Box elder bugs in my house in winter — where did they come from?

They entered your home the previous fall, most likely through small cracks on a sunny south- or west-facing wall, and have been dormant in the wall voids ever since. Boxelder bugs don't breed or feed indoors at any point during the winter — they're strictly sheltering, waiting for warmer weather. When a mild day or your furnace warms the wall cavity, some wake up and wander into living space, which is why they seem to appear out of nowhere in the middle of winter.

When do boxelder and stink bugs invade New Hampshire homes?

Boxelder bugs and brown marmorated stink bugs mass on sunny walls in late September through early October, while western conifer seed bugs continue seeking shelter into late fall. All three concentrate their entry around New Hampshire's first hard frost, which averages about October 19 in Manchester. Once temperatures drop consistently, the bugs that haven't found a way inside die outdoors, while those that made it into wall voids and attics settle into winter dormancy until spring.

Do stink bugs damage my house or breed inside?

No. Brown marmorated stink bugs do not bite, breed, or cause structural damage indoors — they are strictly a nuisance while overwintering, even though they can be a serious agricultural pest on crops outdoors. Once inside a wall void, a stink bug simply sits dormant until spring warmth prompts it to move again. The main downside indoors is volume during a heavy year and the unpleasant odor they can release if disturbed or crushed.

What is the loud buzzing bug in my house in fall?

That's almost certainly a western conifer seed bug in flight — it makes a distinctive, loud, bumblebee-like buzzing sound as it moves through a room looking for a place to settle. Despite the startling noise, it's completely harmless: it doesn't bite, sting, or damage the house, and it's usually seen singly or in small numbers rather than in the large masses typical of boxelder bugs. It feeds on conifer seeds outdoors and only enters homes to overwinter.

When will boxelder and stink bugs leave my house?

Brown marmorated stink bugs typically emerge from their overwintering shelters between late March and mid-May, while boxelder bugs tend to leave a bit later, as spring buds open in late April and May. Warmth is the trigger in both cases — once temperatures climb consistently, the bugs that spent winter dormant in your walls wake up, work their way out through baseboards, trim, or light fixtures, and head back outdoors to host plants like boxelder maples or conifers to feed and lay eggs.

Why do these bugs always target the south side of my house?

South- and west-facing walls catch the most direct sun on cooling fall afternoons, making them noticeably warmer than shaded sides of the building. Boxelder bugs, seed bugs, and stink bugs are all cued by that warmth to gather there in large numbers before working their way into cracks around siding, window casings, and rooflines. Homes with prominent, sun-exposed walls and minimal landscaping shade tend to see heavier fall aggregations than similar houses with more tree cover.

Should I spray inside to get rid of them?

No. Treating wall voids with insecticide after these bugs are already inside just leaves dead carcasses that attract secondary pests like carpet beetles, trading one problem for another. The better approach is exclusion the following summer — sealing likely entry points before the fall aggregation begins — combined with vacuuming any bugs that do make it into living space. Avoid crushing western conifer seed bugs or stink bugs directly, since both release a noticeable defensive odor when disturbed.

Are the red-and-black bugs on my siding boxelder bugs?

Yes, most likely. Boxelder bugs are roughly half an inch long, black with three distinctive red stripes on the thorax and red markings along the wing edges, and they're true bugs rather than beetles. They gather in large, visible groups on sunny siding, decks, and walls in late summer and early fall as they search for winter shelter. Despite the alarming numbers they can form, they are harmless — they don't bite, sting, or damage paint, siding, or wood.

How do I tell a brown marmorated stink bug from a native stink bug?

The clearest identification marker is the antennae: brown marmorated stink bugs have distinct alternating light and dark bands on their antennae, along with light and dark marbling along the edge of the abdomen that's visible from above. Native New Hampshire stink bug species lack these banded antennae. Brown marmorated stink bugs are also a mottled brown-gray overall, roughly shield-shaped, and about five-eighths of an inch long — noticeably marbled rather than a solid, uniform color.

