These are the most common Bonsai Tree insects
How do I know? Little yellow speckles on leaf surfaces. When you turn a leaf over, tiny, oval shaped mites, about pin head in size are scurrying around. Their eggs, best seen with a magnifying glass, will be scattered around at random. Spider Mite eggs are all perfectly round, the same size, ranging from clear to amber in color. With larger infestations a fine webbing, crawling with mites, covers the plant tops. Soon, all the leaves are browning and dying. Spider mites are small and can be hard to spot on your Bonsai, but an easy way to know for sure is to place a piece of white paper under the branches and flick the leaves gently with your finger. It'll be easy to spot the mites because they'll try to make a getaway and you can easily see them move on the paper.
How do I know? Tiny, pure white "moths" resting on leaf surfaces. When disturbed, these moth-like flies quickly flutter up, then settle back down onto the plant. Leaves may appear shiny with honeydew. A magnifier reveals clear-white "scales" (the pupae) on the underside of leaves. All stages of Whiteflies suck plant juices.
How do I know? Wilted-looking plants that aren't thriving. Looking closer, dense colonies of tiny (1/32" - 1/8") soft bodied, pear-shaped insects are seen, especially on tender growing tips and undersides. Young Aphids look like miniature adults and the whole family will be found chowing down together.
How do I know? Distorted leaves, generally weakened plants. Clusters of Mealybugs look like a cottony mass. It's only on close inspection that you see that they're individual insects. They're soft bodied and very slow moving.
How do I know? Leaf surfaces finely speckled with yellow spots. Later, a silvery metallic looking sheen may cover leaf surfaces. Not all Thrips create this sheen, but with or without sheen, you'll also see black specks. Only on close inspection is the pest itself found. About 1/16" long, thrips can move quite quickly for their size. Many gardeners report thrips as a small "worm with legs". Larvae and adults look similar, but adults have wings and can fly.
How do I know? Small, dingy grey flies, flying around your Bonsai tree aimlessly. Fungus Gnats are easy to identify because they look just like, well... gnats. If these "flies" are also seen coming out of the soil around your tree, you probably have Fungus Gnats. Gnats are a pretty common problem with Bonsai trees because of the constant watering and therefore moist soil conditions. But they're virtually harmless and easy to control. Most of the time, just letting the soil dry out a bit gets rid of them.