Common house insects: how to tell what is living in your home, room by room

Where you found it tells you more than what it looks like. A bug in the bathroom is almost always a moisture lover; a bug in the kitchen is after food; a bug at the window is trying to leave. So this guide is sorted by room. Find the room, run down the short list, and use the photo identifier if two of them still look alike.

Kitchen

• Fruit flies. Tiny, tan, red-eyed, hovering over the fruit bowl or the recycling. They breed in anything fermenting: overripe bananas, a splash of wine, the sludge under the bin bag. Remove the source and they are gone in a week. Card #132.
• House flies and bluebottles. The medium grey fly and the big metallic-blue one. They come in through doors and lay eggs on meat, pet food and bins. A bluebottle indoors in winter usually means something has died in a wall or loft. Cards #124 and #127.
• Ants in a line. A trail from a crack to the sugar or the cat food is the black garden ant or the pavement ant: a nuisance, not a danger. Large black ants with no trail, seen one at a time, can be carpenter ants, which nest in damp wood and are worth tracing back.
• Cockroaches. Small and tan with two dark stripes behind the head is the German cockroach, the one that breeds in kitchens. Big, reddish-brown and fast is the American cockroach, which usually wanders in from drains. Both are a hygiene problem rather than a bite problem.
• Pantry beetles and moths. Small brown beetles in the flour, or little moths fluttering out of the cereal cupboard, mean a packet has hatched. Find the packet, bin it, wipe the shelf. Mealworm beetles turn up in old stored grain.

Bathroom and laundry

• Silverfish. Silvery, carrot-shaped, fast, wriggling like a fish when they run. They eat starch and paper, love humidity, and are harmless to people. Card #140, and there is a full silverfish guide.
• Drain flies. Tiny, fuzzy, moth-shaped, sitting on the tiles near the shower. They breed in the film inside drains. A stiff brush and hot water fix it; bleach mostly does not.
• House centipedes. The long-legged sprinter that makes people scream. It eats silverfish, cockroach nymphs and flies, it does not want anything to do with you, and a nip is rare and mild. Card #170.
• Springtails. Pinhead-sized, pale, jumping when disturbed, in damp corners and plant pots. Moisture indicators, completely harmless. Card #141.

Bedroom

• Bed bugs. Flat, oval, reddish-brown, the size of an apple seed, hiding in mattress seams and bed frames by day. The signs are small blood spots and dark specks on the sheets, and bites in lines. This is the one indoor insect where you should not wait: professional treatment works, DIY sprays mostly spread them.
• Fleas. Dark, tiny, and they jump. Almost always the cat flea, even in houses with dogs. Treat the pet and vacuum daily; the eggs are in the carpet, not on the animal.
• Carpet beetles. Small round beetles at the window in spring, and fuzzy little larvae in wardrobes and under furniture. The larvae eat wool, feathers and lint. Annoying for jumpers, harmless for people.
• Clothes moths. Small, plain, golden-brown moths that scuttle rather than fly to the light. The larvae make the holes. Pheromone traps show whether you have them; washing and freezing kills the eggs.

Windows and walls, especially in autumn

• Ladybirds. Clusters of them in October are the Asian lady beetle, which gathers on sunny walls and slips in to overwinter. They do not eat anything indoors and they leave in spring. They can stain if squashed, so sweep them into a jar and put them outside.
• Stink bugs and boxelder bugs. Shield-shaped brown bugs (brown marmorated stink bug) and black-and-red ones (boxelder bug) do the same thing for the same reason. Same answer.
• Cluster flies. Sluggish, slightly larger than a house fly, appearing in numbers at upstairs windows on warm winter days. They overwinter in lofts and do not breed indoors.
• Wasps. A common wasp indoors in late summer is a forager that came in for the jam. Open the window and wait. Many wasps at once, or wasps coming out of a wall, means a nest, which is a job for someone with the right suit.

Cellar, garage and dark corners

• Cellar spiders. The wispy, long-legged spider in the ceiling corner that trembles when touched. It eats other spiders, including the ones you would rather not have. Card #156.
• Jumping spiders. Small, compact, with two big forward-facing eyes, hunting on window sills in daylight. Curious rather than dangerous. Card #149.
• Crickets. A house cricket chirping behind the boiler, or a camel cricket (humpbacked, silent, huge back legs) in a damp basement. Both harmless.
• Woodlice and millipedes. Curling up or crawling slowly across the garage floor after rain. They came in with the damp and will die of dryness indoors. Card #171.

The short list that deserves action

Everything above is a matter of preference except these five:

1. Cockroaches: food hygiene, and they breed fast.
2. Bed bugs: bites, and they are hard to remove without help.
3. Fleas: bites, and they cycle through pets and carpets.
4. Termites and carpenter ants: structure. Termite swarmers indoors in spring mean a colony in the building.
5. Pantry pests: stored food. Cheap to fix if you find the packet.

Front desk note: the single most useful habit is a photo before the shoe. Once it is flat, nobody can tell you what it was, and the answer decides whether you need to do anything at all.

Two things to check before you panic

• How many? One of anything is a visitor. Dozens of the same thing is a population, and populations have a source: a drain, a packet, a damp wall, a nest.
• What season? Autumn crowds at the windows are overwintering guests, not an infestation. Spring swarms of winged ants or termites are colonies sending out founders, which is the moment to look at the building.

For the ones you cannot place, the Field Guide has a page per species with where it lives and whether to keep your distance, and the identifier will give you a name from a photo in seconds.