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

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 Amazon about $36.

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 produce honeydew - a sugar-rich sticky liquid which attracts ants. It can also feed sooty mold (fungus).

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)

Scale

Scale - If you notice the foliage on your tree starting to yellow check for scale. Scale is a tiny insect that lives under a hard shell. It's mouth penetrates the bark of the plant so that it may suck the tree's sap which contains sugar. They often secrete honeydew which attracts ants - and can produce a black mold. Adult scale are usually less than 1/8" in diameter and can appear on almost any species. The shell may be black, white or other mottled.

Scale (from Washington State University, Whatcom County Extension)

Scale go thru two distinct stages in their life. First they are crawling insects and then as adults they become immobile under their shell. We normally don't see the crawling stage because the insect is so small. These photos show the adult stage after they've acquired their shells - which may be soft or hard. Females hide their eggs underneath the shell until they are ready to hatch. Then they become a crawling inspect and move to another part of the plant. At crawling stage they can be controlled by spraying with insecticidal soap - but you have to do at least two applications a week apart because not all eggs hatch at once. Once you see the adult stage we recommend using a 2% oil-based spray to suffocate the insect. Alternatively you can use a systemic insecticide that is absorbed by the plant.

When there are only a few bugs you might try dipping a cotton swab in 71% alcohol and then painting the bug with alcohol.

Juniper Scale

- photo from Ohio State University Extension

Female Juniper Scale

Magnified View

- Juniper Scale (Carulaspis juniperi)United States National Collection of Scale Insects Photograph Archive, USDA Agriculture Research Service, Bugwood.org

Juniper scale are typically white and found on the underside of junipers - and related species such as eastern red cedar (Juniper virginiana) and cypress..

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)

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 (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. It generally 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 wiping with a wet paper towel. But you also need to remove the pest that is damaging the leaves. After removing the mold look for aphids or scale - and treat as described above.

Sooty Mold (from www.clemson.edu)

Others to Look For

Rust (From hawaiiplantdisease.net)

Powdery Mildew (from www.organicgardening.com)

Black Spot (from wikimedia.org)

Treating Fungus

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. It treats most fungus and is safer than older chemicals. It’s good as a preventative treatment but it doesn’t treat every kind of fungus that might infect your tree.

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)

Minimizing the Spread of Fungus

If you water your trees from the top they you may be helping to spread fungus around the foliage. It’s better to only apply water to the soil. Watering Basics on our website explains.

To reduce fungus spreading between trees don’t put trees of the same species next to each other on your benches. Fungus that attack junipers don’t attack pines or oaks. Pines and Junipers are especially susceptible to this fungus spreading this way. This will also minimize spread of insects because the pests that attack pines don't usually attack junipers, etc.

Dormant Sprays Explained

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)

All Seasons Oil (from Summer Winds Nursery)

When to Spray

During the fall and winter we have shorter days and less light so the trees aren’t as able to fight back against insect damage. It’s a good idea to kill insects and mites before this happens. You should also spray in spring to kill off any new infestations. If you see insects or fungus at any other time during the year you should spray again.

Throughout the year you should occasionally look closely at your trees. In spring aphids like to attack maples. And, during the summer heat spider mites can be be a big problem.

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.

Tools for Spraying Insecticide and Fungicides

If you only have a few bonsai, or only need to treat one or two, then you might not want to mix up chemicals in a 1 or 2 gallon sprayer. There are a couple of options for you.

You can buy chemicals premixed in a disposable spray bottle from a nursery, Home Depot, etc. Or you can buy small pump sprayer in a 1 liter / 0.5 gallon size. These are available in sizes around 1 liter (half gallon). They cost $12-15 and are easy to use and to clean up.

However, getting spray underneath foliage can be tricky with the small sprayers. Full-size sprayers have a long wand that is easy to reach inside a tree to cover all foliage. It’s connected to the tank via a 36-inch or longer hose.

Disposable spray bottle

Small pump sprayer

Full-sized Sprayer

I’ve used the "Full-sized" Hudson sprayer for many years. It has a brass wand and adjustable nozzle. Brass nozzles have a reputation for being longer lasting than plastic but as infrequently as we use them I’m not sure that is critical.

Some sprayers, including this one, have a “funnel top” which means it’s easy to add chemicals and water to the tank without spilling down the side of the tank. This model also has a pressure release valve that allows you to release air from the tank once you’re finished spraying. Not all tanks have this, which means you either open the tank (unscrew the pump) and risk getting blasted with chemicals, or you must turn the tank upside down and spray until there’s nothing left in the hose and wand. The pressure release valve may also tell you when you’ve pumped too much air into the tank.

How to Spray

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.

It’s important to spray until the foliage and bark are fully covered. If you miss spots there’s a chance that the pest or fungus won’t be wiped out. So, spray until you see the liquid dripping from the foliage. You don’t want to spray when it’s windy.

Begin by spraying your tree from one side. Have your nozzle a few inches from the foliage and angled down a bit - say at a 45-degree angle. Move the nozzle so you can spray the top layer - moving around the tree (or rotate the tree) until all sides have been sprayed. (#1 below) Spray lower layers (#2)

Now, turn the nozzle upside down and spray UP into the tree. (#3) Lots of bugs hide underneath foliage so this is an important step.

Recommended way to spray a tree

After Spraying

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