Black fly on broad beans

My Sister-in-law reports black fly on her broad beans (?Maybe black aphids?) in previous years.   She is in the UK and it has been very wet, and unseasonably warm, so the lawn has been mowed into paths around the edges of the suburban backyard to the raised gardens, leaving a large section in the middle where the grass is "very long".  This year the broad beans have no little black visitors, and she is crediting the long grass proving a distraction.  It would be interesting to know if this has been experienced elsewhere, with other plants acting as attractants, rather then the veggies.