Aphids and Whitefly: How to Spot Them and Get Rid of Them Indoors

Aphids and whitefly are the two sap-sucking pests most likely to turn up on almost anything you grow indoors, from tomatoes and peppers to herbs, salad and houseplants. Both breed fast, both hide under the leaves, and both leave sticky honeydew behind them. The good news is that one plan deals with both: spot them early with yellow traps, spray the undersides of every leaf with a contact spray, repeat before the next generation hatches, and fix the conditions that let them in.

Aphids or whitefly? How to tell them apart

Aphids (often called greenfly or blackfly) are soft, pear-shaped insects 1-4mm long. Depending on the species they are green, black, grey or pink, and most have two small tubes sticking out of the back of the body. They do not run or fly off when disturbed; they sit in clusters on soft stems, growing tips and the underside of young leaves, often with pale cast skins scattered around them.

Whitefly are tiny, 1-2mm insects with white, powdery wings held flat like a tent. The giveaway is that they lift off in a small white cloud when you brush the plant, then settle again within seconds. Turn a leaf over and you will also find their young: flat, pale, oval scales that look more like specks of wax than insects, because once they settle they do not move again until they emerge as adults.

• Tap test: hold white paper under a stem and tap it. Aphids drop and crawl slowly; whitefly fly straight back up.
• Not spider mites or thrips: mites leave fine pale speckling and webbing, thrips leave silvery patches with black specks. If that is what you see, use our spider mite rescue plan or our thrips guide instead.

The signs to look for

• Sticky, shiny leaves. Both pests excrete honeydew, a sugary waste. Leaves and surfaces below a colony turn shiny and tacky to the touch.
• Black sooty mould growing on the honeydew. It does not infect the leaf, but it blocks light and is a sure sign of a sap-sucker above.
• Curled, puckered or stunted new growth, typical of aphids feeding in the growing tips.
• Yellowing, weak leaves and slower growth where whitefly numbers are high.
• White cast skins on and under leaves, left behind as aphids moult.

A few insects are a warning. Honeydew on the leaves, curled tips or a sticky trap catching more each week means the population is established. Both pests can also carry plant viruses from plant to plant, which is another reason to act early rather than wait.

Why their numbers climb so fast

Aphids indoors are almost all females that give birth to live young without mating. Those young can start breeding in about a week when it is warm, so a few aphids on one plant become a colony within a fortnight. When a colony gets crowded or the plant declines, they produce winged adults that fly off to start new colonies nearby.

Whitefly lay their eggs under the leaf. The young hatch, crawl a short way, then settle as flat scales and feed in one spot until they emerge as adults, roughly three to four weeks after the egg was laid in a warm room. For most of that time they are fixed under the leaf, and the later scale stage is well protected against many sprays. Generations overlap, so adults keep appearing for weeks after any single treatment.

That tells you what to do: reach the underside of every leaf, accept that one spray only clears what it wets, and repeat at short intervals until the next generation has nowhere to come from.

Step 1: monitor, so you know what you are dealing with

Yellow sticky traps are the single most useful tool for both pests. Adult whitefly and winged aphids are strongly drawn to yellow, so a card shows you early whether they are arriving and, counted weekly, whether your treatment is working.

• Hang cards just above the plants, about one per square metre, plus one near the intake and the door.
• Count on the same day each week and write the number down. Zoom in with your phone to tell whitefly from fungus gnats and thrips.
• Check leaves by hand too. Turn over leaves from the top, middle and bottom of a few plants once a week; aphid clusters on growing tips will not show on a trap until they grow wings.
• Watch the room. Both pests breed faster in warm, still conditions. A min-max thermo-hygrometer shows you what the room does while you are not there.

Step 2: spray properly, especially underneath

For a light aphid problem, start simple: a firm spray of plain water knocks aphids off the plant, and small clusters can be squashed between finger and thumb. For anything more, or for any whitefly, use a contact spray and make coverage your priority, because a contact product only works on the insects it actually wets.

1. Use a sprayer that mists finely and can be angled upwards. A pump sprayer reaches leaf undersides far better than a small trigger bottle.
2. Spray the underside of every leaf first, then the tops, the stems and the growing tips, until the foliage is evenly wet but not dripping.
3. Spray with the lights off or low, never onto leaves under hot running lamps, and keep the air moving so the canopy dries rather than staying wet for hours.
4. Treat every plant in the area, not just the ones you can see insects on. Whitefly in particular move freely between plants.
5. Wear gloves, mix only what you need, and follow the label for the dose and the interval.

Which product

Physical contact sprays are the best place to start because pests cannot build up resistance to them. SB Plant Invigorator works on contact against whitefly, aphids and several other sap-suckers, is mixed at 10ml per litre, is sprayed weekly over both leaf surfaces and has no harvest interval. Maven Skram Plant Wash is a non-toxic contact wash mixed at 15ml per litre and sprayed every 3-10 days; its maker states it is safe on edible crops and ornamentals.

For a heavy or mixed infestation, Guard'n'Aid PestOFF Plus is a natural pyrethrum and rapeseed oil contact insecticide that covers whitefly, aphids and a long list of other pests, and its maker says it acts on adults, larvae and eggs. It is a registered insecticide, so use it only on the plants and situations its label lists. Neem oil is better used as a routine between outbreaks than as a rescue.

Step 3: repeat on time

The first spray clears what it wets. The repeat sprays are what actually win, because they catch the aphids born since and the whitefly that emerge from eggs and scales the first spray could not touch.

