Pests and Diseases

All living things are eaten by other living things and your Bonsai are no exception.

A healthy tree will be able to shake off most diseases, keep it well fed use sharp tools to make cuts that will heal over better, treat all major cuts with a sealant and you should have very few problems.


The type of insect taking an interest in your tree will vary with where you are in the world, however your response will tend to be the same. Man's ingenuity has provided us with chemicals to really put them off their lunch. Some of you will favour organic methods of growing your trees, however good these methods may be their response time will not be as fast as a good dose of Malathion.

Friend or Foe?


Remember that not all insects are harmful, Ladybird, or ladybug beetles and their larvae, eat aphids all day. If you must spray the plant remove any you see before doing so.

A Ladybird larvae

They may vary greatly from the example shown, both in the number of spots and the shell colour

The Lacewing is another friend. Both it and it's larva, which look a bit like a beetle lava are voracious aphid eaters.


Hoverfly's are considered to be beneficial insects as the adults pollinate plants and the larvae eat greenfly.

The Larvae resemble a maggot or caterpillar, being about a cm long and tapered. They may often be seen alongside aphids.

This is the last view most insects get of the Mantis. Those of you lucky enough to live where they do should be careful to remove any you come across before spraying either insecticides, or fungicides.


Wasps get a bad press that they really do not deserve. It's true that in exceptional circumstances a wasp's sting can kill humans, but on the whole they are friendly creatures and love to join you for a picnic.

The 'Normal' wasps are ravenous eaters of small caterpillars and other grubs. 

The Hornet "Vespa cabro" is the largest European wasp; the female measures 25 to 35 mm long, the males and workers are smaller. In males, the antennae have 13 segments, while females have 12. The male abdomen is composed of 7 visible segments, while that of the female has 6; females are equipped with an ovipositor.


The parasitic wasps, the Ichneumon Wasps are also widespread

Adults vary in size from 1/8 to 1-1/2 inches with long, many-segmented antennae and long, slender bodies. Females have long ovipositors (egg laying tubes), the longest of which is six times the body length. Legs are long and slender. Yet again they, or rather their larvae eat caterpillars, or grubs.

An ovid female will capture a grub and paralyse it, it will then using its egg laying tube (ovipositor), insert an egg into it and the lucky grub will then be eaten from the inside.

The females of those species with long  tubes, will listen for grubs moving in wood and probe with their tube until they locate their prey, injecting an egg into it.



Other than Locusts, Aphids are probably the best known pests of plants on the planet.

The damage they cause is twofold. Firstly they suck the sap from the plant, inhibiting its flow to and from the growing shoots and not allowing leaf development. The leaves it should be remembered produce the sugars the plant needs to grow.

Secondly they wound the tree, allowing fungal attacks.

Many people will baulk at the idea of spraying insecticides because they kill friendly insects as well as harmful ones. This is undoubtedly true, however a bonsai is to a large extent isolated from the environment, so spraying will have a minimal effect.

The ants in the image are guarding them against predators. Ants 'milk' them for the sweet liquid they excrete.

The life cycle of the Aphid is really complex. if indeed you can use the term life cycle, normally associated with an individual.

As the year progresses the type of aphid and its method of reproduction alters dramatically.

The first generation emerges from eggs, overwintered in bark, they reproduce asexually, that is to say they give birth to live young without mating. This allows a rapid build up of the population. They are wingless. All of these qualities apply to the second generation.

Within the third generation, some are born with wings, rapidly spreading to plants both near and far. Yet again this generation reproduce asexually.

The next generation produces both winged females and males, mating and laying the overwintering eggs.

Woolly Aphids on an Apple tree

Spraying with an insecticide is really the only way to hold them in check. A good preventative is to spray your deciduous trees with one of the many commercially available winter washes. As the name implies it's sprayed on overwinter and will kill any eggs on the tree. Personally I only use them on my deciduous trees, as they tend to be based on 'Tar oil' and I don't like the idea of applying an oil-based product onto trees in leaf.

Keeping a eye on your trees in spring will allow you to spray early and get them before they can harm your trees,


Adelgids are closely related to Aphids and can be a major pest on Pines. They can be dealt wilt as you would Aphids but as tey're covered in a waxy hairy coat you may have to treat your tree more often.



