Black bean aphid
A small, matt black aphid, up to about 2 mm long, that packs itself so densely onto shoot tips and stems that the plant looks sooted. It is the classic "blackfly" of broad beans, runner beans, nasturtiums, dahlias and poppies. Heavy colonies stunt growth and spoil pod set on broad beans; elsewhere the damage is mostly tolerable, and on the shrubs where it spends the winter the plants usually recover.
Wingless adults and nymphs of Aphis fabae on broad bean, with attending ant. Photo: pending.
Identification
Wingless adults are 1.2–2.9 mm, pear-shaped and dull, matt black to dark brown, sometimes with a greenish tinge; the nymphs, and sometimes the adults, carry small white wax spots on the back. The RHS puts garden specimens at up to 2 mm. The colony is the giveaway: dozens to hundreds packed shoulder to shoulder on a tip or stem, usually with ants walking over them.
Several aphids look black to the naked eye. Cowpea aphid is shiny black with whitish legs, where bean aphid is duller and slightly larger (UC IPM). Under a lens, black bean aphid has the middle segments of the antennae pale; melon aphid, which also has dark forms, has no dark markings on its back and a cauda usually paler than its cornicles.
Easily confused with
- Melon aphid (dark forms)Also a pestUnder a lens, black bean aphid has dark cornicles and a dark tail, and larger ones have broken dark bands across the back. Melon aphid has no dark markings on the back, and its tail is usually paler than its cornicles.
- Aphid mummiesBeneficial — leave itA parasitised blackfly swells into a crusty, golden-brown mummy, later with a round exit hole where the wasp left. Leave mummies on the plant: they mean parasitoids are working.
- Hoverfly larvaeBeneficial — leave itHoverfly larvae of most species are legless and maggot-like, tapering towards the head end, 1/5–4/5 in (5–20 mm) long when mature. Aphids have legs and a pair of cornicles.
Damage symptoms
Damage follows density. A few colonies on a nasturtium are cosmetic; a solid sleeve of aphids on a broad bean tip weakens the plant and cuts pod set. The RHS notes that the beans inside the pods stay undamaged and perfectly edible. On the winter hosts the spring curling is usually minor and the shrub recovers.
- Shoot tips and stems coated in dense black colonies
- Leaves curled and puckered at the growing point
- Stunted growth and poor pod formation on broad beans
- Damaged or distorted flowers on dahlias, poppies and nasturtiums
- Sticky honeydew, then black sooty mould, on leaves below
- Ants running up and down the stems
Lifecycle & degree days
In cool temperate climates it alternates hosts. Eggs overwinter on a woody shrub; in spring the first generations feed there, then winged females move to beans, poppies, beet and other summer hosts, where females breed without mating and give birth to live young. The RHS puts garden activity from April to August; numbers build in spring, before predators are active in large enough numbers. Sexual forms appear in autumn and lay the winter eggs back on the shrub. In warm regions it can skip the egg and breed all year.
- Winter host
- Eggs on spindle, viburnum, mock orange (UK)
- Summer hosts
- Beans, beet, poppies, nasturtium, dahlia
- Active
- April–August; builds in spring (RHS)
- Reproduction
- Live young without mating; sexual forms in autumn
- Viruses
- Implicated in over 40 (USDA AphID)
Host crops
One of the broadest host ranges of any aphid: USDA's AphID records it from nearly 120 plant families, including beans, peas, beet, crucifers, cucurbits, potato, tomato and tulip. In gardens it is noticed on broad, French and runner beans, nasturtium, dahlia and poppies. Its winter hosts are shrubs — spindle, viburnum and mock orange in Britain — where the spring curling is short-lived.
Monitoring & action thresholds
We found no extension home-garden number, and UC IPM says measurable thresholds are not available for beans. The RHS advises tolerating it where you can. The practical trigger is broad bean tips going solid black — that is when pinching them out pays.
Check bean tips, the undersides of young leaves and flower stems often from spring, as the RHS advises, so you can act before damage develops. Ants climbing the stems are often the first sign. Look for predators before acting: hoverfly larvae, ladybird larvae and bloated parasitised aphids inside a colony mean control is already under way.
Biological control
Outdoors, conserve rather than buy. The RHS names ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs as the natural enemies to encourage; in spring the aphids build up before these are active in numbers, so waiting often gives control. UC IPM adds lacewings for beans. Ants protect colonies from predators, so keeping ants off matters. Bought beneficials make sense only under cover: the RHS notes that aphid predators such as the gall midge Aphidoletes, lacewing larvae and several parasitoid wasps are sold for greenhouse use.
Cultural controls
The RHS approach is to accept some blackfly, remove the worst of it by hand, and let predators do the rest.
- Pinch out heavily infested shoot tips, where the colonies are concentrated (RHS)
- Squash colonies between finger and thumb, or knock them off with a strong spray of water
- Keep ants off plants, since they defend the colonies from predators
- Use nitrogen sparingly; soft, fast growth feeds aphids
- Grow open flowers nearby to feed hoverflies and parasitoid wasps
Chemical, last resort
- Insecticidal soapREI 12 h
- Narrow-range oilREI 4 h
- Neem oilREI 4 h
- PyrethrinsREI 12 h
Sources
RHS Advice: Blackfly (black bean aphid) (accessed Sept 2026)
UC IPM Pest Notes: Aphids, UC ANR Pub. 7404 (M.L. Flint; updated 07/2013)
UC IPM Pest Management Guidelines: Dry Beans, Aphids, UC ANR Pub. 3446
UC IPM Floriculture & Nurseries Guidelines, Pub. 3392: Aphids treatment table (updated 01/2022)
UC IPM, Relative Toxicities of Pesticides Used in Floriculture to Natural Enemies (updated 01/2022)
USDA AphID: Aphis fabae (Jensen & Gammelgaard Nielsen; accessed Sept 2026)
InfluentialPoints: Aphis fabae, black bean aphid (accessed Sept 2026)
Kati & Hardie 2010, J. Insect Physiol. 56: 14–20 (A. colemani in A. fabae)
Hafer-Hahmann & Vorburger 2024, Ecol. Evol. 14: e11090 (symbiont protection in A. fabae)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Black bean aphid by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.