"Blackfly" and "greenfly" are garden shorthand, not different pests — both are aphids, small pear-shaped sap-suckers about 2–4mm long, and the colour just tells you which species and which crop is under attack. Blackfly usually means dark aphids clustered on broad beans and other legumes; greenfly covers the pale green aphids found across brassicas, roses and most other vegetables.
What makes aphids worth acting on quickly, rather than most garden pests, is speed. In warm weather a generation can complete in 10–14 days, aphids reproduce asexually, and more than 20 generations are possible in a single year. A few aphids on Tuesday can be a colony by the following week.
Identification
| Sign | Likely aphid | Typical host |
|---|---|---|
| Dense black or dark brown cluster on soft new growth and pods | Black aphid ("blackfly") | Broad beans, other beans, sometimes citrus |
| Pale green, scattered along stems and leaf undersides | Green aphid ("greenfly") | Wide range: brassicas, roses, most vegetables |
| Greyish-green with a powdery, waxy coating | Cabbage grey aphid | Brassicas — kale, broccoli, cabbage |
| Sticky, shiny film on leaves, with black sooty patches | Honeydew and sooty mould, not the aphid itself | Any aphid-affected plant |
| Ants moving up and down stems | Ants farming aphids for honeydew | Any aphid-affected plant |
| Small, bloated, papery brown aphids that don't move | Parasitised aphids ("mummies") — a good sign | Any aphid-affected plant |
The last row matters: a mummified aphid means a parasitic wasp already laid an egg inside it, and a new wasp is about to emerge. Don't spray a colony full of mummies without checking first — you may be about to kill the predators doing your work for you.
Why Aphids Build Up So Fast
Aphids feed by piercing plant tissue and sucking sap, which on its own weakens growth, twists leaves, and can stop buds opening properly. The bigger risk on food crops is that aphids can pass plant viruses from one plant to the next in their saliva while feeding — this is often the real reason a virus-affected plant shows up in a bed with no other obvious cause.
As they feed, aphids excrete a sugary waste called honeydew. Honeydew makes leaves sticky and feeds a black fungus called sooty mould, which coats leaves and can reduce how much light they take in. Ants are drawn to the honeydew and will actively protect aphid colonies from predators like ladybirds to keep the supply going — an ant trail on a stem is often the first visible clue that aphids are present higher up.
Blackfly on Broad Beans
Broad beans are the classic blackfly host. Dense black aphid clusters typically form on the soft growing tip and the top few inches of stem, not spread evenly over the plant. Left alone, a bad infestation can stunt growth and reduce pod set on the affected stems.
The standard non-chemical response is to pinch out the soft growing tips once plants have set a good number of flower trusses — this is common horticultural advice for broad beans generally, removes the tender growth blackfly prefers, and has the side benefit of encouraging the plant to put energy into the pods that have already set rather than more leafy growth. Check the removed tips before composting them if the colony is heavy; a light infestation can often be squashed by hand or hosed off before it needs anything stronger.
Greenfly Across the Vegetable Garden
Green aphids are the generalists — they turn up on brassicas, roses, cucurbits like courgettes and pumpkins, and most soft new growth in the garden. Because they are less host-specific than blackfly, greenfly pressure tends to move around the garden through the season rather than concentrating on one crop.
Check the undersides of leaves and the growing tips of any plant putting on soft new growth, especially in spring when aphid numbers build fastest. Catching a small colony on brassica seedlings before it spreads is much easier than dealing with a heavily infested, sticky, sooty-mould-covered plant later.
What to Do About Either
Start with the least disruptive option and step up only if needed.
- Squash or hose off small colonies. A firm jet of water knocks most aphids off without harming the plant, and they are slow to climb back.
- Encourage predators rather than spraying on sight. Ladybird larvae — small, dark, and easy to mistake for bird droppings — are voracious aphid eaters, and parasitic wasps are already working if you see mummified aphids. A garden with some flowering plants nearby tends to hold more of both.
- Watch for ants and deal with the ant trail, not just the aphids, if ants are actively farming a colony — otherwise the ants will keep defending it from the predators above.
- Use a soap or oil-based spray for stubborn colonies. Organic pyrethrum-and-oil combination sprays are commonly used against aphids and are gentler on beneficial insects than broader insecticides; cover the underside of leaves as well as the top, since that is where most aphids feed.
- If spraying anything stronger, use only a product currently labelled for the crop and pest in NZ, and follow the withholding period. Aphids are on food crops as often as ornamentals, and label directions are the source of truth for rate, timing and harvest interval.
Repeat any spray after about a week rather than relying on a single application — aphid life cycles are short enough that a colony can rebuild from survivors within days.
Prevention for Next Season
- Avoid heavy nitrogen feeding right before an aphid-prone season; soft, lush new growth is exactly what aphids prefer.
- Keep an eye on broad beans and other legumes from early flowering onward, since blackfly tends to arrive as soon as there is soft tip growth to colonise.
- Don't clear every flowering weed or companion plant from around the vegetable garden — many host the parasitic wasps and hoverflies that keep aphid numbers down between outbreaks.
- Check new transplants and seedlings before planting them out; a small hitchhiking colony on a punnet is an easy way to start the season with a problem already established.
The Bottom Line
Blackfly and greenfly are the same kind of pest wearing different colours, and both share the same underlying risk: they multiply fast enough that a small colony can become a real problem within a couple of weeks. Check regularly, especially on broad beans and soft new growth, act early with water, handpicking or a gentle spray, and let ladybirds and parasitic wasps do as much of the work as you can before reaching for anything stronger.
Growing broad beans, brassicas or other aphid-prone crops this season? Check timing for your area: NZ Temperate Planting Calendar or NZ Cool/Mountain Planting Calendar.