Daniel Brady · NH-Licensed Pesticide Applicator · License #782664

|Reviewed by Daniel Brady

Box Elder Bugs in the House in Winter — NH Timing Guide

Overwintering Invaders

These three unrelated bugs converge on the same New Hampshire homes for the same reason: as nights cool in late September and October, they mass on sun-warmed south- and west-facing walls looking for a place to spend winter, then squeeze into wall voids and attics. Boxelder bugs favor voids and door or window casings, western conifer seed bugs hide under siding, and brown marmorated stink bugs usually get inside the walls themselves. All three sit dormant until spring, when warmth triggers them to emerge and head back outside to host plants — none of them bite, breed, or damage the house while they're inside.

At a glance

Peak entry: Late Sept–early OctBoxelder and stink bugs mass on sunny south walls before the first frost.
Winter location: Wall voids & atticsStink bugs are usually inside walls; boxelder bugs in voids and casings; seed bugs under siding.
Spring exit: Late March–mid-MayBrown marmorated stink bugs emerge from shelters late March through mid-May.
Breeds/damages indoors?: NoAll three sit in diapause; no biting, breeding, or structural damage occurs indoors.

Peak entry
Late Sept–early Oct
Boxelder and stink bugs mass on sunny south walls before the first frost.
Winter location
Wall voids & attics
Stink bugs are usually inside walls; boxelder bugs in voids and casings; seed bugs under siding.
Spring exit
Late March–mid-May
Brown marmorated stink bugs emerge from shelters late March through mid-May.
Breeds/damages indoors?
No
All three sit in diapause; no biting, breeding, or structural damage occurs indoors.

The timing answer

The short of it

The verdict · overwintering invaders

Dormant in walls until spring

Boxelder, brown marmorated stink, and western conifer seed bugs seen indoors in winter are overwintering adults that entered last fall — harmless and dormant.

NH activity calendar

When are they active — the whole year at a glance

Boxelder bugs, western conifer seed bugs, and brown marmorated stink bugs all follow the same broad New Hampshire pattern: outdoors on host plants all summer, a hard fall entry window, then winter dormancy indoors.

12-month activity · New Hampshire

Peak · OctYou are here · Aug

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

PeakActiveBuildingDecliningDormantCurrent season · Summer

Best window to act · Summer through early September — UNH Extension recommends sealing boxelder bug entry points before roughly September 10 — by caulking and screening gaps on south/west walls, repairing screens, and adding door sweeps.

What else peaks thenOct

Peak reflects the late-September through October entry window, anchored to Manchester's first frost (~Oct 19, 50% probability — NOAA 1991–2020 Climate Normals).

Where they go in winter

Winter behavior

All three of these bugs answer the 'where do they go in winter' question the same way: into the walls, not away from the property.

Overwintering strategy

Hides indoors

All three overwinter as dormant, diapausing adults in wall voids and attics — brown marmorated stink bugs are usually found inside the walls themselves, boxelder bugs settle into voids and door or window casings, and western conifer seed bugs hide under siding and in other protected cavities. None feed, breed, or lay eggs while overwintering.

Move in
Late September through October, favoring warm south- and west-facing walls, concentrated around the first frost.
Hides indoors
All three overwinter as dormant, diapausing adults in wall voids and attics — brown marmorated stink bugs are usually found inside the walls themselves, boxelder bugs settle into voids and door or window casings, and western conifer seed bugs hide under siding and in other protected cavities. None feed, breed, or lay eggs while overwintering.
Spring exit
Warmth triggers movement out through baseboards, trim, and light fixtures. Stink bugs emerge late March through mid-May, boxelder bugs as buds open, and both then head back outside to host plants.

Season by season

A year in the life

Boxelder bugs, western conifer seed bugs, and brown marmorated stink bugs share one seasonal story in New Hampshire even though they're unrelated species.

