I would be concerned about them harming the roots ... but I'm not sure if they will or not ... perhaps someone else my have a better idea ... you may have to find a way to get rid of them without harming the roots ...
yes it is...watch the main stem of ure plant and if you get these little black spots that means there or theyre carry'n some pest to ure crop....ants carry alot of garden pest and yes harm roots....kill'em or theyll cause problems...
they will eat all the leafs with in days, you will see 100s of them carrying pieces down to their nest. get some ant killer and mix it in the soil and a good sprinkle around the base of the plant.