Introduction: What are blackfly?
Blackfly is a general classification for over 2,000 different species of flies in the family Simuliidae (which includes aphids, midges, mosquitos, and lakefly). The only trait they share is that they are all black or dark grey. What most gardeners are referring to when they talk about blackfly on broad beans though, is the black bean aphid.
Black bean aphid (Aphis fabae) is one of the few species of Simuliidae that does not feed on the blood of mammals and instead feeds on the plant sap from young shoots and growing tips. In this article, we’ll explore how to get rid of blackfly and how to prevent blackfly on broad beans in the first place, by talking about the unique biology of these frustrating insects.
What is the blackfly lifecycle on broad beans?
The blackfly lifecycle is incredibly complex and involves multiple changes to the body to increase their likelihood of survival, including the ability to clone themselves, select the gender of their offspring and reproduce asexually. The parent blackfly (or founding females) over-winter as eggs, hatch in late spring and can reproduce up to 8 times in its 25-day lifespan.
While it might sound complicated to us, it’s essential to their success, and why aphids and blackfly have become such prolific pests in vegetable gardens in every corner of the earth.
Blackfly lifecycle table:
| Blackfly Eggs | Blackfly Eggs are laid in late autumn and stay in the soil, compost, or under leaf litter from dropped plants. They hatch in early spring, usually when temperatures are over 12oC to give them the best chance of finding newly germinated plants and young shoots. | |
| Blackfly ‘stem mother’ | The first blackfly to hatch are all female, and referred to as stem mothers. Stem mothers are wingless blackfly, which gather together in clusters on host plants as soon as new growth appears. After 4-7 days on their host plant, these wingless blackfly are mature enough to reproduce, and do so through cloning, and rather than laying eggs, they actually carry these cloned infants in the same way as mammals.Aphids can clone themselves up to 80 times in a single life span. They usually live for up to 25 days. | |
| 2nd generation, spring blackfly | Wingless female blackfly If the parent aphid is happy on their host plant, and decides to continue reproducing and increasing the population in that spot, they clone wingless versions of themselves to continue eating the plant. These clones live 25 days, and can reproduce up to 80 times. Winged female blackfly (emigrant) If there is not enough new growth on their host plant, the clones of the mother develop wings before they are born and fly to new plants, where they clone themselves, either producing winged, or wingless clones again. | |
| Winged female blackfly (emigrant) | If there is not enough new growth on their host plant, the clones of the mother develop wings before they are born and fly to new plants, where they clone themselves, either producing winged, or wingless clones again. | |
| 3rd – Final generation, summer blackfly | The offspring of these cloned females continue producing either on their own plants, or to emigrate to other plants until late summer. | |
| Final generation, late season blackfly | The penultimate generation of blackfly are born in late summer when the day length begins to shorten and their plant hosts stop producing new shoots.This generation lay a final generation of winged female blackfly, followed closely by a generation of winged male blackfly.The female blackfly lose their wings when they find a final host plant, and are joined by males from other families to create a wider gene pool. The males die in autumn after mating and the females lay eggs. | |
| Overwintering blackfly eggs | The blackfly eggs remain dormant in the soil, compost, or hidden in leaf litter until spring. |
What does blackfly damage look like?
Early signs of blackfly damage will be beneath the clusters of flies so are imperceptible until the infestation is removed.
For ongoing infestations, where blackfly have been present for over a month and the plant has continued growing, your broad beans will have yellowing stems, followed by white, washed-out leaves that are curled and crisp looking, but excessively damp.
How to prevent blackfly on broad beans
There are two different ways that blackflies find broad beans to land on. The first is relatively simple. Like most plant pests, they are attracted to weaker plants as weaker broad beans are pumping more sugars up the stem to attempt to recover. The blackfly can sense this struggle and take advantage.
The blackfly will continue feeding on this plant for as long it produces sap and will move further and further up the stem as new shoots appear.
The second way blackfly get around is thanks to the ingenuity of ants. To understand this, first, we need to understand the blackfly diet:
- Blackfly eat sap, a substance that is full of sugar, in such high quantities that the blackfly can’t actually process it.
- This creates secretions called honeydew, which is highly attractive to ants.
- Ants, as we know, are enterprising creatures, and they know that blackfly = honeydew. To help create a more constant supply of honeydew, the ants literally pick up the blackfly and move them to other plants. The blackfly that has been moved quickly realize that they have a new plant to themselves, and within a matter of days can clone themselves to create a whole new infestation, and a massive new supply of honeydew for the ants.
So how, with all the odds stacked against us, should we gardeners tackle this mounting problem?
Does pinching out broad beans prevent blackfly?
Pinching out broad beans is by far the most effective way to prevent blackfly. Blackfly loves the new shoots of broad beans, and once broad beans have flowered it’s best to pinch out growing tips so they continue putting their energy into growing bigger pods, rather than trying to flower again.
