How Long Do Flies Live?

The question has a better answer than the number

People search for this when flies will not go away, and the honest response is that the lifespan is the least useful figure in the whole business.

What matters is the replacement rate. UC IPM’s pest note on flies records that under optimal summer conditions a house fly completes development from egg to adult in as little as seven days, and that a female lays several batches of roughly 100 to 150 eggs. Set those two numbers beside each other and the arithmetic is unforgiving: anything breeding on your property is producing new adults faster than any amount of swatting removes them.

Which is why a fly problem that has lasted more than a fortnight is not a fly problem. It is a source problem, and the flies are the symptom.

House flies, and the seven-day clock

The adult stage is a few weeks. The part worth your attention is the week before it.

House flies lay in warm, moist organic material — UC IPM names manure, garbage, lawn clippings and decaying produce, along with soil contaminated with any of those. In this county that is a bin that does not get rinsed, grass clippings heaped against a fence, a dog run, livestock bedding, or a dead rodent in a wall void. In food premises it is almost always a drain, a floor junction or something under a fixed unit.

Twenty feet is the rule of thumb we use on a job. Adult flies concentrate near the thing that produced them, so a persistent cluster of flies at one window or one corner of a kitchen is pointing at something within a short distance of itself. Finding that is the whole job; the rest is cleaning.

Cluster flies live much longer, and that is the whole difference

A cluster fly behaves nothing like a house fly, and in Whatcom County it is the one most people are actually asking about when they call in October.

These do not breed in buildings at all. The larvae develop inside earthworms in the soil outdoors, which is why no amount of cleaning changes anything. What happens in autumn is that the adults look for somewhere to overwinter, find the warm south and west faces of a building, and work their way into attics, wall voids and the gaps around upstairs window frames.

Then they stay alive all winter, dormant, and wake up on any sunny day — which is why people find dozens of slow, clumsy flies at the same upstairs window in November, clear them out, and find more in January. They were never gone. The situation in detail, including why spraying the window achieves nothing, is on cluster flies in the upstairs windows, and the species itself is on cluster fly.

The practical upshot: a long-lived fly in a wall is a sealing job in August, not a treatment in November.

Fruit flies and drain flies: short cycle, small source

The reason fruit flies feel immortal is that the source is usually smaller than people are looking for.

A film of fermenting residue in the bend of a drain, the underside of a recycling bin lid, a splash behind the bin, one potato at the back of a cupboard or the drip tray under a fridge is enough to support a population indefinitely. The adults are not long-lived; they are simply being replaced continuously from a square inch of something nobody thinks of as food.

Drain flies — the small furry gray ones that sit flat on bathroom walls — are the same story in a gelatinous film inside a drain. Bleach does not remove that film, which is the single most common reason a drain fly problem survives being treated, and it is covered in full on fly and gnat control.

Fungus gnats: the adults are not the problem

Short-lived adults, a longer larval stage, and a breeding site sitting in your living room in a decorative pot.

Fungus gnat larvae live in the top inch or so of permanently damp potting compost, feeding on fungus and organic matter. New adults emerge continuously as long as that top inch stays wet, so killing the ones flying around the lamp changes the picture for a day. Letting the surface dry properly between waterings changes it permanently. That one is on fungus gnats in houseplants.

What this means if you have flies right now

Work out which of four situations you are in, because the answer is completely different in each.

A sudden flush of large flies indoors, out of season. Suspect something dead — a rodent in a wall or under a floor, most often in late winter after somebody put bait out. These arrive together and stop together.

Steady flies in a kitchen or around bins all summer. A breeding source within a short distance. Find it, remove it, and the population falls within one life cycle.

Slow, clumsy flies at upstairs windows from October onwards. Cluster flies overwintering. Sealing in late summer is the fix; nothing done in winter helps much.

Small flies hovering over a plant or a sink. Fungus gnats or drain flies. Both are about a wet medium rather than about flies.

When to call us

When you cannot find the source and the flies keep coming back, which is the most common reason people phone about this. We treat fly work as source-finding rather than spraying, because against a seven-day life cycle, killing adults is a week of relief and nothing more.

Also call if this is a food business. Flies in a commercial kitchen are a compliance problem as well as a pest one, and the documentation matters as much as the treatment.

And if what you have turns out to be a dead rodent in a wall, we would rather tell you that on the phone than sell you a fly treatment. The full picture is on fly and gnat control.