Aphids suck sap from young shoots and the undersides of leaves, and few pests multiply as fast. In the green peach aphid (Myzus persicae), common on houseplants, a whole generation takes 10-12 days. Here you will find out how to spot an infestation, why ants make it harder to clear up, and what works indoors and what works outdoors.
How to recognise an infestation
The aphids are a couple of millimetres long and sit most densely on the top 5-10 centimetres of a shoot, in the buds and on the undersides of the leaves. Colour tells you nothing about the species, they come in green, black, pink and grey.
White cast skins: aphids moult several times before they are adult, and the dry shells collect on the leaves below the colony.
Sticky surfaces: a sticky table or floor under a houseplant comes from the aphids above it.
Curled leaves: aphids introduce substances into the leaf as they feed, and the leaf grows crooked.
Curled shoot tips are not always down to aphids, high nutrient levels give the same symptom on tomato. More pests are covered in the article on pests and diseases.
Why the population explodes in a couple of weeks
Through the summer half of the year the colony consists almost entirely of females that give birth to live young without having mated. The young one already carries the makings of the next generation when she is born, so three generations are present at the same time.
For the green peach aphid the figures are concrete: a female has her first young at an average age of 10.8 days, gives birth over about 14.8 days and lives around 23 days. In a mild climate the species gets through more than 20 generations in a year, and for the cabbage aphid up to 41 generations in one season have been recorded.
Winged individuals are produced when the colony gets crowded, and also when it is attacked by ladybirds, green lacewings or parasitic wasps. An infestation you disturb without clearing up therefore spreads on to the next plant.
The aphid year, month by month
Most species overwinter as eggs outdoors, while the aphids on houseplants indoors breed all year round. Around 10 percent of the species change host plant during the year, from a woody winter host to herbaceous summer hosts. The green peach aphid overwinters on Prunus: peach, apricot and plum. The months below follow a temperate season in most of the UK and northern Europe, and in milder areas everything runs a few weeks earlier.
| Period | What happens | What you do |
|---|---|---|
| October to March | The eggs sit on woody hosts. Indoors the breeding carries on without a break. | Check the houseplants every second week. |
| April to May | The eggs hatch and the first females start giving birth. | Pinch out the first colonies by hand. |
| May to June | Winged females move over to vegetables and summer flowers. | Put your measures in place now. |
| June to August | The population is at its largest and the predators catch up. | Leave light infestations outdoors alone. |
| September to October | The aphids return to the winter host. Short days send the aphid midge into rest. | Do not buy beneficial insects now. |
Ants are part of the problem
Ants milk the aphids for honeydew by stroking them with their antennae, and in return they drive off the aphids' enemies. A colony guarded by ants is left in peace by ladybirds and parasitic wasps.
If you see ants running up and down a stem, do not count on the beneficial insects to settle the infestation. Cut the route off: stand the pot so that no leaves reach the floor, and put a grease band around the trunk of trees and shrubs. We put the band above the lowest branches, because a branch hanging down to the ground otherwise becomes a shortcut past it. Check the band every 14 days, it clogs up with dust.
Honeydew, sooty mould and viruses
Aphids take in more sugar than they need when they feed on phloem sap, and the surplus comes out as honeydew. Sooty mould, a black fungal coating, grows on that sticky film. The sooty mould does not attack the leaf, but it shuts out the light so the leaf works less well and in the end withers.
The second kind of damage does not show at all. The green peach aphid transmits more than 100 plant viruses and is the most versatile virus spreader among the aphids. Viruses cannot be treated away, and that is the reason to take even a small infestation seriously in the kitchen garden. The sugar stays where it is until you wipe it off.
Houseplant indoors or a bed outdoors
The same pest, two different situations. Indoors the natural enemies are missing altogether and the infestation grows until you do something. Outdoors they work for free.
| Question | Houseplant indoors | Bed and kitchen garden outdoors |
|---|---|---|
| Natural enemies | None. Nothing slows the colony down. | Ladybirds, green lacewings, hoverfly larvae, parasitic wasps. |
| Bought beneficials | Work in a closed room or a greenhouse. | Often fly off. Attract the wild ones instead. |
| Rinsing off | Shower in lukewarm water every 3rd to 4th day. | Rinse in the morning so the leaves dry. |
| Timing | Act straight away. No season breaks it off. | At the first colonies in May. |
| Soft soap solution | Cover the compost and rinse off afterwards. | Never in sun or above 32 degrees. |
Beneficial insects: which ones work and when to release them
Timing decides it. The beneficial insects need weeks while the aphids need days, so they go out at the first colonies and not when the plant is covered.
Aphid parasitoid wasp (Aphidius): a wasp under 3.2 mm that lays an egg inside the aphid. The aphid swells and turns into a yellow-brown mummy that the finished wasp chews its way out of. If you see mummies the work is under way and you should not spray.
Aphid midge (Aphidoletes aphidimyza): the female lays about 70 eggs, and each larva needs at least 7 aphids but can eat up to 80. It works best at 20-26 degrees and high humidity. Short days at the end of summer send it into rest, so autumn is the wrong time to buy it without extra lighting.
Green lacewing (Chrysoperla carnea): the larva eats between 100 and 600 aphids during its 2-3 weeks as a larva.
Outdoors it pays off more to attract enemies than to buy them. Hoverfly larvae eat aphids, while the adults live on nectar and pollen and have short mouthparts that suit open shallow flowers in white and yellow. Pot marigold is singled out for hoverflies, and you get the same flowers if you let some of the dill and coriander run to flower. More on sowing is in the article on growing pot marigold.
Five mistakes that make the infestation worse
Spraying something broad acting: a product that takes the aphids also takes the ladybirds, the wasps and the aphid midge larvae. The aphids have a generation in 10-12 days, a ladybird needs 2-3 weeks from egg to adult. The aphids win the race back.
Treating once only: soft soap solution acts on contact alone and has no residual effect against aphids that arrive after the spray has dried. Count on repeating at 7-14 day intervals.
Spraying in sun and heat: the risk of leaf damage rises above 32 degrees and on plants that are drought stressed, newly planted or carrying soft new shoots. Spray in the evening and test on one leaf first.
Ignoring the ants: as long as the ant trail runs up the stem the aphids' enemies are kept away, however much you pick off by hand.
Releasing the beneficials too late: once the whole top is covered nobody catches up. Order at the first colonies.
What you are allowed to use at home
Which products a home gardener may use is decided country by country, and the rules are not the same across Europe. A plant protection product has to be authorised in the country where you use it before it may be sold or applied there, and the label states whether the product is cleared for amateur or home garden use. In Great Britain that authorisation sits with HSE, and in EU countries the national authority decides under Regulation (EC) No 1107/2009.
We do not point to individual products. Authorisations change, and the label states which plants and which uses are covered. Start with what costs nothing and spares the beneficial insects: pinch out the first colonies, cut off infested shoots, rinse the plant down and keep an eye on the ants. The same order applies against thrips indoors.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-09-25