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Pest and Fungus Control Basics

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

How to Identify Bugs

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.

Recommended book on Pests
Recommended book on Pests

Helpful Tool

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.

Loupe
Loupe

Common Bugs to Look For

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 and even red.

Aphids (from University of Florida)
Aphids (from University of Florida)
Red aphids and sap they’ve extracted from red maple branch

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 HomeDepot and Lowes)

Caution: Once opened, insecticidal soap will begin to oxidize. If it has turned brown you should not use it because it may kill your trees. To be safe, throw away any product 6 months after you’ve opened it. It helps if you write a discard date on the bottle at first use. The cost of this product is far less than any trees it may damage.

Adelgids

Adelgids are related to aphilds. Sometimes called “adalgia”. There are variations, for example pine bark adalgids, pine leaf 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. A small 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 soap 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 (from US Forest Service)
Adalgid on Hemlock (from US Forest Service)

Scale

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)
Scale (from Washington State University, Whatcom County Extension)

When there are only a few bugs then you might try dipping a cotton swab in 71% alcohol and then painting the bug with alcohol. Some people spray a solution of alcohol, water and insecticidal soap onto plants to kill bugs but you need to be careful. First some plants may be sensitive to this solution so you’ll need to test on a single leaf first and wait 24 hours before applying. Second, this isn’t healthy to breath in, so do outdoors, use a mask, etc. Third, avoid getting it on the soil. Given these concerns, when there are too many insects to treat by painting with alcohol we recommend spraying with an oil-based spray.

Neem oil is a good choice because it kills the insect and the mold – but make sure Neem oil is safe for the particular species.

Red Spider Mites

Red spider mites often invade and ruin the color of pines and shimpaku during hot weather. The recommended method to capture these bugs is to place a sheet of white paper underneath foliage you suspect may have spider mites then tap or gently shake the branch. Spider mites, and other bugs, will fall onto the paper. Use your loupe to examine anything that moves.

There are a variety of solutions. Red spider mites prefer a warm, dry climate so 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 (from redspidermites.net)
Red Spider Mites (from redspidermites.net)

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

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. Use insecticidal soap to kill thrips.

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

Fungus

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

Sooty mold is a type of plant mold that grows in the secretions from pests like aphids or scale. Itgenerally looks like a fine black powder on the surface or underside of the leaves. But it may be large enough to look bumpy. It might be dull or shiny. 

The soot itself can usually be washed off by spraying with water or using a wet paper towel. But you also need to remove the pest that is damaging the leaves. After removing the mold look for any scale – and treat as describd above. If you don’t see scale then assume aphids and treat as described above.

Sooty Mold (from www.clemson.edu)
Sooty Mold (from www.clemson.edu)

Others to Look For

Rust (From hawaiiplantdisease.net)
Rust (From hawaiiplantdisease.net)
Powdery Mildew (from www.organicgardening.com)
Powdery Mildew (from www.organicgardening.com)
Black Spot (from wikimedia.org)
Black Spot (from wikimedia.org)

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)
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 (from Summer Winds Nursery)
Neem Oil (from Summer Winds Nursery)
All Seasons Oil (from Summer Winds Nursery)
All Seasons Oil (from Summer Winds Nursery)

Application of Pesticides

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 TypeTypical Radio (product to water)When to Apply
Deciduous20 to 1Half of the foliage remains
Conifers30 to 1End of November to Beginning of December

Check for soil dryness the day before spraying. The soil should be moist.

If you’re using lime-sulphur cover the pot with newspaper to protect the pot and any moss from damage.

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

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