The Flea Life Cycle, and Why It Decides Everything

Egg

An adult flea feeds on the animal and lays its eggs in its coat. The eggs are pearly, smooth and about half a millimetre, and crucially they do not stick to anything. Within hours they roll off wherever the animal happens to be.

So the eggs end up as a map of the animal's day: its bed, the sofa it is allowed on, the warm spot by the radiator, the stair carpet, the bedroom if it sleeps up there, the boot of the car if it travels. A well-fed female keeps this up steadily, day after day, which is how a small problem becomes a large one in a few weeks.

Photographs on this page are for illustration purposes only.

Larva

The eggs hatch into small pale larvae, and the larvae want nothing to do with the pet at all.

They avoid light. They wriggle down to the base of the carpet pile, in under the gripper rail at the skirting, into the gap between floorboards, under a heavy sofa that has not been moved in years, and into the dust beneath a fitted wardrobe. What they eat there is flea dirt — the dried blood that adult fleas pass and that falls off the animal alongside the eggs.

That is the part people find hard to believe and it is the most useful thing to know. The larvae are not on your pet and they are not sitting on the surface of the carpet. They are down in the dark at the edges, which is exactly where a treatment has to go, and deeper than a vacuum reliably reaches.

Pupa — the cocoon, and the whole problem

When the larva is ready it spins a silk cocoon. The silk is sticky, so it immediately picks up carpet fibres, dust and grit until it is camouflaged and effectively part of the floor.

Two things about that cocoon govern how every flea job goes.

First, it is far more resistant to insecticide than any other stage. No treatment available reliably kills a flea inside its cocoon. That is not a shortcoming of one product; it is the stage.

Second, the adult inside does not come out on a timer. It waits for evidence that something worth biting has arrived: vibration through the floor, warmth, and the carbon dioxide of something breathing nearby. With none of those, it can sit there for weeks, and in a cool empty room for considerably longer.

That is why the first person into a house that has stood empty gets swarmed at the ankles, and why fleas appear a few days after a treatment that seemed to have worked.

Adult, and what it means for the treatment

The adult emerges, finds a host, feeds, and starts laying. In a warm house the whole circuit can turn over in a few weeks; in a cold one it stretches out considerably.

Put the four stages together and the arithmetic is uncomfortable: the overwhelming majority of the fleas in a house at any moment are eggs, larvae and cocoons in the floor, not adults on the animal. Treat the animal alone and you are dealing with the visible minority while the supply continues.

Which gives the three rules the rest of these pages keep coming back to. Treat the property as well as the pet. Expect to see fleas for a while afterwards as the cocoons empty. And keep vacuuming, because the vibration brings them out early, into a treatment that is waiting for them.

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Other Flea Questions

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