01
Spring
Overwintered adults leave the walls — stink bugs emerge late March through mid-May, boxelder bugs slightly later as buds open in late April and May — and return to host plants outdoors to feed and lay eggs.
02
Summer
All three are entirely outdoors on host plants: boxelder bugs on female boxelder and maple trees, seed bugs on conifer cones, and stink bugs on a wide range of fruit, vegetable, and ornamental plants. Nymphs develop through the warm months.
03
Fall
The invasion season. Cooling nights drive heavy aggregation on sunny walls from late September through October and entry into voids just ahead of the first frost. Western conifer seed bugs may buzz loudly in flight, and all three can release a defensive odor if disturbed.
04
Winter
Dormant in walls and attics. Warm days and indoor heating cause occasional sightings — usually one or a few seed bugs at a time, but larger numbers for boxelder and stink bugs.

New Hampshire context

Boxelder, seed, and stink bugs in southern New Hampshire

In Manchester, Nashua, and Concord, these three species converge on homes for the same architectural reason: sun-exposed south and west walls warm up fastest on cool fall afternoons, making them the preferred staging ground before bugs squeeze into gaps around siding, windows, and rooflines. Boxelder bugs are tied to the relatively uncommon boxelder maple locally, while brown marmorated stink bugs — first confirmed in New Hampshire in Portsmouth in 2010 — have since spread and are also a significant agricultural pest outdoors, even though indoors they're strictly a nuisance.

Key local data

Manchester's first hard frost averages October 19 (50% probability), the point by which most fall wall-massing and entry has occurred (NOAA 1991–2020 Climate Normals).

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When to act

Timing your treatment

Because all three species enter as adults through the same kind of exterior gaps, one seasonal sealing effort addresses all of them at once.

The right window

Summer through early September — UNH Extension recommends sealing boxelder bug entry points before roughly September 10 — by caulking and screening gaps on south/west walls, repairing screens, and adding door sweeps.

Heavy annual wall masses every fall

Bugs streaming in around windows or siding

Loud buzzing flights from western conifer seed bugs

Winter emergence into living space

Persistent odor from disturbed or crushed bugs

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Learn more about this pest

Common Questions

Frequently asked

Sources & references

UNH Extension — Boxelder Bug Fact Sheet
~Sept 10 seal-by date, south-wall massing behavior
UNH Extension — Brown Marmorated Stink Bug (BMSB)
September massing timing, late-March–mid-May spring exit, no indoor damage
Maine Dept. of Agriculture, Conservation & Forestry — Western Conifer Seed Bug (NH)
life cycle, buzzing flight, late-fall entry
Penn State Extension — Western Conifer Seed Bug
host conifers, overwintering behavior, don't-crush guidance
University of Minnesota Extension — Boxelder Bugs
no indoor breeding, small-numbers seed bug behavior
EPA — Brown Marmorated Stink Bug
does not bite, breed, or damage buildings
NC State Extension — Brown Marmorated Stink Bug
diapause biology, no winter feeding
Garden.org — Manchester, NH Frost Dates
~Oct 19 Manchester first-frost anchor (NOAA 1991–2020)

Editorial disclaimer: This content is provided for informational purposes only and does not constitute pest control advice. Every property is unique — consult a licensed professional for guidance specific to your situation. Anchor Pest Services is licensed in New Hampshire (#782664).

Written & reviewed by

Verified

Daniel Brady

NH-Licensed Pesticide Applicator · License #782664 (NHDAMF, RSA 430)

Daniel Brady is the New Hampshire–licensed applicator behind Anchor Pest Services, a family-owned pest control company serving the Manchester area since 2017.

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Last updated Jul 2026

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Keep Exploring

Box Elder Bugs in the House in Winter — NH Timing Guide

The short of it

When are they active — the whole year at a glance

Winter behavior

A year in the life

Spring

Summer

Fall

Winter

Boxelder, seed, and stink bugs in southern New Hampshire

Timing your treatment

Frequently asked

Sources & references

Whatever the season — we handle the timing for you.

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