Aphids include a variety of small, soft-bodied sap sucking pests about 1-2mm long. Species feeding on conifers reach 4-5mm long. these insects are very wide spread and can be in abundance in horticulture, aphids can be disatrous and can also spread other diseases from one plant to the next.
in small numbers they can still be quite devastating to a tree sucking the life right out of the tree, in large numbers they can take out a lot of plant life.
sometimes they are called blackfly, green fly etc. although they have a wide range of colours from grey, pinks and yellows.
most aphids will feed on above soil areas of the tree/plant, there are some known species that will burrow into the soil/potting media these are known as wooly aphids (one of the many in the genus that will feed on roots) these root eating aphids will also feed on stems and fruits, the protect themselves with a white waxy coating and can often be mistaken for mealy bug.
damage
aphid damage can be noted by yellowing and discoloured, wilting leaves and shoot tip damage, distorted flowers and leaves. aphids exrete a sticky substance from excess sugars called honey dew that falls and adheres to leaves below, which in turn encourages sooty mould. Infestations usually begin in growing points, leaf undersides, bark fissures and shaded parts of plants. Spread is most rapid on soft new growth in spring and autumn.
over fertilised plants/trees with high nitrogen fertilisers and constant water encourages soft, sappy new growth which will attract aphid.
another common problem with aphid is ants, they will "farm" the aphids excretion of honey due and even protect the ahpid so they can have a constant supply of the honey due, the ants will also move them from one tree to another to increase availability.
control
aphids have a natural predator that is quite a nice little bug, lady beetles, or lady bird/bug. the ladybirds will attack and eats aphid, they are quite voracious from an early stage of there life.
you can also rub aphids off with your fingers and thumb, pressure spray them with water, or using natrasoap is a good control method that wont endanger the tree.
you can also use a lime sulpher mix on the deciduous trees in winter when the tree is bare, this will kill of any eggs and break the cycle, only do this once though as that should be enough. when using lime sulpher always protect the soil with towels and/or plastic, lime sulpher can be deadly to a tree if it gets into the trees system.
so in summary aphid being a common pest should be quite controlable with a little bit of knowledge
if you have any experiences with aphids and bonsai please feel free to add to this topic, also if you have any questions post them here as it will allow us to build a good data base on pests and disease jamie
note- information and picture sources are as follows- re written.
information- safersolutions.com
pictures- several varying sites.
taking the top half of trees of since 2005! 
