What are the Black Bugs on my Runner Beans?

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Having decided to grow runner beans this year, I was concerned when I saw several areas of black bug infestations on both the stems and on the underside of the runner bean leaves. So I set about trying to find out exactly what these black bugs were, why they were there and how I could best get rid of them. Here is what I’ve found out!

Black bugs on runner beans are most likely to be black bean aphids (also known as blackfly). Blackfly grow to around 5mm in length and feed on the sap of the plant. They secrete a sticky substance that can cause mold and disease to take hold and become established, causing damage to the plant.

What Attracts Blackfly to Runner Beans?

Blackfly are not unique to runner beans. They feed on sap and can be found on many different trees, shrubs and plants – but runner beans are one of one of their favourites!

They can be found anywhere on the plant but the most common locations are on stems, new buds and the underside of leaves. The below pictures are what I have found on my runner beans:

Blackfly found on the Underside of Runner Bean Leaves
Blackfly found on Runner Bean Stem

What Damage do Blackfly do to Runner Beans?

The damage caused by blackfly to your runner beans is two fold.

Firstly, because blackfly feed on sap, they take energy from your runner bean plants which in turn can stunt their growth. Not good news!

Secondly, blackfly produce a sticky secretion which can lead to the establishment and development of mould and disease on the runner bean plant.

If you Have a Blackfly Problem, you may Find That you Have an ant Problem as Well.

Ants enable the spread of blackfly

The presence of blackfly often goes hand in hand with the presence of ants.

Ants are attracted to the sticky secretions of the blackfly as it is a source of food or them. Whilst in the process of finding and consuming this food source, ants actively facilitate the spread of blackfly by physically moving them to new areas of the plant (like new leaves and stems) where they then go on to establish new colonies. And this spread can be rapid!

In fact, it is quite possible that the ants may have introduced the blackfly to your runner beans in the first place.

How do you get rid of Blackfly on Runner Beans?

Checking your runner beans regularly and catching any blackfly issues at an early stage, before they have had a chance to spread, will make the problem much easier to deal with. The blackfly population will grow exponentially so it is important to take action as soon as you spot them on your beans – if you do nothing, the infestation will only get worse.

There are several different treatment options to consider once you have identified a blackfly presence – and using one or more of these will help keep blackfly numbers to a minimum. Lets take a look at them below:

-Remove by hand – running your fingers over the blackfly will get rid of them, although this can be a bit messy and is not the most pleasant of jobs!

-Cut off infested stems and remove infested leaves – Removing infested stems and leaves will get rid of the blackfly attached to them and will help stop any future spread. Catching them early can stop the infestation in its tracks.

-Remove (healthy) growing tips of the beans – the tips of the beans are especially susceptible to blackfly. When your beans are tall enough, pinching them out and removing unwanted tips before they are affected will help reduce the number of of potential places available for blackfly to establish themselves.

Pinching out tips of broad beans will help prevent blackfly spread

-Spray with water – Targeting affected leaves and stems by spraying them regularly with water can help deter blackfly, although you will need to keep repeating this every few days to stay on top of the problem otherwise the blackfly will just come back. If you employ this method, it is best to do it late in the day.

-Spray with weak soap solution (washing up liquid) – This is another spraying option and again it is best to do this late in the day. This is likely to be more effective than simply spraying with water as a foamy soap solution will actually stop the blackfly from breathing and kill them off. Again, you will probably need to administer several repeat applications in order for it to be fully effective, and ideally these applications should be one or two days apart from each other. Another thing to bear in mind is that you will need to ensure that the soap solution that you are using will not be detrimental to the health of your runner beans.

-Kill any ants – If ants are present, one option is to kill them by using ant powder or ant spray (ensuring that the product used is safe for use on food crops).

-Grow companion plants to distract the ants from your runner beans – if there are ants present it is unlikely that you will be able to completely eradicate them so planting companion plants to your runner beans that are attractive to ants, such as sunflowers, can help attract them away help protect your crop.

-Grow companion plant to attract blackfly predators – Blackfly have a number of natural predators (such as ladybirds). If you grow plants such as marigolds next to your runner beans, for example, they will attract ladybirds which should in turn help keep blackfly numbers down.

Ladybird – predator of blackfly

What Other Pests can you Encounter When Growing Runner Beans?

Slugs and Snails

Slugs and snails can wreak havoc on runner beans. Tell tale signs that you have a problem are partially eaten leaves and trails of slime across the stems and leaves of the plant.

For tips on getting rid of slugs from your garden, follow our handy guide.

Leaf damage from slugs and snails