Black bean aphids, more commonly known here as Blackfly, affect a wide range of garden plants, trees shrubs and certain vegetables mainly beans (broad beans & runners) and peas.
Blackfly breathe through their bodies so you can squirt them with any long lasting foamy product to combat an infestation -providing it is harmless to your plants - as this will stop them breathing.
That is the most economical method but if you don't have the right product for the job or you are uncertain if what you have is harmless to your plants then go for a proprietary brand.
They are commonly found en masse on the undersides of leaves as well as on soft new shoot tips or buds. Black bean aphids cluster together and are noticeable because of their dark colouring. As they feed, they secrete a sticky, honeydew substance which drips onto lower foliage and often becomes covered in a sticky black mould.
This summer (2018) we are really suffering with infestations of Black Fly which are more often than not managed by ants in your garden who 'farm' the aphids and 'milk' the sticky honeydew that they produce. Ants will often carry young aphids onto new plants to establish new colonies.
If you have a black fly problem then don't neglect it because in the warmer months these pests can each produce 5 new offspring and that being the case you can see that they can soon make short work of an irreplaceable plant.
The ants which "farm" these aphids are harmless to your plants, but if you can get rid of the ants then you have at least got rid of some of the problem.
Most garden centres and seed producers supply sprays such as Bug Free which will do the job but please note that you really have to adhere to the manufacturer's instructions as too many applications can kill the very plants you are striving to protect.
The organic treatment method is just as reliable but has no repercussions in possible damage to your plants, so:-
Inspect plants regularly and deal with early symptoms immediately.
Remove as many aphids as you can by hand or, if possible, cut off infested shoots and stems on susceptible shrubs.
Spray the infested areas of a plant with a strong jet of water to keep aphid numbers down. I find that a "pump up" sprayer helps enormously when tender plants such as runner bean flowers are concerned.
Use nettings and fleeces to stop aphids spreading to susceptible plants though we accept that this option is not always possible as you need to give the bees plenty of access to your plants and flowers.
Encourage natural aphid predators such as ladybirds, Aphidoletes, hoverfly and lacewing larvae. These can be obtained from commercial suppliers and released on to affected plants outdoors.
For greenhouse plants use parasitic wasps of aphids such as Aphidius matricariae and Aphidius ervi which can be purchased for release in a contained space.