Ants round runner beans is a common problem, but it's likely to be slugs or
something like that that's doing the chomping of lower leaves.

The ants are/will be a problem though. They are very good at farming and have
herds of aphids that they collect up and herd into their nest over winter to
make sure they don't die off in any chill weather. Much like any other farmers
they take their herd out to pasture when the weather is warm enough and let
them graze, collecting the sugary secretions from the aphids and taking it back
to the nest. They are very consciencious and look after their herd and protect
it from attack by anything that might harm it, such as landbirds. I suggest
you look at the top of your beans where the ants know that the grazing for
their herd is lushest, It's probably covered with black aphids, or soon will
be.

Hope this helps.

Dave.