are you sure its ants ?
more likely aphids but you see the ants trying to assist in eating them
Try spraying with a strong solution of washing up liquid or soap solution

"Len Buffey" wrote in message
...
Can anyone tell me how to prevent my Snowball Tree, viburnam opius
'Roseum'
from being attacked by black ants ? They seem to be associated with some
sort of black deposit on the underside of the leaves which curl quite
tightly. I would like to avoid a chemical solution if possible