Invisible Invaders: Confronting Bonsai Tree Pests

Like all plants, bonsai can fall prey to a number of parasites, which feed off the sap, at the expense of the tree. These pests benefit from the regular watering and health care given to bonsai. The following are some common pests, although indoor bonsai can be susceptible to attacks by white flies as well.

Key Takeaways

  • Aphids, including the wooly aphid, are a major threat to bonsai trees, weakening them by sucking sap and encouraging diseases. They can be controlled with insecticides or methylated spirit.
  • Various caterpillars can cause significant foliage damage. They are resistant to some insecticides, but can be manually removed or controlled with insecticide powders.
  • Scale insects cling to branches and trunks, sucking out sap and spreading diseases. They can be treated with oily insecticides and physically removed using methylated spirit.
  • Red spider mites and insect larvae are resistant to traditional insecticides. Mites require special treatment or pruning of infested branches, while larvae in the root ball can be removed during repotting.
  • Ants can exacerbate pest issues by ‘farming’ aphids and damaging root systems. They can be controlled by traps or by disturbing their nests during repotting.

Common Bonsai Tree Pests

Here, common bonsai tree pests, tiny yet formidable, embark on a silent siege against these miniature marvels.

1. Aphids (Aphidoidea)

As most gardeners will know, these pests are hard to eliminate. They are small, green, yellow or brown insects which live in colonies on the leaves and stems. Extremely prolific, they quickly multiply to form clusters that cover the whole plant.

They weaken the tree by sucking its sap and also encourage fungal and viral diseases, which move in quickly when the plant is weakened. These also attract ants which establish their nests in the root ball.

Bonsai Tree Pests
Colony of Black Aphids

Aphids can be quite easily eradicated by spraying with insecticide. Old bark should be scrapped, as it often harbors insect eggs.

One of the most persistent of aphids is the wooly aphid. This has a white fur coat which protects the aphid from contact with insecticide. Although systemic insecticide should control this pest, a better method is to paint it with methylated spirit. If your tree looks as though it is developing tiny pieces of cotton wool, then it has wooly aphids.

2. Caterpillars

There are many different kinds of caterpillars, all with a voracious appetite for foliage, which attack plant leaves, to the point of total destruction.

Some of them develop in cocoons on the undersides of leaves before appearing in the daylight. They twist themselves into silky webs, hence their name, tortrix caterpillars. Sometimes they develop in the leaf and flower buds.

Bonsai Tree Pests
Lime Butterfly Caterpillar Eating the Leaf of Bonsai Tree

Caterpillars are not always easy to eliminate, as they are sometimes resistant to insecticides. Remove them by hand where you see them and destroy them. An insecticide powder, which stays on the leaves longer, can give good results, as the caterpillars absorb it as they eat their way through the leaves.

It may be possible to catch some species in a saucer of glue placed at the base of the tree. This will prevent moths from climbing the tree at night in autumn and laying their eggs on the leaves.

3. Scale Insects (Coccoidea)

Minuscule insects which cling firmly to branches and trunk, protected by a scaly outer shell. These insects are destructive pests. The female penetrates the plant’s skin and sucks out sap. Like aphids, they encourage the spread of fungal and viral diseases.

Control with an oily insecticide in winter, then scrape away any dead bark. Eggs will continue to develop under the shells of dead insects. It is vital to burn all debris from the scraping.

Bonsai Tree Pests
Scale Insects (Coccidae) on a Magnolia in The Garden

Another method is to dip a small paintbrush or cotton bud in methylated spirit. This can then be painted on and around the scale.

Tiny, red spider mites are visible to the naked eye because of their bright color. The tetranychus group are particularly voracious, the adult mite piercing the backs of the leaves to suck out the sap. They most frequently attack conifers, the needles of which then turn yellow and drop.

Mites are quite resistant to insecticides (which are formulated to kill insects, but not necessarily mites). Special products are hard to find. The most effective treatment is to prune away and burn infested branches.

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4. Insect Larvae

Bonsai Tree Pests
Insect Larvae

Some insects may lay eggs in the bonsai compost, that is, in the root ball. The larvae which hatch are especially fond of young roots, which they devour, depriving the plant of nourishment.

So keep a watchful eye open during repotting, when you can destroy larvae with your fingers. You could also use a suitable insecticide like HCH.

5. Red Spider Mites (Tetranychus Urticae)

Red spider mites, a particularly troublesome variety of bonsai tree pests, pose a significant threat to the health of these intricate miniature ecosystems.

These tiny arachnids, often identified by their characteristic webbing and the reddish hue of infestation, primarily attack by sucking sap from the leaves, leading to a mottled appearance and eventual leaf drop.

Red Spider Mite Infestation
Red Spider Mite Infestation

Combatting these pests requires a vigilant approach; increasing humidity around the bonsai can deter them, as they prefer dry conditions. Regularly inspecting the undersides of leaves and employing natural predators like ladybugs can be effective.

For more severe infestations, using a miticide or a homemade solution of water and mild soap to spray the bonsai can help in controlling and repelling these persistent mites, ensuring the bonsai’s health and aesthetic appeal are preserved.

6. Ants

Ants are formidable pests in several ways. Due to their fondness for aphids, on whose excretions they feed, they can almost be said to ‘farm’ them, since they convey them from one plant to another plant and protect them from predators such as ladybirds.

Ants Swarming on The Berries

In order to be near their food, ants also tend to congregate in nests in the soil around the roots, causing havoc in the root system and cutting through roots as they carve out tunnels.

The sole remedy is to set a trap for them on the surface of the root ball. If an ant’s nest is set up in the root ball, the only solution is to lift the plant from its tray. Brush off the soil and eliminate as many larvae as possible. When disturbed, ants are easily dislodged.

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Final Thought

In the delicate art of bonsai cultivation, the tiny yet formidable bonsai tree pests pose a significant challenge, turning gardeners into vigilant protectors of these miniature natural wonders. Battling against aphids, caterpillars, and mites, each bonsai enthusiast learns the intricate balance of nature and the importance of persistent care. These pests, while small, bring a profound understanding of the fragility and resilience inherent in the world of bonsai.

As we navigate the complexities of maintaining these living sculptures, we’re reminded of the delicate dance between beauty and adversity. In the end, our fight against bonsai tree pests is not just about preservation, but a deeper appreciation of the subtle yet powerful life forces within each bonsai.

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