Ants like trees for one of two reasons. They are searching for food or making a home. Generally, ants don’t damage a tree but instead provide a warning sign that your tree is in trouble.
Scales and aphids are sap-sucking insects and feed on the sap of trees and plants. Sap has more sugar than the insects can handle, so they discard the extra sugar called honeydew. Ants love this sticky, sweet substance.
"Sawdust on he tree or at the base means these guys are active"
In large numbers, scales and aphids may harm your trees and plants.
As a tree softens and rots, carpenter ants move in, removing the rotten wood and opening up space to nest. Sawdust on the tree or at the base means these guys are active. The ants may extend the damage into sound wood and an injured tree may be further losing structural strength. This weakens the tree and could be at risk of falling. Control of carpenter ants inside a tree may not make any difference to the tree but it may be warranted as one way to reduce invasion of the ants into adjacent structures.