• Spray every five to seven days, or at the interval on your product's label, for at least three to four weeks for whitefly. Aphids usually clear faster, but keep going until the tips stay clean.
• Remove the worst leaves. Older lower leaves carpeted with whitefly scales are a nursery for the next generation. Bag them up and take them out of the room.
• Keep counting your traps. The count should fall week on week. If it does not, check your coverage of the leaf undersides before you blame the product.
• Rotate products with different modes of action if you use insecticides, and never exceed the number of treatments a label allows.

Step 4: fix what let them in

Aphids love soft, sappy growth, and the most common cause of soft growth indoors is too much nitrogen. If your plants are dark green, lush and floppy, ease back on the feed; our nutrient burn guide shows how to read the signs of overfeeding. Still, warm air helps both pests settle and breed, so make sure your air circulation moves every part of the canopy and that the room is not running hot; our humidity guide covers the targets.

Once the outbreak is under control, a weekly deterrent spray makes the leaves a less attractive place to settle. BioBizz Leaf Coat dries to a thin, breathable film that repels insects, Aptus Dislike is an essential-oil repellent used weekly at 4-8ml per litre, and Element 115 is an organic concentrate whose targets include aphids and whitefly.

Commercial glasshouses also release beneficial insects, such as parasitic wasps against whitefly and ladybirds or lacewings against aphids. They work well in a stable room but are sensitive to many sprays, so choose one approach or the other rather than mixing them blindly.

A four-week plan that covers the whole cycle

1. Day 1: confirm which pest it is with a close look and a tap test. Hang yellow traps and note the count. Rinse or squash aphid clusters and remove the worst whitefly-covered leaves.
2. Days 1-2: spray every plant thoroughly, undersides first, with a contact spray.
3. Day 7: second spray. Check the growing tips for new aphid clusters and recount the traps.
4. Days 14 and 21: third and fourth sprays. For whitefly, this is the stretch where adults from the old scales emerge, so do not stop early.
5. Day 28 onward: if traps are near zero, switch to a weekly deterrent spray and keep one trap up permanently as an early warning.

Keep them out next time

• Quarantine new plants for a couple of weeks with a yellow trap beside them, and check the leaf undersides before they join the rest. Bought-in plants are the most common way both pests arrive.
• Filter the intake. Winged aphids and whitefly can be drawn straight in through an open intake. A filter over the inlet stops most of them.
• Visit the grow room before the garden. Whitefly and aphids hitch a ride on clothing from outdoor plants and glasshouses, especially in summer.
• Clean properly between crops. Leftover plants, leaves and debris carry the next generation into the next crop. Clear everything out, clean and sterilise, and treat the empty, sealed room with a fumer after a bad outbreak; our guide to cleaning between crops shows the order.

For the bigger picture on keeping every pest out, read our grow room pest prevention guide.

Six mistakes that keep aphids and whitefly coming back

1. Spraying once. New aphids are born daily and whitefly scales survive. Plan four sprays over four weeks.
2. Only wetting the tops of the leaves. Both pests live underneath.
3. Treating only the plants you can see insects on. Treat everything in the area.
4. Leaving infested lower leaves on the plant. They are where the next generation of whitefly is coming from.
5. Overfeeding nitrogen. Soft, lush growth is exactly what aphids look for.
6. Using a product on a crop it is not authorised for. Read the label, especially for anything you will eat.

Shop plant health

Quick answers

How do I tell aphids from whitefly?

Aphids are soft, pear-shaped insects 1-4mm long, usually green, black or pink, with two small tubes at the tail; they sit still in clusters on stems and under leaves. Whitefly are tiny white moth-like insects 1-2mm long that fly up in a cloud when you brush the plant, with flat, oval young fixed to the underside of the leaf.

What is the sticky stuff on my leaves?

Honeydew: the sugary waste that aphids and whitefly excrete as they feed on sap. It makes leaves shiny and sticky, and a black sooty mould often grows on it, which cuts the light the leaf receives. Find the pest above the sticky patch and the honeydew stops.

Why do aphids appear so suddenly?

Indoors, female aphids give birth to live young without mating, and those young can start breeding themselves in about a week when it is warm. A handful of aphids can become a colony in a fortnight, and crowded colonies produce winged adults that fly to the next plant.

Why does whitefly keep coming back after I spray?

Because eggs and the flat, scale-like young are fixed to the underside of the leaf, and the later scale stage is protected from many sprays. Adults keep emerging for weeks after one spray. Repeat a contact spray every five to seven days for three to four weeks, and keep yellow traps up to catch the adults.

What colour sticky trap catches whitefly and aphids?

Yellow. Adult whitefly and winged aphids are both strongly drawn to it. Hang cards just above the plants, about one per square metre, and count the catch weekly so you can see whether numbers are rising or falling.

Can I just wash aphids off?

For a small outbreak, yes. A firm spray of water knocks aphids off the plant, and you can squash small clusters with your fingers. It will not stop a big population or touch whitefly scales, so follow up with a contact spray and repeat.

Can I use these sprays on vegetables and herbs?

Only if the label allows it. Some contact washes are labelled as safe on edible crops; insecticides are authorised for specific crops and situations. Always check the label for the crops it covers, the dose and the interval before you spray anything you will eat.

Is this the same for tomatoes, peppers and houseplants?

Yes. Aphids and whitefly are standard pests of glasshouse tomatoes, peppers, cucumbers, aubergines, herbs, salad crops and houseplants. Commercial growers manage them with traps, hygiene, contact sprays and beneficial insects, exactly as described here.