Aphids are also known as greenfly and blackfly and come in many colors and varieties.
By Andreas Eichler, CC BY-SA 3.0, Link
They feed on the soft parts of bonsai, like shoots, soft stems, around flowers and the underside of leaves. Their sap sucking habits can cause blotchy, blistered leaves and transfer viruses between plants. They excrete a sticky substance called honeydew that attracts ants but can also cause sooty mold outbreaks.
Female aphids can produce more female clone nymphs all by themselves and therefore one female aphid can multiply itself very rapidly. One should be constantly vigilant and inspect the tree for aphid every time one waters, prunes or otherwise maintains the tree. Inspection for pests should become a standard routine coupled to any other maintenance.
They can be removed manually or with a hand held spray gun but it is typically easiest to spray them with a suitable insecticide fit for indoor use on bonsai. The should be sprayed as soon as noticed and repeat sprays must be utilized to ensure eradication. All mold can be cleaned from leaves with a suitable damp cloth taking care not to damage already weakened leaves.
They have been around as a species for millennia and have even been found encased in Amber millions of years old.
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By Anders L. Damgaard - www.amber-inclusions.dk - Baltic-amber-beetle , CC BY-SA 4.0, Link