Blackfly

Anyone got any remedies for blackfly on dahlias and beans.  I'm resorting to squishing. The ants are having a lovely time farming them. I'm thinking of sprinkling some semolina on the ground beneath the plants, supposedly the ants take it as food and when it gets eaten it swells in their tummies.  I'm not sure I believe it but if the ants aren't farming it might reduce the blackfly. Then again at least the ants put the blackfly all on one place ready for squishing   so I might be making matters worse.