What is causing scales on citrus plants? Preventable?

[jth]

I've had two citrus trees in my apartment in the past few years. Each time the tree looks great when I get it, but soon after being in my apartment it starts to show signs of scale and I eventually have to get rid of it. I have many houseplants and none of them have ever been affected by scale, just the two citrus trees. Is this just bad luck, or could there be some insect in my apartment that only attacks citrus and this will keep happening whenever I bring in a new tree? I love keeping citrus trees and I want to make it work, but I hate to bring in plants only to have them infested. Is there a way to prevent scale, or should I just avoid citrus until I'm in a new place?

[applestar]

Are you seeing ants on the citrus -- going up and down or clustered on underside of leaves? They tend to the scales that usually latch onto the main leaf vein.

I'm dealing with these right now

-- this is not too bad yet, but a sign that shouldn't be ignored.

Scales can be removed with cotton swabs dipped in alcohol, but you have to get rid of the ants (I use ant bait made with borax) because they will bring more and move them around on the tree.