Ants and their Lavae and Pupae
Ants nests are uncommon in bonsai and penjing, however a flying queen may decide that your pot is an ideal nest site. Ants themselves don't actually harm the trees, indeed the tunnels they make in the soil allow air to the roots they are 'Farmers' of aphids, milking them for the sugar they excrete after dining on sap. They will collect aphids and protect them from predators.

They are not really harmed by the insecticides applied to kill aphids. A trip to your Garden store will produce several chemical remedies to get rid of them. I've tried immersing the pot in water for a few days and all this does is to drive them up into the branches.

Ants are members of the order Hymenoptera the order of insects that also includes the bees, wasps and sawflies


There cannot be a plant on the planet that is not prone to attack by caterpillars. Even the hard, resin filled needles of the Pine have their own special caterpillar.

To the left you see eggs on the underside of a maple leaf, those of you who have the patience can look under the leaves and remove the eggs by hand. Spraying with a systemic insecticide at this stage will probably not have any effect, as in common with all eggs, the developing embroyo is feeding off its own yolk sac, not eating poisoned leaf.

The catterpillar of The Oak Eggar Moth. You're no t likely to get this on your Bonsai and as it's (when grown) some 6cm long you'll notice it!




Galls are an abnormal growth of plant tissues, which may take many forms, caused by living 
organisms such as fungi, mites, And minute gall wasps.

Here we see Galls caused by the pustule gall Aceria Ulmicola.

And Oak Galls which are caused by a Parasitic Wasp


Leafcutter Bee

Over the past few years I've noticed an increase in attacks from the Leafcutter bee. This little pest cuts crescents out of leaves, rolls them up and takes them back to build nests for their grubs.

As insecticides are not absorbed by either the adults, or lava, spraying is of no use. The adults are agile in flight and a sod to try and swat. I use a pair of small nets for use in Aquaria and trap the bee between them. I then give it a good telling off !.

The bee and the damage it causes


Another sap sucker, however due to it's size this one can cause a lot of damage. Below you can see the 'nest' of bubbles the young blow around themselves as cover.

The easiest way to deal with them is to pick them off, however a systemic insecticide will give them a very nasy headache

The young and adult stages of the leafhopper.


There are several types of leafmining moth, not all produce the track like tunnels shown. The larvae of Phyllonorycter (Lithocolletis) coryli, eat patches, which show up as a clear blotch on the leaf.

Scale Insect

This cute little killer not only sucks the sap of your trees, but also creates nice little holes allowing infections in. Although there are lots of chemical remedies to deal with them, nothing is quite as satisfying as squashing them by hand.

Try to get them before they get to the reproductive stage, where I've heard them described as looking like a "bogey on a bit of cotton wool". They seem to erupt with a wool like substance and die.

There are thousands (about six) of species of scale insect scattered around the world, so the insect may vary from that shown.

You usually only see the wingless femails.

The reproductive stage, the 'cotton wool' is alive with baby scale insects and they're beginning to move out to the rest of this and other nearby trees. At this point spraying is the only option.

And here they are, about to make their way out into the world, cute arn't they?padding text in white.

Slugs and Snails

Slugs and Snails, it must be said are not that much of a problem to the Bonsai keeper. They tend to damage fresh growth on deciduous trees. Slug pellets are the best way to deal with them, however if you don't want to use pellets, consider using 'beer traps' and no I'm not joking, Slugs and Snails are attracted to the beer, drink it, get drunk and drown in it. My advice is to use an American beer (see there had to be a good use for it!).

The way to do it is to get a small container, of about 5cm deep and half fill it with beer. Place it (them) near your trees and wait for the little buggers to drown.

Both Slugs and Snails lay eggs, about 3mm diameter and like little pearls. How you dispose of them is up to you.

Slugs and their eggs.


The spidermite, or rather mites, there are several types, are common garden pests. They can cause serious damage to your indoor or outdoor bonsai. Here in the UK and in most areas subject to hard winters they tend to attack indoor trees.

These microscopic spider like pests are found on the underside of leaves and in the crotch of branches. They suck fluids from the leaves, leaving traces of yellowish white spots. Inspection of your plants with a magnifying glass should reveal tiny spider webs on stems and leaves.

The mites thrive in dry, warm conditions and in temperatures above 80 degrees, they can reproduce in as little as 5 days, making early detection a necessity.

