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.

This article contains affiliate links. If you buy through one of my recommended links I may earn a small commission at no extra cost to you. I only recommend products I genuinely think are worth buying.

The first year I grew brassicas properly I did everything right except the one thing that mattered. Beautiful cabbages, well fed, well watered, and I stood back rather proud of them in late July. Ten days later I lifted a leaf and the underside was crawling. Not a few caterpillars, hundreds, and the plants were more hole than leaf. I had watched the pretty white butterflies flitting about the plot all month and thought nothing of it. That is the whole lesson of this article in one story: the butterfly you ignore in June is the ruined crop you find in August.

Which Butterfly, Which Caterpillar

Two species do nearly all the damage, and it helps to know them apart because they behave differently. The large white is the bigger butterfly, with bold black wing tips, and it lays clusters of yellow, skittle-shaped eggs on the undersides of leaves. Its caterpillars are yellow and grey-green with black blotches, hairy, and they feed in gangs out in the open, often stripping the outer leaves first. The small white is slightly smaller with less black on the wings, and it lays single eggs rather than clusters. Its caterpillars are velvety green, well camouflaged against the leaf, and they burrow into the heart of a cabbage where you cannot see them, which makes them the sneakier of the two. There is also the cabbage moth, whose brown caterpillars do the same job at night. In practice you treat all of them the same way, but the large white is the one you usually spot first because it is so much more obvious.

How to Tell Caterpillars Are the Problem

The damage is distinctive once you know it. You will see ragged holes eaten through the leaves, often starting at the edges and working in, and on a bad infestation only the tough leaf veins left behind, giving that skeletal look. Look for the caterpillars themselves on the undersides of leaves and tucked into the growing heart, and for scatterings of dark green or brown frass, which is a polite word for caterpillar droppings, on the lower leaves and around the plant. Above all, check the leaf undersides for the eggs before they even hatch: the large white lays neat clusters of upright yellow eggs, and finding and rubbing those off is ten seconds of work that saves you a hundred caterpillars later. If the holes are accompanied by silvery slime trails instead of frass, you are dealing with slugs and snails rather than caterpillars, which is a different problem with a different answer.

The Only Thing That Really Works: Netting Them Out

Prevention beats cure so completely with this pest that it is worth saying plainly: the single most effective thing you can do is stop the butterflies ever reaching the plants. A physical barrier of fine insect mesh, put over the brassicas from May and kept on through summer, means no eggs, no caterpillars, and no work beyond checking it now and then. Ordinary butterfly netting with a mesh around 7mm is enough to keep both cabbage whites out, and it is the one piece of kit I would not grow brassicas without after that first disastrous year. The two things that matter in using it: hold it clear of the foliage on hoops or canes, because a butterfly will happily lay through mesh that is resting on a leaf, and tuck the edges right down to the soil or weigh them with stones, because they will find any gap. A tunnel of netting over a bed of cabbages is not the prettiest thing on the plot, but it is the difference between whole leaves and lace.

What to Do If They Are Already Here

If the caterpillars have already arrived, and in a British summer they will find unprotected plants within weeks, the honest answer is hands and vigilance rather than a magic bottle. Go over the plants at least twice a week, more in warm weather, and check the undersides of leaves and deep into the hearts. Rub off any egg clusters, and pick off and squash or drop into soapy water every caterpillar you find. It is not glamorous but it genuinely works if you are consistent, because you are breaking the cycle before the next generation lays. On a heavy infestation, a strong jet of water knocks many off, and standing the outer leaves back to expose the caterpillars to birds helps too. The biological control Bacillus thuringiensis, a naturally occurring bacterium sold for caterpillar control, is the one genuinely effective treatment beyond hand-picking, though availability to UK amateurs varies, so check what is currently sold before relying on it. What does not work well is most general bug sprays, which struggle to reach caterpillars tucked into a cabbage heart and which also kill the ladybirds and hoverflies you want to keep.

Working With Nature Instead of Against It

You do not have to wage total war. Cabbage white caterpillars have plenty of natural enemies, and a garden that encourages them suffers less. Birds, especially in spring, eat huge numbers, so leaving some access rather than netting absolutely everything has a place. A tiny parasitic wasp, Cotesia glomerata, lays its eggs inside large white caterpillars, and those sickly caterpillars surrounded by little yellow cocoons are the wasp doing your job for you, so leave them be. Growing some flowers among the veg to bring in hoverflies and other predators helps the whole plot, which is one of the arguments in my piece on plants for pollinators. Some gardeners plant nasturtiums as a sacrificial crop that the whites lay on in preference, though in my experience that just breeds more butterflies unless you then remove the caterpillars, so treat it as a decoy you still have to police, not a fix.

Keeping Brassicas Healthy Enough to Cope

A strong plant shrugs off light damage that would set back a weak one, so good growing is part of pest control. Firm planting, steady moisture and sensible feeding all help, and the brassica cluster on this site covers each in turn: growing brassicas for the overview, feeding without soft growth, since soft sappy growth from too much nitrogen is exactly what pests love, and watering brassicas properly. Net the plants, keep them growing steadily, check them twice a week through summer, and you will eat the cabbages instead of watching the caterpillars eat them. It really does come down to those few habits, and the netting is the one that does most of the work.

Frequently Asked Questions

A Sensible Place to Start

If you grow brassicas, buy the netting before you need it, not after. Put it over the plants in May on hoops that hold it clear of the leaves, tuck the edges down to the soil, and you have solved the problem before it starts. If the caterpillars are already among your cabbages this summer, do not despair and do not bother with sprays: get into a rhythm of checking the leaf undersides twice a week, rubbing off the yellow eggs and picking off the caterpillars, and you will save the crop. It is ten minutes that feels like a chore and pays you back in whole, unlaced cabbages. I learned that the hard way, standing over a bed of skeletons one August, and I have never grown a brassica uncovered since. Getting the netting on early, right from when you plant your brassicas out, is what keeps them clean.