Bugs that come into the house in autumn: ladybirds, stink bugs, boxelder bugs and cluster flies

Every year, somewhere in late September or October, a warm afternoon turns a sunny wall into a crowd. Ladybirds cluster in the window frame, a brown shield-shaped bug turns up on the curtain, and the loft window has a row of sleepy flies on it. None of them have come for your food, and none of them plan to breed in the house. They are looking for somewhere dry and sheltered to wait out the winter, and your house is a very good cliff.

Why they all arrive at once

These insects spend the cold months in a slowed-down resting state, and in the wild they do it in rock crevices, under bark and in dead trees. Buildings offer the same thing with better insulation. What sets them moving is shorter days and cooler nights; what brings them to your house in particular is warmth. They gather on the walls that catch the afternoon sun, usually south- or west-facing, and then work their way into any gap: window frames, eaves, vents, roof tiles, where cables come in.

Once inside they mostly stay out of sight in wall voids and lofts. The ones that end up in your living room are the ones that got confused by the heating. So the second wave is often on a mild day in January or February.

Ladybirds and the Asian lady beetle

The ladybird most likely to come indoors in numbers is the Asian lady beetle, card #002, known in Britain as the harlequin ladybird. It was brought to North America and Europe to eat aphids and is now common across both. It is very variable: orange, red or black, with anything from no spots to many. The most reliable clue is on the white area just behind the head, which often carries a black mark shaped like an M or a W.

The native seven-spot ladybird, card #001, does spend winter in sheltered places but prefers leaf litter and hedge bases, and rarely turns up in crowds inside houses. There is more on that one in our seven-spot ladybird guide.

Two things people notice about lady beetles indoors: a musty smell and small yellow stains. Both come from reflex bleeding, a defensive fluid they release from their leg joints when disturbed. It can mark pale fabric and wallpaper, so it is worth not squashing them. Asian lady beetles can also give a small pinch on the skin, which is a property of the beetle rather than a reason for alarm.

Stink bugs

The one that comes indoors most in North America and parts of Europe is the brown marmorated stink bug, card #109, originally from East Asia. It is about the size of a fingernail, mottled brown, shaped like a shield, and has pale bands on its antennae and alternating light and dark patches around the edge of the body. The green stink bug, card #108, is its bright green North American relative and sometimes comes in too.

The name is honest. When a stink bug is disturbed or crushed it releases a sharp, pungent smell that some people compare to coriander. It is unpleasant and it lingers, but it is not poisonous. Stink bugs do not sting and are slow, clumsy fliers that buzz loudly around lamps.

Boxelder bugs

The boxelder bug, card #111, is a North American true bug: black, narrow, with red or orange lines along the edges and a red body underneath that shows when it spreads its wings. Young ones are smaller and mostly red. They feed on the seeds of boxelder maples and some other maples, so a house near one of those trees can see hundreds on the sunny wall in October. They can leave small reddish-orange spots of droppings on curtains and walls, but they do not damage the building or eat anything indoors.

Cluster flies

Cluster flies look like a slower, slightly larger house fly, with a dark grey body and fine golden hairs on the thorax that you can see in good light. They are not in our Field Guide yet, but they are easy to separate from the house fly: house flies are active and come in through the door all summer; cluster flies are sluggish, turn up in autumn, and gather in lofts, spare rooms and on upstairs windows. Their young grow up as parasites of earthworms in lawns and fields, which is why they are commonest in houses surrounded by grass. They do not breed indoors.

Quick comparison

Getting them out gently

Front desk note: the rule for all four is the same. Do not squash them. Squashing is what makes the smell and the stains.

• By hand. Ease them into a jar or onto a sheet of card and take them outside. A cold porch or a shed works well in midwinter, since warm sun will only send them straight back to the wall.
• By vacuum. Put an old stocking or a thin sock inside the nozzle and hold it with a rubber band. The bugs collect in the stocking and you can take them outside without them ending up in the vacuum bag, which is where the smell lives on.
• Leave the loft alone. A cluster in a loft or wall void is not harming anything and will leave in spring. Disturbing it in winter mostly spreads it into the house.
• Skip the sprays. They kill the few in the room and change nothing about the gaps, and a pile of dead insects in a wall can feed carpet beetles later.

Keeping them out next year

The only thing that really helps is sealing, and timing is everything. Check the sunny side of the house in late summer, before the first cool nights: gaps around window and door frames, cable and pipe entries, vents without fine mesh, loose fascia boards and roof edges. Once the insects are already in the wall, sealing only traps them in. If you have a boxelder maple close to the house, expect boxelder bugs every year; the tree is the reason, and it is still a good tree.

If you cannot tell which autumn guest has moved in, a photo in the identifier will give you a name and a safety read, and the Field Guide has each species’ seasonal pattern from iNaturalist records. There is a room-by-room list of the rest of the year’s visitors in our house insects guide.