black fly and green fly suck the juice from the plant making it weak . the also multiply very quickly, though runner beans are very sturdy,
mix a little washing up liquid with water and spray them making you spray under the leaves as well. happy bean eating.

I use this method for my veg and other plants it works well but may need to be repeated. On my non veg plants such as roses I add a drop of cooking oil to the mixture as this makes it sticky so therefore lasts a bit longer between sprays

... And the good plus side of using soap/cooking oil is they are, cheap, they are to hand and no toxic chemicals.

these little insects are born pregnant and give birth every hour .