Why Plants Die
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Aphids: crowded sap-feeding insects on shoot tips and buds

Check your plant with the questions

Aphids pierce the soft tissue of a plant with slender mouthparts and drink its sap. Nearly every plant has at least one kind that feeds on it, houseplants included. In a garden they come and go with the seasons; indoors a colony can keep breeding all year. The crowds look alarming, yet an established plant seldom dies from aphids; they cost it vigor and looks, and now and then a virus.

Spotting a colony

Start with the newest, most tender growth. Colonies settle on the growing tips, on flower buds and on young leaves, mostly underneath, packed close and visible to the naked eye. Each insect is soft-bodied and shaped like a pear, its legs and antennae long, and it measures a quarter inch at most. Depending on species and diet, they can be green, black, yellow, pink, brown, red, white or mottled. The green and black ones are what British gardeners call greenfly and blackfly.

A close look settles any doubt. Most aphids have two small tubes, called cornicles, sticking backward from the tail end, and no other insect carries them. Aphids also stay put when nudged, whereas leafhoppers and plant bugs dart off.

Often the clues show up before the insects do:

  • Sticky leaves. Aphids pass sugary waste called honeydew, which glazes the leaves beneath a colony.
  • Black film. Sooty mold, a fungus, grows on that honeydew and turns it dark.
  • White flakes. Aphids molt as they grow, and their pale cast skins collect on the upper faces of leaves.
  • Ants. Ants climb plants to collect honeydew and in return drive off the insects that eat aphids.

Most of a colony has no wings. Winged individuals develop when the group gets crowded or the plant declines as food, and those fly off to start colonies on other plants.

Aphid or something else?

Some species coat themselves in white or gray wax and look fluffy. Those woolly aphids get mistaken for mealybugs, scale or whitefly. If the wax hides the body, compare against those pages before picking a treatment.

The damage

Small or moderate numbers do little. Large colonies drain enough sap to yellow leaves, stunt new shoots and weaken the plant. Certain species release a toxin while feeding, and the leaves curl and grow twisted. The sooty mold is unsightly but harmless in itself, and a damp cloth takes it off.

The more serious risk is disease. Some aphids carry plant viruses from one plant to the next, and ornamentals on the RHS list of affected plants include dahlias, lilies, tulips and pelargoniums (the plants sold in the US as geraniums). An infected plant shows mottled, yellowed or curled leaves and stunted growth. Controlling the aphids does little to stop this, because a feed lasting only minutes can pass the virus on, even from a handful of insects. The RHS advises destroying an infected plant so the virus spreads no further.

Why a few become hundreds

Through most of the year a colony consists only of females, which bear live young without mating. The young shed their skins about four times and are adults, with no pupal stage in between. In warm conditions that takes seven or eight days, and a single adult can bear as many as 80 young in one week. Rich feeding helps them: heavy nitrogen fertilizer speeds their breeding, so a slow-release fertilizer used sparingly suits a plant that has had aphids.

Clearing them off a houseplant

The RHS suggests tolerating a few aphids, and UC IPM puts nonchemical methods first. When a plant does need help, go from the gentlest step upward:

  1. Prune. If the colony sits on a few shoots or curled leaves, remove those and bin them. Curled leaves deserve removal anyway, since they shield the aphids inside from sprays.
  2. Rinse. A forceful jet of water knocks aphids from a sturdy plant, and most of them cannot climb back.
  3. Use a smothering spray. Insecticidal soap and oils (horticultural, neem or canola) kill only the aphids they wet, so soak both faces of each leaf. Nothing lingers afterward, and aphids born later need a second round. Leave out plants that are dry and wilting, avoid hot spells, and because some plants are damaged by these products, try a few leaves first and wait several days.
  4. Deal with ants. Ants tending a colony protect it from its predators, so controlling them is part of controlling the aphids.
  5. Bring in predators. Lacewing larvae, hoverfly larvae and small parasitoid wasps can be bought for greenhouse and indoor plants.

Broad-spectrum insecticides also kill the insects that feed on aphids, and repeated use can breed resistance. The RHS does not recommend pesticides against aphids at all.

Checking your progress

Examine the plant at least two times weekly while it is growing fast, turning leaves over as you go. Two things mean the colony is failing on its own. Parasitized aphids swell into tan or golden-brown husks called mummies; once those appear, numbers tend to drop steeply over the next week or two. Aphids killed by fungal disease look reddish or brown, shriveled and fuzzy. Since a generation matures so fast, keep checking until fresh shoots stay clear and no new stickiness appears. Look over the shoot tips of anything newly bought before it goes on the shelf beside your other plants.

Plants whose sources name aphids

This list is generated automatically from the site's records: a plant appears on it when its own entry, from RHS or NC State Extension, includes aphids.

Sticky leaves with no aphids on them point to some other honeydew-making insect; the scale, mealybugs and whitefly pages cover three of them. For anything else a plant is showing, start the diagnosis.

Answer a few questions about your plant

Sources

This page is written for this site from the sources above; it quotes none of them. It has not seen your plant.