As temperatures drop, many pests begin seeking warmth and shelter — often finding it inside homes and commercial buildings. These overwintering pests don’t reproduce indoors or feed heavily during winter, but they can still cause headaches for property owners. When spring arrives and they emerge en masse, it can quickly turn into a nuisance — or worse, a sanitation issue.
Let’s take a closer look at the most common overwintering pests across the Pacific Northwest, Mountain West, and Intermountain regions — and how Sprague can help your business stay protected year-round.
Box Elder Bugs (Boisea rubrolineata and Boisea trivttata)
Western Box Elder Bug. Photo courtesy of iNaturalist: https://www.inaturalist.org/photos/29810323
Box elder bugs are black with three distinctive red or orange markings on their thorax, a key feature that sets them apart from similar-looking insects. During the fall, they often gather in large numbers, clustering on sunny exterior walls before sneaking into cracks, gaps, and vents to overwinter indoors. While their lookalikes lack the orange stripe, they may also seek shelter inside your facility through the same entry points.
While they’re harmless, their sheer numbers and tendency to stain walls, drapes, and furnishings with their excrement make them a serious nuisance. Once they settle in, they can reappear every warm winter day and again in spring when temperatures rise.
Sprague’s Approach:
Our specialists perform exterior exclusion services to seal entry points around windows, siding, soffits, and utility lines. Strategic perimeter treatments in late summer or early fall stop box elder bugs before they migrate indoors.
Western Conifer Seed Bugs (Leptoglossus occidentalis)
Western Conifer Seed Bug. Photo courtesy of iNaturalist https://www.inaturalist.org/photos/344303585
These long, leaf-footed insects are often mistaken for stink bugs, but they’re actually seed feeders that live on pine and fir trees. As temperatures cool, western conifer seed bugs fly toward buildings seeking warmth. Once inside, their loud buzzing flight and pungent odor when crushed make them unwelcome guests.
Sprague’s Approach:
We identify exterior attractants such as nearby conifers and perform targeted exterior building treatments to reduce populations before they enter. Our experts also inspect for openings around siding, attic vents, and soffits to prevent infestations from recurring.
Lady Bugs (Asian Lady Beetles) (Harmonia axyridis)
Asian Lady Beetle. Photo courtesy of iNaturalist https://www.inaturalist.org/photos/452640195
Lady beetles, or as they are more commonly known lady bugs— particularly the non-native Asian lady beetle— are beneficial outdoors but problematic indoors. In the fall, they gather by the hundreds or thousands on sunny building exteriors. When they crawl into walls and attics to overwinter, they can leave behind allergens and odors and become a persistent issue each season.
Sprague’s Approach:
Our field teams use integrated pest management (IPM) strategies that combine exclusion, vacuum removal, and limited treatment applications to safely manage lady beetle populations without disrupting beneficial insects outside.
Cluster Flies (Pollenia rudis)
Cluster Fly. Photo courtesy of iNaturalist https://www.inaturalist.org/photos/674260
Cluster flies look similar to house flies but behave differently. They lay eggs in soil where earthworms live (their larval stage is a parasite of earthworms), and when cold weather hits, the adults seek protected indoor spaces — especially attics, wall voids, and upper-story windows. They tend to “cluster” together in large groups, creating a mess of dead insects and fly spots.
Sprague’s Approach:
We focus on preventative structural inspections, attic treatments, and sealing gaps in upper-level eaves and windows. If cluster flies have entered your facility, we recommend the installation of insect light traps. Because cluster flies feed on decaying matter, eliminating other fly species from the area helps remove their food source. Continuous monitoring then ensures an remianin flies are controlled before they can reappear in spring.
Brown Marmorated Stink Bugs (Halyomorpha halys)
Brown Marmorated Stink Bug. Photo courtesy of iNaturalist https://www.inaturalist.org/photos/251561287
The brown marmorated stink bug (BMSB) is one of the most destructive overwintering pests. Native to Asia, it feeds on fruits, vegetables, and ornamental plants, causing extensive agricultural damage. When the weather cools, BMSBs invade homes, offices, and warehouses in search of shelter. Disturb them, and they release their trademark odor — a defense mechanism that’s tough to forget.
Sprague’s Approach:
Our custom integrated pest management (IPM) programs are designed to protect both structural integrity and stored goods. By combining exclusion, light management, and precision perimeter treatments, our teams stop stink bugs before they breach your facility.
Protect Your Business Year-Round
Overwintering pests may not bite, chew, or breed indoors, but they can still disrupt operations, create sanitation issues, and damage customer perceptions. Prevention is key — and timing is everything.
Sprague helps businesses across the western United States implement seasonal pest management strategies tailored to their property and risk level. From proactive fall treatments to spring emergence inspections, Sprague keeps your facility pest-free and ready for business every season.
Don’t let overwintering pests take shelter in your space.
Contact Sprague Pest Solutions today to schedule an inspection or develop a customized prevention plan before the next cold snap.