Fly Pupae: What the Brown Capsules Are, Where They Came From, and How to Get Rid of Them

What are fly pupae?

Fly pupae are flies in the stage between maggot and adult, inside a hard brown case. A house fly pupa is about 1/3 inch (8 mm) long and rounded at both ends, like a grain of brown rice. It does not eat or move, and in 2 to 6 days of warm weather a fly pushes out of one end.

The case is the maggot’s own last skin. When a maggot is fully grown, it stops eating, shrinks into a barrel shape, and its skin hardens and darkens around it. Inside, the true pupa forms and turns into a fly. That case is called a puparium. Orkin compares it to a butterfly cocoon, and the Illinois Department of Public Health calls it a cocoon, but it contains no silk: it is the maggot’s own skin. Our guide to what a pupa is explains how a puparium differs from a cocoon or a chrysalis.

New cases are cream or yellowish, then turn red, brown and finally nearly black as they age. Anglers call them “casters” and use them as bait. Pet owners buy them to hatch as live food for lizards and spiders. In a house, though, they mean flies are on the way.

Does one maggot make one fly?

Yes. Each maggot forms one pupa, and each pupa releases one fly. So twenty full cases on the floor mean up to twenty flies. Empty cases, with a round cap missing at one end, have already hatched. Those flies are probably at your windows now.

Fly pupae or something else?

Brown capsules on a floor can also be mouse droppings, cockroach egg cases or carpet beetle skins. Pupae are the only ones that are smooth, shiny and rounded at both ends, about 1/3 inch (8 mm) long. Mouse droppings have a pointed end, and roach egg cases have a ridge along the top.

Sizes are typical, from the sources listed at the end. If yours match a row other than the first, our guides to German cockroaches, carpet beetles and drywood termites take it from there.

Fly pupae vs mouse droppings

This is the most common mix-up, and people argue about it online. On Reddit’s r/CleaningTips, one thread with more than 70 comments split between the two answers. The test is the ends. Mouse droppings have at least one pointed end and look dull, like small dark grains of rice. Pupae are rounded at both ends, smooth and a little shiny, with faint rings.

Mouse droppings also turn up along walls, in drawers and behind appliances, where mice run, rather than in one patch. If they are droppings, do not sweep or vacuum them. That can lift dust from mouse urine and droppings into the air. Follow the CDC clean-up steps further down instead.

Fly pupa or cockroach egg?

A cockroach egg case is shaped like a small purse, with a ridged seam along one edge where the young will split it open. A German cockroach’s is light brown and about 1/4 to 1/3 inch (6 to 8 mm) long, and the female carries it until just before it hatches. An American cockroach’s turns almost black and is glued near food. A fly pupa has no seam, no ridge and no flat side.

If you are still not sure, our photo bug identifier can tell you in a few seconds.

Which fly made them?

Most pupae found indoors belong to house flies or blow flies. House fly pupae are about 1/3 inch (8 mm) long. Blow fly pupae are a little longer and usually mean a dead animal nearby. Fruit fly pupae are tiny, about 1/8 inch (3 mm), and sit near fruit or drains rather than on open floors.

Sizes are typical. Cluster flies are not on this list because they never breed indoors. Three of these have their own guides: phorid flies, drain flies and fungus gnats.

House fly pupae

These are the most common kind on a floor. They are about 1/3 inch (8 mm) long, bluntly rounded at both ends, and change from yellow to red, brown and black as they age. The flies that come out are gray, about 1/4 inch long, with four dark stripes on the back. They came from moist rotting matter, most often a trash can, compost pail or pet waste.

Blow fly pupae and a dead animal

Blow fly pupae look like house fly pupae, only a little bigger: about 3/8 inch (9 to 10 mm) for the common green bottle fly. The flies that hatch are shiny metallic blue or green. Finding them indoors, says the University of Wisconsin’s insect lab, “often indicates a dead mouse”. Purdue adds dead rodents in wall voids or ceilings, and traps or glue boards left too long.

Flesh flies come from the same places. They are unusual because the female does not lay eggs. She deposits live young maggots, 40 to 80 at a time.

Fruit fly pupae

Fruit fly pupae are tiny, about 1/8 inch (3 mm), yellowish brown and seed-like, with two little horns at one end. Fruit fly maggots crawl out of the food to a drier spot to pupate, so you find the pupae on the sides of the fruit bowl, the rim of a bin or the corner of a sink. The whole cycle takes about 10 days in a warm kitchen.

To stop them, throw out overripe fruit, rinse recycling, and clean the drain and the bin. Wipe the pupae off with a damp cloth. They will not survive in the trash.

Black soldier fly pupae in compost

Large, dark, flattened maggots and pupae in a compost bin are usually black soldier flies. The larvae reach about 1 inch (27 mm) long. The adults are black and wasp-like, but Oregon State Extension notes that they do not bite or carry disease, and they rarely come indoors. They are good for compost, so leave them.

Why are there fly pupae in my house?

Pupae on a floor mean maggots fed somewhere close by and then crawled away to pupate. A full-grown house fly maggot can crawl up to 50 feet (15 m) looking for a dry, cool, dark spot. So the source is usually in the same room or the next one, not where the pupae are.

The usual sources:

• A trash can or kitchen bin, especially one with meat scraps, used diapers or a torn bag underneath.
• Food waste: a forgotten bag of potatoes, spilled pet food, a compost pail, food under an appliance.
• Pet waste: litter boxes, a dog run, a missed accident on a rug.
• A dead animal: a mouse, rat or bird in a wall, ceiling, attic, chimney or crawl space, or in a trap that was never checked.
• Damp, dirty fabric: a wet towel or rug with food on it, as one Reddit user found.

