An Aphid Is Carrying Her Granddaughters.

Pick a leaf with aphids on it and you are not looking at a population. You are looking at one animal and its copies, most of them pregnant, none of them waiting for a mate.

That is the whole of it. Aphids are not unusually fast eaters, they are not unusually hard to kill, and nothing about an individual aphid is impressive. What makes them the pest they are is arithmetic — and the arithmetic is stranger than it looks.

What an aphid is, and how to be sure

Aphids are small, soft, pear-shaped insects that feed by pushing a needle-like mouthpart into a plant’s phloem and drinking the sap. They come in green, black, pink, yellow, grey and white-waxy, they range from about a millimeter to about four, and some have wings and most do not. None of that identifies them, because plenty of other things on a plant are also small and soft and green.

One feature does. University of California’s Pest Notes names it exactly: aphids have “a pair of tubelike structures called cornicles projecting backward out of the hind end.” Nothing else you will find on a houseplant has them. If you can see two short tubes pointing backwards off the rear, it is an aphid, whatever color it is.

They are worth a second look with a hand lens, because the two things people most often mistake for aphids need completely different products. Root mealybugs are covered in the same kind of white wax and have no cornicles. Whitefly nymphs sit flat and still on the underside of a leaf and do not walk. And the tiny things running around on the soil when you water are usually springtails, which jump, eat mold, and are not hurting anything.

Why the numbers behave like that

Most insects lay eggs, the eggs hatch, the young take weeks to mature, and a proportion of the adults are male and do not lay anything. Aphids skip nearly all of that.

Through spring and summer an aphid colony is usually all female and entirely asexual. The adults do not lay eggs — they give birth to live daughters, which are clones. No males are involved, so nothing has to find a mate, and no part of the population is spent producing individuals that cannot reproduce.

Then it goes one step further, and this is the part that is genuinely odd. Because the embryos develop inside the mother rather than in a laid egg, a daughter starts developing her own daughters before she is born. Ogawa and Miura put it in one sentence: “the granddaughters of a female aphid (mother) are already developing within the daughter embryos of the mother.”

The consequence is that a colony has no generational pause. There is no period where this year’s young are too small to breed and the population is flat. University of California gives the clock: “from newborn nymph to reproducing adult in seven to eight days,” and “each adult aphid can produce up to 80 offspring in a matter of a week.”

Seven days, eighty daughters, no mating, and the daughters were already under construction before their mother was born. That is the pest.

Compared with what you would release against it

The awkward part, for anyone buying a predator, is that nothing you can buy runs on that clock. Here is how long one generation takes, for the aphid and for the two predators most commonly sold against it:

The aphid turns a generation over in roughly a third of the time. That single fact is why releasing predators once, against an established colony, does something real and then stops — and why every published release protocol that worked involved releasing more than once.

What the damage actually is

Aphid damage arrives in three separate ways, and people usually notice them in the wrong order.

The virus point is the reason aphid tolerance should be lower than aphid numbers suggest. Everything else on this list is proportional to how many aphids there are. That one is not: it takes one.

Where the winged ones come from

Sooner or later some of the aphids on a plant have wings, and shortly after that there are aphids on a plant across the room. Where the winged ones come from is worth getting right, because the usual explanation is not the one the evidence supports.

The tidy story is that aphids grow wings when the food runs out. The better-supported story is that they grow wings when it gets crowded. A review of aphid wing dimorphism reports that increased density triggers wing formation, that “the proximate mechanism mediating these environmental conditions appears to be increased tactile stimulation between individual aphids” — they can feel how many of them there are — and, pointedly, that “more than half of 38 examined studies in 12 different aphid species did not confirm the hypothesis that poor nutrition is responsible for an increase in winged morph production.”

UC IPM’s summary of what produces them covers both anyway, plus the autumn signal: winged aphids appear “when plants become heavily infested and the aphids crowded or plant quality declines and when day length shortens during the fall.”

Where they came from, and whether they leave

Two things are commonly stated about aphids and the year that are true of some species and not of most.

Host alternation is the exception, not the rule

Some aphids spend winter as eggs on a woody plant, hatch there in spring, then migrate to a completely unrelated herbaceous plant for the summer and fly back in autumn to mate and lay. It is a genuinely strange life cycle and it gets written about a lot, which makes it seem universal.

It is not. A study of aphid life-cycle evolution puts it plainly: “Host alternation is found in about 10% of the 4500 described aphid species, the other 90% of aphids using the same group of host plants throughout their entire life cycle.” So for nine species in ten, the aphids on your plant did not come from a poplar tree. They came from another plant like that one.

