Pests and Diseases

Just as children can catch a cold, your bonsai can get an insect infestation or a disease. A strong, healthy tree is less likely to be attacked by fungus, disease and pests.

The best way to prevent pests and diseases to foster good health by providing the best possible conditions: good light, good air circulation and the proper environment and proper watering, among many other things. Yet even under the best conditions you may encounter a problem.

First of all, you need to identify what the problem is. This is important because you want to treat the actual problem and not operate on guess work. DON’T HESITATE. If something doesn’t feel right or look right about your tree, bring it in to us as soon as you can.

Let’s start with insects.


Small sucking insects are mostly visible to the naked eye and almost always visible with a magnifying glass. Here are the most common ones you may encounter...


Aphids appear as green or very dark gray (usually) small rice kernel shaped insects that congregate on the new growth and the underside of leaves.


Scale insects are harder bodied and look like little discs, helmets or turtles and will appear on the leaves and branches.


Mealy bugs will leave their white cottony residue in the elbows of the trees and the white disc shaped insects can also be easily seen crawling on the tree.


Spider mites will leave dense webbing on the top of the foliage (do not mistake for a regular spider web, which can be helpful for your tree). Mites will also cause the foliage to look bleached out because they are so tiny and numerous that as they suck on the leaves or needles, they drain the color.


Fungus Gnats look like fruit flies and live in the soil rather than on the tree. They may appear when you pick up the tree to water it. The small flies will circle around and go back into the soil They are most common when the soil stays moist all the time. So although it is critical you never let your bonsai get bone dry, it is also important to let the soil get some air and not to keep it too wet, since this will encourage the gnats.

Aphids, scale and mealy bugs will leave some kind of sticky residue as they suck the sap from the plant and excrete it. So they are easily identified. Mites are harder to detect. To test for mites take a clean white sheet of paper and hold it under the foliage. Lightly agitate the foliage and watch for small particles falling on the paper. Give it a few seconds and see if any of these particles start crawling around. If they do they are most certainly mites. (Sometimes red in color but not always).



For mites, aphids and mealy bugs wash the foliage with warm water-like a shower. This will wash off many of the insects. After that spray with a mild insecticidal soap. There are many safe sprays you can buy at any garden center. Always follow directions. You may need to treat once a week for three weeks. Then keep a close eye out for further infestations. Sometimes multiple treatments are necessary. But don’t spray continually or you can further weaken the bonsai and make the problem worse. These sprays also do not function as preventatives. 

For scale insects, who have a harder exterior it is best to pick them off or rub off with a cue tip and alcohol, one by one. Its labor intensive but it does the trick. Then, if you see more later- remove them again.


Home Remedy Spray

Fill a quart mister ¾ full of water.


  • ¼ cup rubbing alcohol
  • 1 teaspoon vegetable oil
  • 2 squirts mild Castile liquid soap or mild dish soap

Shake it up and apply as indicated above.



Fungus and diseases are much harder to identify. If your bonsai leaves show a white powdery film on the leaves or black or dark blue spots (especially on Chinese elms) or a rusty red blotches it may be a fungus.

There are many other symptoms of fungus. In general be aware of blotches on the leaves-yellowing, browning and sudden die back. Mold and fungal diseases can sometimes be wiped off the leaves. However be careful not to spread from tree to tree in the process. Once again you should treat immediately with an over the counter fungicide and ALWAYS follow directions.

If you can bring your bonsai to a professional for a diagnosis this is always recommended. If you self diagnose just remember to be as thorough as possible and treat as minimally as possible. Base your treatment on your best information rather than assumption.

Remember it is important to use these sprays and substances responsibly.

Please let us know if you need further help.