bonsai and  these are of an accidental nature and are not caused by pests or diseases.

Withered Leaves

This symptom may be caused by:

  1. Lack of water
  2. Too much water
  3. Too strong a food moisture

Buds Dropping

This is usually caused by a lack of water when the buds are forming.

Leaves turning brown

This symptom may also be caused by one of the several things:

  1. Scorch due to hot sun on wet leaves --  usually through glass.
  2. If dogs make a habit of ‘watering’ any plant or tree, it will gradually show signs of scorch.

If leaves or branches do die for any reason, they should be removed carefully with a sharp knife, scissors, secateurs, according to thickness of the injured part. Sometimes a new shoot can be trained to replace one that has had to be removed, and occasionally the style of the tree may be altered to offset any serious damage.


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