BonsaiPests and Diseases

Bonsai - Controlling Pests & Diseases:

Problems are usually noticed sooner, before they become widespread

Bonsai are as susceptible to pests and diseases as normal sized plants and trees of that variety. However, being a more manageable size and receiving closer attention than the average houseplant or garden tree means that problems are usually noticed sooner, before they become widespread. On the other hand, due to their diminished size, bonsai may not be able to withstand a determined attack. Vigilance is of the utmost importance.

There are many available insecticides, each suitable for a specific insect and not necessarily suited for them all. Sometimes the first thing to try is to try picking or washing the insects off. A good rule to follow is to use the least dangerous method that will do the job. An insecticidal soap, for example, is very safe for humans, but must be rinsed off plants after spraying, as the fatty acids will eventually cover the pores on the leaves. It has to be sprayed fairly often, as it must contact the insect and has no residual effect. It must not be used on some plants (notably Serissa), so read labels.


Some of the more common insects and appropriate methods of control:

  • APHIDS - small, juicy-looking insects, usually light green, but can be white, black or grey, that suck sap from young leaves and flower buds. Most houseplant sprays will kill them on contact, but must be repeated every four to six days to be effective. Aphids produce live, female, pregnant aphids, so it is a challenge! Sometimes removing all the flowers and buds at the same time will make it easier to control them. Insecticidal soaps will only kill the aphids they come in contact with.

  • SPIDER MITES - tiny, hard to see, look like very small, fine grains of white pepper on the underside of a leaf along the mid-vein. Later, fine webbing, which looks like angel hair on a Christmas tree, is seen. Spider mites are not insects, but closely related to spiders. They have eight legs, not six. An insect killer will not work.

  • MEALY BUGS - look like little clusters of damp cotton wool, usually in a crack where a leaf joins a stem. Dabbing each insect with rubbing alcohol on a Q-tip will kill that insect but is rather time consuming! Very careful watching for months will be necessary to be sure that a stray insect doesn't take over again.

  • SCALE - looks like a drop of dried glue on the stems or undersides of smooth-leaved plants. It can be scraped off with a fingernail. There are no chemicals that will kill scale established on leaves or stems. It is possible to kill individual scales by rubbing each one off with rubbing alcohol, but this will not kill them all, and they will be back! It is better to eliminate that plant, as it can spread to other plants and is all but impossible to eradicate.

  • FUNGUS GNATS – are tiny little black insects hovering around plants. The adults do no harm, but the larvae in the soil can damage fine root hairs if in large numbers. They prefer dying plant roots and are an indication that the soil is being kept too wet (which kills the roots). The plant may be watered too often, have poor drainage or be sitting in water. A soil dust will temporarily eliminate the problem but modifying watering habits will be the only way to completely eliminate them.

  • WHITEFLIES - tiny white flies on the underside of the leaves that fly up into the air when disturbed. Sprays are not very effective, unless you can hit them when they are flying. Try a sticky yellow card near the plant, and shake the plant frequently to encourage them to fly to the card.

  • POWDERY MILDEW - is a fungus infection, which is most prevalent in cool areas with high humidity. As this is a common practice with some bonsai, it can be a problem. It looks like a dusting of fine icing sugar on the leaves and causes distortion and browning of the edges. Warmer temperatures discourage mildew, but over wintering plants need cooler temperatures!


If you are not sure what the problem is, do not spray to see if it will work! It is very easy to do more damage than the bugs could! Ask our certified pesticide dispensers, or bring a leaf to us we'll be able to help. Be cautious with chemicals, and READ LABELS!