Pest and Fungus Control Basics

Bonsai trees are susceptible to both insects and fungus. This article will help you with both.

Which Bug Is It?

Our library has a copy of The Gardener's Book of Pests and Diseases. Unfortunately this seems to be out of print now, so if you want your own copy you'll need to buy a used copy - or find an alternative. We don't have a recommendation for an alternative text yet.

Book Cover

Identifying Pests

One of the things you need to do is see the pest that's eating your tree. You can use a magnifying glass, but a better solution is a 10-power loupe. You may think if 10-power is good, then a 15- or 20-power must be better, but that's not true. First the image will tend to shake more and they are just harder to use.

Many geologists recommend the BelOMO 10x Triplet Loupe. It's available from Amateur Geologist for about $32. If you'd prefer to order from Amazon, they also carry it - although it really ships from Amateur Geologist.



Aphids - In the spring aphids can be a real problem, especially on newly emerging maple leaves. Most of the ones we see are green (like in the following image) but some are dark gray / black.

From University of Florida

Fortunately aphids are easy to get rid of by washing them off while watering. Simply turn the water pressure up a bit more than normal and spray the undersides of leaves. You may have to do this every few days but eventually the aphids will disappear. Insecticidal soap is very safe and effective. Or you can use one of the oil-based sprays (see Dormant Sprays).

Insecticidal Soap
From Home Depot

Adelgids are related to aphilds. Sometimes called "adalgia". There are variations, for example pine bark adalgids, pine leav adelgid, hemlock wooly adelgid, etc. These adelgids are often confused with woolly aphids or mealybugs or even fungi because of the fluffy secretions that cover the adelgids. During the winter immature females hide in crevices and rough places on the bark. In late winter / early spring the female lays up to 24 eggs under and around her body in her fluffy, white secretion. After laying her eggs, the female dies. The newly hatched insects spread out to eat and reproduce. Some sources say five generations a year. Asmall infestitation can quickly become a major one if not treated. The white fluffy "wool" is waxy, and water repellant so these bugs can't be washed off. Insecticidal soal or oil-based sprays are best. You may need to treat two or three times every couple of weeks to kill all the bugs.

Adalgid on Hemlock
Adalgid on hemlock (from US Forest Service)

Scale - Scale is an insect that lives under a hard shell. The shell may be black, white or other mottled.

Scale (from Washington State University, Whatcom County Extension)

Red spider mites often invade and ruin the color of pines and shimpaku during hot weather. There are a variety of solutions. Red spider mites prefer a warm, dry climate. Increasing humidity thru misting and keeping trees well watered should minimize infestation. But when they strike you should quickly spray - either with insecticidal soap or one of the oil-based sprays. Remember to move sprayed trees to a shaded area for a week to avoid damaging the tree. These bugs have a very short reproduction period so you'll need to repeat the spraying two or three times at ten-day intervals, as the eggs will keep hatching.

Red Spider Mites
Red Spider Mites (from

Juniper Twig Girdler - This pest is a major problem for many club members. The real damage is done by the larve of a small moth. Oil-based sprays may not be effective on the larve. Pyrethrin spray may work better, and is fairly safe – except you need to avoid exposing cats, fish and bees. You need to spray twice - in early June and mid-July. Capitol Bonsai (Sacramento, CA) has a great article about how to inspect and treat for this problem - see article.

Thrips - are another insect that feeds on the undersides of leaves. Thrips puncture the leaves, flowers, or stems and suck up the exuding sap. The first indication may be when fine yellow spots appear on the leaf surfaces. Later the foliage may take on a silvery appearance, eventually browning and dying. Leaf tips may wither, curl and die. The undersides of leaves are spotted with small black specks. Flowers become flecked, spotted, and deformed and many buds fail to open.

This illustration below shows a green thrip, but there are many varieties - and different colors. As treatment you can use insecticidal soap.

Thrip Damage Thrip on Leaf
Thrips (from University of Hawaii) Thrips (from University of California, Riverside)



In addition to insect control bonsai can be attacked by various fungus diseases, like sooty mold, rust, etc. Here are a few common ones:

Sooty Mold Rust
Sooty Mold (from Rust ( from


Powdery Mildew Black Spot
Powdery Mildew (from Black Spot (from

For many years Lime Sulphur was used because it treated common fungus and insect infections. But Lime Sulphur is now illegal in California so when you need to treat fungus a copper fungicide is the preferred choice. Neem Oil can also be used for Powdery Mildew, Black Spot and other fungus. Spray during the cooler months, or move your tree to a shady area for a couple of week after spraying.

Alternatively, Peter Tea recommends using Cleary 3336 DG Lite - a granular systemic fungicide. It can also be used on junipers. For pines he uses 1 tablespoon per square foot of pot space. Apply every 2 months. Note, this comes in a 30 pound bag and isn't cheap. It may not make sense to use if you only have a few trees to treat.

Copper Fungicide
From Summer Winds Nursery

Dormant Sprays

Although the colder weather of autumn and winter will kill off some bugs, like aphids, some will survive to attack our trees - esp. in California. It's important to continue treating for insects and fungus so spray trees after leaves drop.

In previous years manufacturers sold a variety of different pest controls with names like dormant spray, summar spray, all-seasons, etc. This naming was supposed to help consumers know when to use the spray. Often the dormant spray, used when the leaves have fallen, would be a bit too harsh to use on growing leaves. But this seems to be less of a concern because the oils used in such sprays are more highly refined.

That brings us to choosing which type of spray to use. Oil-based sprays kill insects by somthering the bugs. Many are mineral oils (derived from petroleum) although some are vegetable oils (canola, cottonseed, Neem). Common product names are Ultra-Fine and Volk. Some literature suggests that cottonseed oil is a slightly better insecticide. But Neem oil may be even better because it also attacks fungus and bacterial infections.

Caution: Some tree varieties can damaged by some oils, so be safe and read the Precautions on the label first. For example, Do not use lime-sulphur to spray ume. And, do not use a solution containing volck oil for cedar varieties.

Neem Oil All Seasons Oil
From Summer Winds Nursery From Summer Winds Nursery


Never spray any tree that you plan to display in an exhibit within the next few weeks. The oil can change the color and shine of the tree and make it look unnatural. Here's when to spray

Tree Type Typical Ratio When



Half of the foliage still remains



End of November - Beginning of December

Check for soil dryness the day before spraying. The soil should be moist.
Cover the pot with newspaper to protect the pot and any moss from damage from lime-sulphur.

After spraying move your trees to a shady area of your yard for a week or two. Otherwise you risk damaging the leaves.