There are several chemical remedies to deal with these bugs, pyrethrum being one.

As Iíve said above they like dry warm conditions, so indoor trees are particularly prone, however there are a couple of things that can be done to protect your trees.

Regular inspection of your trees will allow you to deal with an attack early. Providing a moist microclimate around your trees will help. This can be done in two ways, firstly regular spraying with water will help. This may mean moving your indoor tree outdoors, avoiding damaging your furnishings. Another way is to stand your trees above a tray of water, allowing it to evaporate past the tree. The latter can be applied to both indoor and outdoor trees.

Vine Weevil

An adult vine weevil. These are not so much a problem as the soil dwelling grubs, who feed on the roots.

The grub of the Vine weevil, with an example at actual size.

These are the most dangerous of all the pests you are likely to come across. In most cases you only know you've got them when your tree dies. Just as caterpillars eat leaves, Vine weevil grubs eat roots.The Adults eat little notches out of the edges of the leaves, so keep an eye out for this.

Over the years all sorts of chemicals have been tried to eradicate them, however a new one has recently appeared in the garden centers, here in the UK it's sold as 'Provado Vine Weevil Killer'

Those of you who favour a 'natural' remedy, should consider nematodes, small worms that attack and kill the weevil grubs. Nematodes are available from specialist suppliers, who usually advertise in gardening magazines.

The problem with using Nematodes is that you must apply them to an already infected pot, or they will starve to death rapidly.


Vine weevils cannot fly, or swim, so a good way of stopping them getting to your trees is to stand them over water. This has the added benefit keeping all crawling pests away from your trees.

In addition, as the water in the trays evaporates on hot days, the 'steam' provides a moist microclimate around the trees.

Leatherjacket grubs

Another root eater is the leatherjacket, the grub of the Crane fly, or 'daddy long legs'. The adult looks like an overblown mosquito and is about 3cm long. The adults are harmless, the grubs are not. A dose of a systemic insecticide will see to them.

The Crane fly, or 'daddy long legs'. The adult looks scary but is totally harmless to us.

The cranefly, at about 1.5X its actual size.

Fungus and Molds

I should start by saying that not all fungi are harmful to your trees, all plants have a symbiotic root fungus called Mycorrhiza which benefits the tree. It can usually be seen as a thin white sheet around the roots on repotting and is particularly noticeable on Pines, it is as I said beneficial. However most fungi, and molds will kill your tree, or disfigure the leaves.


Mychorriza on a young Pines roots. You should not be alarmed at this as it's a sign of a healthy root system.

Fungi enter your tree through wounds and particularly those wounds below soil level, so when repotting your trees the first watering should contain a fungicide. Other fungi, such as mildew or blackspot will attack your leaves, disfiguring them and greatly reducing their ability to feed the tree by clogging the stomata, throught which the leaves breathe. Regular spraying with a fungicide will help to prevent this.

Not all fungi have the typical mushroom
fruiting body, many have the round fruit above.


Scab is a disease commonly associated with Fruit trees. It is often seen in trees continually fed with high nitrogen feeds. It's unlikely to be seen in a bonsai fed with a well balanced fertiliser.

High Nitrogen feeds promote rapid, soft growth and this, if damaged may allow scab to enter, so watch out.

Scab itself is typified by the shrinking and drying of an area of bark. If you are the victim of such an attack you will find a remedy, usually a spray in your garden center.

Assuming you've stopped the attack in time, you might consider applying a 'Dead wood' effect to the damaged area.

The tree to the right shows both Scab (the Diamond shape at the top) and its associate Canker.


Worms are not likely to be a problem in bonsai pots, they may however get in if the pot is placed on the ground.

On balance worms will benefit the tree in the same way they do other plants in the soil, by allowing air to circulate in their holes. I'm not advocating introducing them into your pots, but you should not panic if you do find them.

Other pests


Cats are not a problem if dealt with in a sympathetic and understanding manner.

My own late moggie had a spot on my Bonsai racks where he liked to sun himself. I left a space for him, aren't cats quick when you turn the hose on!.

Little Helpers

The last thing I would suggest is that you keep your children away from your trees, however very young children will mimic their parents as part of the growing process. If they see you working on a tree they may decide to 'help' you, with perhaps disastrous results for your trees. Keep your tools away from them.

© Allen. C. Roffey May 19, 2018 18:54