Unfortunately bonsai are susceptible to pests, as all plants are. Besides looking unsightly, pest insects feed of the sap of the tree, which can damage it and prevent it from thriving.

This article discusses some of the common bonsai pests, and approaches to bonsai pest control.

Common Bonsai Pests

Aphids/ greenfly

Aphids are tiny green or yellow insects that live on the leaves and stems of plants. There is also a white fluffy variety. They suck the sap out of plants and can be very hard to eliminate. They also multiply very quickly, forming clusters that can cover the whole tree.

Not only do they weaken the tree by sucking its sap, they also encourage disease and fungal infections, which can take over a tree very quickly if it has been weakened from the sap sucking.

Symptoms: A sticky residue on the leaves or stems, and the presence of ants on your tree (which eat the sticky residue). Also aphids will be visible on the underside of the leaves if you check carefully.

Treatment: Treat with insecticide. Also scrape off any old bark, which may contain aphid eggs.

Ants

Ants often go hand in hand with aphids, and have actually been known to ‘farm’ aphids, carrying them from plant to plant and protect them from predators.

Once they have found a source of food, ants will often build nests in the soil around the tree roots. This causes problems with the root system, including cutting through roots as they build their tunnels.

Symptoms: Ants on the tree.

Treatment: A combination of ant bait and soil insecticide should get rd of the ants. If they have set up an extensive nest in the root ball then this should be removed, and replaced with fresh soil.

Caterpillars

There are many different types of caterpillar, but they all destroy the foliage of trees at a great rate, and an completely defoliate a tree if not stopped. Caterpillars are not always easy to eliminate, as they are sometimes resistant to insecticides.

Symptoms: Holes (often irregular) are seen in the leaves. Black or brown droppings may also be present.

Treatment: Remove by hand as you see them and destroy them. A caterpillar-specific insecticide (designed to stay on the leaves of the tree) can work.

Scale insects

Scale are tiny insects which cling tightly to the branches and trunk of a tree. They are protected by a scaly outer shell. They are highly destructive sap-sucking pests. As with aphids they encourage the spread of diseases and fungal infections.

Symptoms: Tree not thriving. Scale insects will be visible on the under side of leaves and on stems.

Treatment: For a small-scale infestation you can carefully paint methylated spirits over and around the scale insects. For a larger infestation use an oil-based insecticide.

Spider mites

These tiny red mites are not actually insects. They are microscopically small, but visible because of their bright red color. They are another species of sap-sucker.

Symptoms: Conifers are most frequently affected. The needles turn yellow and drop off.

Treatment: Mite-specific insecticide (can be hard to find). Alternative approach is to prune and then burn any infested branches.

Pest Prevention

Pest prevention is better than cure, especially when it comes to caring for your bonsai. If you have outdoor bonsai, try to place your outdoor bonsai above ground level away from ground dwelling insects. It is a good approach to use a dedicated display stand or bench.

Not only will this help keep your bonsai collection away from ground dwelling pests, it will also keep them out of harms way of pets (urine is acidic and repeated doses will be harmful).

Controlling an Infestation

Keep your bonsai well spaced and immediately remove any infected plant away from healthy bonsai. If your bonsai is normally kept indoors, placing it outside will help to control an infestation.

Insecticides

Only treat your bonsai with insecticide once you have spotted that there is a problem. Most garden insecticides can be used, but you should dilute so that you use less than the manufacturers recommended dose.

Do not apply as a preventative measure, as insecticides can be harmful to the soil and will be long lasting. Avoid applying insecticides to trees in flower.