How can I save my Calla Lily?

My Calla Lilies have little black bugs (that look like aphids) and ants all over them. There are no holes in the leaves or stems but the plant appears to be rotting. The stems are covered in the black bugs and most of them have become weak (limp) and yellow – they look they are rotting away. I have sprayed them with aphid killer, but never seems to work. Also, I never water them because I was afraid the water was contributing to the problem. Last year I thought I had a rotting fungus so chopped off all of the greenery and dug up the bulbs. I guess I didn’t get all of the bulbs because they came up beautifully this year and looked lovely (until now). Now the same thing is happening again. They are in the front of my house so if I can’t fix this problem, I need to replace them. What should I do? I live in an area where they should do well and survive year round.