What pests “overwinter” inside houses?

Overwintering, used precisely, describes a pest that has a specific seasonal strategy for surviving the cold months — entering diapause, seeking indoor shelter specifically in fall, or otherwise adjusting its behavior around the season. That's a narrower category than "any pest found indoors in winter," which would also sweep in species that are simply present year-round regardless of season, like German cockroaches, and don't have a winter-specific behavior at all.

This list sticks to the narrower, more honest definition: pests with a documented, seasonal overwintering pattern specific to a New Hampshire home. That's why the ranking below covers five species groups rather than every pest that could theoretically turn up in a house during the cold months.

It would be easy to rank this list by which pest shows up in the most dramatic numbers on a warm January afternoon — a wall void of a thousand cluster flies is visually striking in a way a single mouse dropping isn't. But visual drama and actual significance point in opposite directions here. Extension and CDC sources are consistent that rodents are active, gnawing, and potentially breeding all winter, while the visually dramatic overwinterers — cluster flies, lady beetles, stink bugs — are dormant, non-reproducing, and cause no structural or health damage.

Ranking by documented presence and behavior rather than by visibility is the entire point of doing this as a census rather than a scare list. It's also consistent with the honest grading used throughout this hub: rodent exclusion earns do_this_first because it's a real, evidence-backed priority, not because rodents are the scariest thing on the list.

This page and our companion page on which pests to watch out for in winter cover closely related ground, and it's worth being direct about how they differ rather than making you read both to figure it out. This page is a census: an honest inventory of what's actually present and overwintering inside a New Hampshire house right now, ranked by how documented and established each presence is.

The watch-list page asks a narrower, more action-oriented question: of everything that might be present, what actually deserves your ongoing attention and effort this winter? The two lists share the same underlying evidence and mostly agree on rodents at the top, but the watch-list page includes a health-relevant pest not on this census (cockroaches, where present) and frames every entry around what to do, not just what's there.