I spent my first few years at war with greenfly and my last fifteen more or less ignoring them, and the plants have never been better. What changed was noticing what happened when I did nothing. One June I gave up on a badly infested row of broad beans, too busy to deal with them, and came back ten days later to find the colonies crawling with ladybird larvae and the plants cropping perfectly well. That was the year I stopped spraying anything, and the greenfly have been someone else’s problem ever since.

Greenfly, Blackfly and What They Actually Are

Greenfly and blackfly are not different pests, they are different aphids, and there are hundreds of species in the UK. What they share is a way of life: they pierce soft plant tissue and drink the sap, breeding at a speed that is genuinely hard to believe. In warm weather females give birth to live young without needing to mate, and those young are themselves pregnant when born, which is how a handful of aphids becomes a solid colony in a week. Greenfly are the pale green ones you find on rose buds and shoot tips. Blackfly, properly the black bean aphid, are the sooty black clusters that smother the growing tips of broad beans in early summer, and they also hit runner beans, dahlias and nasturtiums. Woolly aphids are a different creature again, appearing as white fluffy patches on apple branches and woody stems.

How to Spot the Damage

Look at the softest, newest growth first, because that is where they always start: the tips of shoots, the undersides of young leaves, and flower buds just before they open. You will see dense clusters of small soft-bodied insects, often with winged individuals among them once the colony gets crowded. The signs that go with them are as telling as the aphids themselves. Leaves become sticky with honeydew, the sugary waste they excrete, which then grows a black film of sooty mould. New shoots curl, pucker and distort, and buds may fail to open properly. And you will often notice ants running up and down the stems, because ants farm aphids for that honeydew and will actively protect them from predators, so a stream of ants on a rose is a reliable sign of greenfly above.

What to Do About Them, Honestly

Start with your fingers. Running a colony between finger and thumb takes seconds and kills hundreds, and on roses and beans that is usually the whole job. A sharp jet of water from a hose knocks off most of what remains, and aphids that fall off rarely find their way back up. For a heavy infestation on a tough plant, a very weak solution of plain soap in water sprayed onto the colonies works by breaking down their outer coating, though use it sparingly and never in strong sun, because it can scorch soft leaves and it kills beneficial insects it lands on just as readily. Pinching out the top few inches of broad beans once the lower pods have set removes the blackfly and the tender growth they want in one go, and it is standard practice on any allotment worth the name. What I would not do is reach for a spray as a first resort. Most of what is sold to home gardeners is either ineffective, short-lived, or kills the ladybirds, hoverfly larvae and parasitic wasps that are your actual solution, which is how people end up with a worse infestation a fortnight later than the one they sprayed.

Let the Ladybirds Do It

The single most useful thing to understand about greenfly is the timing. Aphids wake up and breed earlier in the year than the things that eat them, which is why late May and June feel like an invasion. Give it two or three weeks and the predators catch up, at which point the colonies collapse fast. A single ladybird can eat fifty aphids a day, and its larvae, those odd little blue-grey alligator things with orange spots that people often mistake for pests and squash, eat far more. Hoverfly larvae are even better, and lacewings, earwigs, tiny parasitic wasps and blue tits all pile in too. Learn to recognise ladybird larvae and leave them alone, because killing them is the most common own goal in the garden. Planting flowers among the vegetables brings the predators in earlier and keeps them there, which is one of the practical arguments in bee friendly plants. Hoverflies in particular want open, simple flowers such as calendula, poached egg plant and single marigolds.

Greenfly on Roses

Roses get greenfly every single year and it is worth saying plainly that a rose covered in greenfly in June is not in danger. The damage is cosmetic on an established plant, some distorted buds and sticky leaves, and by July it is generally over. Squash what you can reach, hose the rest, and resist spraying, because roses are magnets for ladybirds precisely because they carry aphids, and a sprayed rose loses that population for the season. If the same rose is badly infested year after year, look at how soft the growth is, since heavy nitrogen feeding produces exactly the lush sappy shoots aphids prefer. Ease off the feed and you get fewer greenfly, the same principle that applies to soft growth on vegetables.

Blackfly on Broad Beans

This is the one aphid problem that genuinely needs action, because a heavy blackfly infestation on broad beans really does reduce the crop. The colonies target the soft growing tip, so the fix is to remove it. Once the plants have set four or five trusses of pods at the bottom, pinch out the top four inches or so, which takes away both the blackfly and the growth they are heading for, and pushes the plant into filling the pods it already has. Do it as a matter of routine in early summer rather than waiting for the blackfly to arrive. Autumn-sown beans usually escape the worst of it altogether because they are further ahead by the time the aphids appear, which is one of the quieter arguments for autumn sowing covered in how to grow broad beans. If a few colonies remain lower down the plant after pinching, squash them and leave the rest to the ladybirds.

Keeping Numbers Down Next Year

Prevention here is mostly about not creating the conditions aphids love. Go easy on high-nitrogen feeds, because soft sappy growth is what draws them in, and a plant fed steadily rather than pushed hard is noticeably less troubled. Water consistently, since stressed plants attract more pests, and inconsistent watering causes plenty of other problems besides, as covered in how often to water. Grow flowers among the vegetables to keep predators resident rather than visiting. Check plants weekly in May and June and squash the first small colonies before they become large ones, which is five minutes that saves an hour. And accept a background level of aphids as normal and even useful, because a garden with no aphids at all is a garden with no ladybirds either, and that is a worse position to be in when the next wave arrives.

Frequently Asked Questions

A Sensible Place to Start

Do the cheap things and wait. Squash the colonies you can see, hose down the rest, pinch the tops out of your broad beans once the bottom pods have set, and then leave it alone for a fortnight. Learn what a ladybird larva looks like so you stop killing your own allies. Ease off the nitrogen feed on anything that gets hit year after year, and put a few open flowers in among the vegetables so the predators are already living there when the aphids arrive. That is the entire strategy, it costs nothing, and it works better than anything in a bottle. The year I stopped fighting greenfly was the year I stopped having a greenfly problem.