The ants unknowingly help to open the buds….
Do not try to get rid of the ants on your peonies. This is a natural and temporary activity. It is believed that peonies produce small amounts of nectar and other ant attractants to encourage ants to help in opening the dense double flower buds found in many peonies. The ants may be found covering certain varieties and avoiding others, this is totally normal.
Once the buds have opened the ants will disappear – also normal.
Some people think ants are REQUIRED to open the flowers, but this does not to appear to be true.
It seems a debatable question whether ants are beneficial or harmful. I think they are neutral.
Should you spray a pesticide to get rid of the ants? That is a definite no. Since the ants are not harmful and some pesticide residues are harmful, why endanger yourself, the plants or thepeony’s pollinator (good insects) with poisonous sprays?
Just don’t spray.
Instead just enjoy the unique interaction of ants and peonies; an evolutionary effect thousands of years in the making and posing no problems in the long run.
[from the www.peonies.org site]