I've been growing a few Cosmos plants for many years because of the wonderfulness they add to a vase arrangement.

HOwever, for the past three years I have had a problem with some sort of tiny black bug which eats at the bottom of the stems of the plant, eating right through and destroying the plant over time.

They look like tiny black ants which move incredibly quickly and there are a big number of them clustered at the stem.

I don't notice them until I realize that one stem is laying on the ground and when I move it, a mass of these tiny insects will run for another stem - they are everywhere at the ground level of the entire plant.

I've tried planting in three totally different areas with the same results.

What could these be? I only messed with one cosmos this year and put it on a side of the house which I've never planted them and again, today I discovered a stem on the ground and there were all those tiny bugs at the base of it. This cosmos plant doesn't have long, they munch right through. I don't know when they begin though, the whole plant flowers and looks very healthy - until the stem is eaten through.

Thanks for any help you might give me.