A dead mouse in the wall

A sudden crop of pupae with no obvious trash, often with big metallic blue or green flies at the windows, usually means a dead animal. You may notice a rotten smell too. In a University of Wolverhampton study inside a house, blow fly maggots traveled up to about 28 feet (8.5 m) from where they started, and they used ceiling spaces to move between rooms and floors. That is why pupae can show up a room away from the body.

Most of the time, the body is in a wall, under a floor, in the attic, or behind a stove or fridge. If you set mouse traps or glue boards recently, check them first. If you cannot find it, the problem ends on its own: Texas A&M says flies may keep emerging for 1 to 3 weeks, after which they should disappear.

Where to look for the source

Start with the spot where the pupae are thickest and work outward. Check every bin, under and behind the fridge and stove, under the sink, the pet’s bowl and litter box, and any cupboard with root vegetables. Then sniff along walls, closets and baseboards. If one area smells worse, or flies gather at one window, the source is usually near it.

How long until fly pupae hatch?

House fly pupae hatch in about 2 to 6 days in warm weather, but they can take 17 to 27 days in a cool room. Blow fly pupae take about 7 to 10 days. Fruit fly pupae take about 4 days. So if you find full cases, expect flies within a week unless you remove them.

The sources agree once you account for heat: a few days in summer, several weeks in a cool room.

Are they empty or full?

Look at the ends. An empty case has a round cap missing at one end, or the cap is hanging open, and it feels light and hollow. The fly pushed it off with a balloon-like sac on its head, called a ptilinum. A full case is closed at both ends and feels firm. Full cases still need to go, because each one will release a fly.

How long does it take a maggot to turn into a fly?

A house fly takes about 7 to 10 days from egg to fly in warm weather. The egg hatches in 8 to 20 hours, the maggot feeds for about 4 to 7 days, and the pupa lasts 2 to 6 days. Blow flies take a little longer. So yes, maggots turn into flies, and the pupa is the step in between.

Can fly pupae crawl?

No. A pupa has no working legs and cannot move. The maggots crawled to that spot before they pupated. If you see something small, pale and moving on the floor, it is a maggot that has not pupated yet. Those will form more pupae within a day or so.

How to get rid of fly pupae

Sweep or vacuum the pupae, then find and remove the source, or new ones will keep appearing. Sprays are rarely needed: UC IPM says that in most home situations, pesticides are not needed or recommended. The fix costs almost nothing. It takes a broom, a bag and some cleaning.

What kills fly pupae?

Removing them is what works. Sweep them up and throw them out, and the flies inside never hatch. Extension guides do not recommend spraying them. They point to the maggot’s food as the target: UC IPM calls removing the place where larvae grow “the preferred method”.

Should you spray for fly pupae?

Usually not. If sanitation and exclusion are successful, insecticide treatments are not required. Texas A&M puts it plainly: insecticides alone are rarely successful. A spray does nothing about the trash or the dead animal, so the next batch of maggots will still come. If flies keep coming after the source is gone, use a product registered by the EPA for flies indoors, and follow the label exactly. The label is the law.

Are fly pupae dangerous?

The pupae themselves are not. They cannot bite, sting or move, and they do not eat. The risk is the flies and their source. House flies can carry germs such as Salmonella, Shigella and E. coli from garbage and waste to food. So clean the source, keep food covered, and wash hands after handling bins or a dead animal.

Keep pets and children away from a dead animal and the area around it until it is cleaned.

Fly pupae on farms, and as bait or pet food

On farms and stables, the pupae are where natural pest control happens. Two kinds of tiny parasitic wasps, Muscidifurax and Spalangia, lay an egg inside a fly pupa, and a wasp hatches out instead of a fly. Farms buy and release them. Cornell warns that spraying can kill these wasps while hardly touching the flies.

Anglers buy bluebottle fly pupae as “casters” for fishing bait. Reptile and spider keepers buy house fly pupae to hatch as live food.

Fly pupae myths

Most myths about them come from how suddenly they appear. One day there is nothing, the next there are dozens. Here are seven common myths, each with what is actually true. The biggest is that they are eggs, when they are really the last stage before the fly.

• Myth 1

  Myth: “Those brown things are fly eggs.”

  Truth: Fly eggs are white, soft and about 1/16 inch (1 mm) long. Brown capsules are pupae, the last stage before a fly.

• Myth 2

  Myth: “The flies came from nowhere.”

  Truth: Flies do not just appear. Every pupa means a maggot fed on something nearby.

• Myth 3

  Myth: “Cleaning the floor solves it.”

  Truth: It removes the pupae you can see. The source is still there, and it will send more maggots.

• Myth 4

  Myth: “Fly pupae are mouse droppings.”

  Truth: Droppings have a pointed end and crumble. Pupae are rounded at both ends and hard.

• Myth 5

  Myth: “Spraying the pupae kills the problem.”

  Truth: Extension guides target the source, not the pupae. A spray leaves the trash or dead animal in place.

• Myth 6

  Myth: “All flies lay eggs.”

  Truth: Flesh flies deposit live maggots instead. Some extension pages still say they lay eggs.

• Myth 7

  Myth: “Cluster flies at the window mean pupae in the house.”

  Truth: Cluster flies grow up on earthworms outdoors and only shelter indoors. They do not breed inside.

When to call someone

Most cases end with a broom and an emptied bin. Call for help when you cannot find or reach the source, when you rent and the source is in the building, or when flies keep coming more than 3 weeks after you removed the source.

Pest control disclaimer. Photo-based identification can help, but it is not always perfect. For dangerous spiders, large infestations, or anything you are unsure about, contact a licensed pest control professional.