Pinch out broad bean tips after the first pods start to appear on old growth. Blackfly typically hatch out in late spring or early summer, so in a colder spring, you can often have beans that are starting to develop in their pods before blackfly emerge.
By pinching out broad bean tips in summer, you’re also removing the parts of the plant that attract new blackfly, reducing the chances of infestation.
How to grow pest-resistant broad beans
Blackfly are attracted to weak plants and broad beans, while tough, are quite temperamental plants, so the more you can harden them off the better. Broad beans are frost hardy plants, but their hardiness also depends on your climate – check this article if you want to learn more about sowing dates.
Start by sowing broad beans indoors from September-November (they can germinate down to 2oC), and plan them in the garden at least 4 weeks before the first frost. For example: For me, the first frost is usually mid-December. So I work backwards:
- Sow – October 23rd (+1 week)
- Germination – November 1st (+2 weeks)
- Plant out – November 15th (+4 weeks)
- First frost – December 15th
Don’t fertilize your broad beans other than with a compost/manure mulch over winter. Over-fed broad beans are often leggy and full of sap, which attracts blackflies.
Companion plants for broad beans
Plants that blackfly hate include oregano, catnip and mint (which attract hoverflies), dill, fennel, angelica, and wild carrot (which attract ladybirds) and buckwheat and poached egg plant (which attract lacewing, whose larvae feed almost entirely aphid and blackfly).
How to treat blackfly on broad beans
Treating blackfly on broad beans is actually pretty simple. Despite their seemingly infinite adaptations, they are incredibly susceptible to soap, and while they are capable of voluntarily cloning themselves, blackfly have never managed to evolve to taste bad. Almost every predatory insect loves the taste of blackfly and aphid, and are attracted by the scent of their honeydew secretions.
Essentially, if you can cover them in surfactants (surface active agents – which form an even liquid layer without suspended gasses) they suffocate almost immediately. The stickier the better too, so adding sugar soap can help. If you have borax in the cupboard, it can literally dissolve the insects from the inside out.
Best homemade treatments for blackfly on broad beans
Soap and water (and bicarbonate of soda) kills blackfly
To create a basic surfactant, you can just add dish soap to water at a 1:10 concentration. The soap creates an even layer of water that quickly suffocates the blackfly.
Soap and water will not fully eradicate eggs though, so to create a more effective pesticide, add bicarbonate of soda (one tablespoon per litre). Bicarbonate of soda works by dehydrating insects, in much the same way we use it to clean our homes.
Borax and sugar kills blackfly
Borax and sugar work by sticking to the insects and forcing them to ingest it. Borax is a naturally derived mineral, mined from the earth, so is an organic pesticide, whereas boric acid, the chemical compound is a much more effective, but not organic, pesticide.
Both work exceptionally well to kill aphids and blackfly but should be used with care as borax and boric acid are indiscriminate and can kill any insect or invertebrate it touches.
Buy Borax on Amazon here to have a go at making your own pesticide.
Use Vaseline to keep ants of broad beans
As we mentioned above, ants actively farm blackfly, moving them from plant to plant and protecting them from other predators. To keep the ants away from blackfly on broad beans, rub Vaseline on the base of the broad bean stem so the ants can’t climb the plant. Birds, ladybugs and hoverflies will quickly pick off the remaining insects.
Spray blackfly off broad beans with a high-pressure hose
If you prefer not to use pesticides of any sort it’s possible to keep control of blackfly populations by simply spraying them with a high-pressure hose or a pressure sprayer. They drop off the plant, onto the soil, and provided you don’t have mature winged blackfly on the plant, that will keep them at bay for a few days.
Best organic pesticides for blackfly on broad beans
Neem oil
Neem oil is the most effective blackfly treatment you can buy. Spray on broad bean foliage every two weeks and repeat the process after rain.
Buy neem oil from Amazon here to try making your own pesticide (it works brilliantly for common houseplant pests too)
Horticultural soap
Horticultural soap is a mix of everything and works as a surfactant, a dehydrator, and an organic chemical pesticide. Most horticultural soaps use 20+ ingredients, so it’s far cheaper to buy it ready mixed. This is the best horticultural soap for broad beans we’ve tried.
Ladybugs
Ladybugs are the most effective blackfly predator, so encouraging them into the garden in any way is a foolproof naturally remedy for blackfly.
As strange as it may sound, you can buy ladybugs online, but I’d much prefer to attract these beneficial insects to the garden with other plants. They’re available on Amazon if you want to take a shortcut though.
Conclusion
Blackfly are fascinating creatures, with adaptations that exceed our expectations, and continue to confuse gardeners, scientists, and their predators. Their survival tactics include self-cloning, gender selection and growing wings at will, but millions of years of evolution couldn’t stop being a tasty snack for a ladybug.
These bizarre creatures are one of the broad beans’ worst enemies, but at least you’ll be confident this year when you see the first signs of blackfly on broad beans in the garden and know just how to tackle them.
