Blackfly (black bean aphid) build up on broad beans as the weather warms, clustering on soft shoot tips, flower stems and the underside of young leaves. A heavy, unchecked colony can reduce how many pods the plant sets, though the beans themselves stay perfectly edible. The single most reliable fix is pinching out the growing tip once plants have flowered and started setting pods — that soft tip growth is exactly what the aphids are after.

Ranked, most likely first

  1. Soft new growth at the tip. Blackfly concentrate on the plant's softest, most actively growing parts — the shoot tip, flower stems, and the undersides of young leaves — rather than spreading evenly over the whole plant. A vigorously growing plant with plenty of fresh tip growth is offering exactly what they want.
  2. Seasonal timing. Aphids move onto bean plants as the weather warms through the growing season, after overwintering elsewhere. A colony that seems to appear overnight is usually this seasonal migration reaching a critical mass, not a sudden new problem with the plant itself.
  3. No natural predators established yet. Ladybirds, hoverfly larvae, and parasitoid wasps take time to find a new colony. Early-season infestations, or plants in a tidy, heavily managed garden with little else growing, often go unchecked longer simply because nothing is preying on them yet.

Why temperate gardens see it build quickly

Our own broad beans planting window is identical across all three NZ zones — direct sow August to October, harvest December to February, Egmont-sourced — so timing alone doesn't explain a zone difference. What does is how fast spring actually warms up. Temperate gardens warm faster in spring than cool/mountain gardens, and aphid populations respond directly to that warmth: a faster warm-up means a blackfly colony here can go from a few aphids to a dense cluster in a shorter window than in a cooler garden. That's the practical argument for checking plants regularly from flowering onward, rather than waiting until a colony is already obviously heavy before doing anything about it.

Fixing it

  • Pinch out the growing tip once the plant has flowered and set its first pods. This removes the soft tip growth blackfly prefer and is the standard control for this crop — it doesn't harm the plant, since broad beans don't need to keep growing taller once they're podding.
  • Squash small colonies by hand or hose them off before they build up, especially on the tips and flower stems where they cluster most densely.
  • Encourage natural predators — ladybirds, hoverflies, parasitoid wasps and ground beetles all prey on aphids — by leaving some flowering plants nearby rather than keeping every bed bare between crops.
  • For a heavy, established colony, a contact insecticide can knock it back — Yates NZ specifically recommends its Nature's Way Vegie Insect Spray Natrasoap, a natural, low-toxicity option based on contact action. Reserve this for infestations that hand-removal and tip-pinching haven't controlled.
  • Don't panic about light aphid presence alone. It's the heavy, sustained infestations that measurably reduce pod set, not a handful of aphids on one shoot tip.

Cultivar notes

None of our sourced Broad Bean cultivars — Coles Prolific, Red Seeded, Scottish, Superaguadulce, or Suttons Dwarf — carry a specific claim about blackfly resistance or tolerance; their supplier notes cover sowing timing and method rather than pest performance. So variety choice isn't a lever here — tip-pinching and timing are what actually control this problem, whichever cultivar you're growing.

If your plants are flowering well but not setting pods and you don't see aphids in numbers, that's a different, pollination-driven problem rather than pest pressure — check the weather at flowering time before assuming blackfly is the cause.

Next season

Get into the habit of pinching tips as soon as the first pods have set, rather than waiting for a visible colony to justify it — in a fast-warming temperate spring, by the time blackfly are obvious the colony has often already had a head start. Checking plants weekly from flowering onward costs little and catches a build-up long before it's dense enough to affect pod set.

More on this zone: see the full NZ Temperate Planting Calendar →.