Like any other living plant, Bonsai trees can be infected by any kind of pest or disease. However, when your plants are healthy and cared for properly, the risk of infection is reduced to a minimum.
In short: make sure your trees are potted using the right soil mixture, you understand how and how often to water, you don’t over or under fertilize and your Bonsai is placed right. Although chances of infection of healthy trees are low, of course they can still get infected. As it is often hard to identify the problem correctly you might want to take pictures and ask for help at our Bonsai forum.
Leaves suddenly turn yellow and fall of the tree. This is often the result of a sudden lack of water. When the leaves slowly turn yellow and die it is most often the result of sustained overwatering, overfeeding or underfeeding
Try to identify the insects either by photographing them and asking for advice at on online Bonsai forum, or by comparing the insects you see with pictures found on the internet.
Chewing Insects: Grasshoppers, Ants Borers, Caterpillars, Carpenter Bees, Cicidas, Leafminers, Slugs and Snails
Sucking Insects: Aphids, Scale, Mites, Spittlebugs, Wasps, Weevils
Soil organisms and parasites: Nematodes, Maggots, Grubs
Beneficial bugs: Ladybugs, Spiders, Lacewings, Praying Mantis, Worms
After you have identified the insect buy an insecticide (chemically or organically) to treat the trees with; be careful to follow the instructions and you might want to start using a half doses first, if necessary you can increase the doses later on.
Viruses can be detected by discolored leaves and sudden dieback of branches. Place the infected tree separately from your other trees to prevent cross-contamination. Treat by removing infected parts from the tree and by applying a fungicide spray.