Brown streak on Stalk

I have a brown streak going up the stalk from the soil and all the bottom leaves are dying. I noticed when I picked the plant up some little white catipillar looking bugs under the pot and the top of the soil in some areas looked like what an ant hill looks like. I have sprayed it with "Safer" a natural catipillar and soft bug killer and I also flushed her but to no avail. Can someone please tell me what may be wrong and what I can do to save her? I am just at my wits end with this
Thanks..