The tips of broad bean plants are something of a magnet for blackfly (black aphids). These will cause stunted growth and affect the quality of the pods.
So what can you do about it?
1. Prevention. When the flowers on the plant start to set (produce pods) at the bottom of the plant pinch of the tips of the plant by about 5-7cm/2-3ins.
2. If you find you have an infestation – and it can happen very quickly – pinch off the tips and destroy or put in green bin. This will also result in an increased yield.
3. If still a problem, spray with an insect control spray such as Bug Clear or Bug Killer (and there are lots of others available too).