The one good thing about Black Fly - or Blackfly - is that they are easy to see. Sometimes the other closely related aphid - the Greenfly, is not seen until it has taken a stranglehold on its plant meal.
As with all other Aphids, Black Fly is a sap sucker. It literally sucks the host plant to death. A little bit of exaggeration, in that it departs the plant that is giving it a feed, before it dies. A dead plant is of no use to this sap sucker!
As Black Fly are easy to see, we have the advantage of being able to deal with it - getting rid of it - before it can do too much damage. Even when we see the seemingly devastating effect of over-wintered broad beans, all is not lost. In this case, it simply puts the plant back a bit. kill the black fly, and the plant soon recovers.
There are three basic ways of dealing with a Black Fly infestation. Spray with an insecticide or puffer; using a biological control; or preventing them in the first place!
Blackly - along with the other Aphids - have a quite complicated life cycle, thus making them more difficult to control. For instance, the Black Fly female, is quite capable of producing live young - and eggs - without the help of the male! Live young are immediately able to feed upon the host plant. They do not have to wean on the mother Black fly, so getting in early with control measures is quite important.
As their name suggests, Black Fly can fly! This is part of their life cycle apparatus, in that the female flies off in the Autumn to lay eggs on a host plant which over-winter. The young are born with the ability to fly immediately in early spring; their first point of call is normally the over-wintered broad beans - which make a succulent meal before most other suitable plants start to grow.
More young are born - with wings - enabling a migration to the likes of Viburnum, Honeysuckle and then the later Dahlias. From there, off once again to perform the Winter Egg Laying off site!
Fortunately, the image shows the Blackfly at many times normal size. They are only a few millimetres long! Image by Joaquim Alves Gaspar,
The quickest way to get rid of Blackfly is to apply a spray as soon as you see the infestation. This will most certainly be on Broad Beans firstly, followed by French and Runner Beans, then ornamentals such as Dahlias, Poppies and nasturtium later. Later still, they will surface on the Autumnal foliage of Mock Orange (Philadelphus). Viburnum opulus types and Euonymus japonicus europus (The evergreen one).