Documents: Special Interest: Bonsai:

Controlling Pests & Diseases on Bonsai
by Ruth Staal
February 13, 2011

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. A chemically impregnated wax strip hung near the plant is worth trying for flying insects such as fungus gnats and whiteflies. It is quite safe for humans and indoor use but must be confined to a limited space. Contact insecticides will kill the insects they touch, or kill insects that eat or suck leaves the chemical is sprayed on. They are short-lived, which makes them reasonably safe for indoor use but they must be repeated regularly to be effective. These products will not kill insect eggs, so must be present when the larvae hatch from the eggs, to kill the next generation before they grow up and lay more eggs! Outdoor insecticides must not be used indoors. If an outdoor tree has an insect problem, an appropriate insecticide for that insect and that tree must be used. 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! Look for insecticides that contain Resmethrin or pyrethrin. 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, that 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 - count them! An insect killer will not work - it must be a mite killer containing Kelthane or Dicofol. It must be repeated once a week for several weeks, spraying the underside of the leaves.

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! A houseplant insect spray containing Resmethrin will kill them on contact but must be repeated regularly. 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. The only chemical registered for use on scale is Sevin. It must be repeated every four to five days for several applications, then the plant must be watched very carefully for months. If you cannot control it after several sessions of spraying, it is better to eliminate the plant, as it can spread to other plants and is very difficult to completely eradicate.

FUNGUS GNATS - tiny little black 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 wax impregnated bar near the plant in an enclosed place.

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. It is controlled with a fungicide, such as Benomyl or Funginex. They are systemic fungicides which can be sprayed on the leaves and/or applied to the soil, as they are carried in the sap to the leaves.

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 a knowledgeable person, read a book (preferably one with pictures of typical problems), or bring a leaf to a club meeting. Be cautious with chemicals, and READ LABELS! Bonsai Tools and Basics of Pruning

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