Aphids usually attack certain varieties of young Thujas. If you do not actually notice the aphid itself you soon become aware of the black sooty mould that is created on your plant. This mould is a fungus that forms in the sugar like excretion deposited by the aphids. Aphids excrete this to attract ants, the ants then protect the aphids from other predators. Ants sometimes cover the stems of effected plants in a crust of soil or potting mix, this also protects the aphid.
The ants do no harm, the sooty mould looks more unsightly than it is harmful, but if left unattended Aphids will eventually suck the life out of an effected plant.
A quick application of one of a number of contact or systemic type insecticides will do the trick.
Ask your resident nursery person as to which one you should use. Again use the one least harmful to yourself and the environment. Read all instructions carefully and use appropriate personal protection.
Here at the nursery we use Scott's Procide. (see disclaimer) This spray has been designed with low toxicity in mind, making it more environmental and people friendly.
Once the aphid threat has been taken care of, the sooty mould can be washed off the effected plant using a solution of soapy water and hose pressure. (You may have to do this a few times).