Indoors, the year does not happen

The autumn switch to sexual reproduction and overwintering eggs is triggered by shortening days and falling temperature together. Warmth alone cancels it: in one well-studied species the sexual response “disappears above 23°C.” A heated room is above that all winter.

So an indoor aphid population never gets the signal. It stays female, stays asexual, and keeps going at whatever rate the temperature allows, indefinitely. There is no season to wait out and no winter that will do the job for you. What changes with the season outdoors, and what does not change indoors, is its own article.

One thing not to conflate, because it is easy to: only about 3% of aphid species have given up sex altogether. That is a different claim from a normal species running asexually year-round in a warm greenhouse, which is common. The species still has a sexual phase somewhere; your plants simply never provide the conditions for it.

What eats them

Aphids are soft, slow, and sit in the open in dense groups, which makes them the most heavily preyed-upon pest in the catalog. Four commercially available organisms work on them, and they work in genuinely different ways rather than being four brands of the same thing.

Which of those is right depends on whether you are treating a problem you have or preventing one you do not, and on which aphid it is. The four of them side by side, with what each is actually good at is the article for that decision.

They are not the only things eating aphids, and most of the rest arrived on their own. Hoverfly larvae, brown lacewings, several parasitoid wasps nobody sells, and a long list of generalists all take aphids outdoors. A garden that has been left alone for a season usually has more of them than a released population would add.

Why this biology decides how you treat them

Everything above collapses into four consequences, and they are the reason aphid advice differs from advice about most other pests.

1. One treatment is a dent, not a finish. The colony replaces its breeding adults in about a week, so anything applied once — predator or spray — gets measured against a population that is already rebuilding. Repeat at intervals shorter than the aphid’s generation, not longer.
2. Early is worth far more than thorough. A colony that has not yet got crowded has not yet made winged aphids, so the problem is still on one plant. After that it is a room problem, and a room problem costs several times as much to fix.
3. Something that breeds beats something that eats, on anything that has been going a while. A predator you release is a fixed quantity. A parasitoid that reproduces inside the colony scales with the colony, which is the only thing that keeps pace with an aphid’s arithmetic.
4. Deal with the ants first. Where ants are farming a colony, they will defend it, and a release into that can fail on those grounds alone.

Where to go next

This article is the biology. The decisions are in the others:

• Which aphid do I have — the species decides which wasp works, and one of them is not controlled by most of what is sold for aphids.
• Ladybugs, lacewings or wasps — the four-way comparison, and what each one is honestly for.
• How many do I actually need — the published rates, which are much higher than the ones usually printed.
• The ants are farming your aphids — why this is a precondition rather than a side issue.
• Aphid season — why the same release works in May and disappoints in August, and why indoors there is no season at all.
• Root aphids — none of the above applies, and the evidence for what does is thinner than the labels suggest.

Common questions

How can I be sure it is aphids and not something else?

Look for cornicles — a pair of short tubes projecting backward from the rear end. UC IPM describes them as “a pair of tubelike structures called cornicles projecting backward out of the hind end,” and nothing else you are likely to find on a houseplant has them. Color, size and whether they have wings all vary and none of them identifies an aphid.

Where did they come from? My plants never go outside.

Usually on a new plant, on cut flowers, or on someone’s clothes. Winged aphids also come in through an open window. Once indoors they do not need anything else to arrive — a single female can start a colony on her own, because no mate is required.

Why do they multiply so fast?

Three reasons at once. They give birth to live young rather than laying eggs; they do it without mating, so the whole population is reproductive; and the embryos start developing before birth, so there is no lag while the young grow up. UC IPM puts the clock at seven to eight days from newborn to reproducing adult, with up to eighty offspring from one adult in a week.

Is the sticky stuff on my floor coming from the aphids?

Yes. It is honeydew — surplus sugar the aphids excrete, because they have to process a lot of sap to get enough protein out of it. Left alone it commonly grows a black sooty mold, which does not infect the plant but does shade the leaf it grows on. Both wash off.

Will the aphids die off in winter?

Outdoors, mostly. Indoors, no. The switch to overwintering eggs is triggered by short days and falling temperature together, and warmth alone cancels it — in one well-studied species the response “disappears above 23°C.” A heated room stays above that, so an indoor population just keeps going.

Do I need to worry about a few aphids?

A few aphids on a robust plant is not an emergency. Two things make it worth acting early anyway: crowding is what produces the winged aphids that move to your other plants, and aphids transmit plant viruses in probes that last seconds, so virus transmission is not proportional to how many of them there are.

Do ladybugs get rid of aphids?

They reduce them, at rates much higher than most labels suggest, and only for as long as they stay — which is usually a day or two. Against an established colony the published protocols that worked all involved repeat releases. Against a problem spreading across several plants, a parasitoid that reproduces inside the colony does something a released predator cannot.