As winter approaches in Denver, many insects and wildlife prepare for the cold by finding places to overwinter—or hide out until warmer weather returns. Unfortunately, homes and businesses provide the perfect shelter. At Animal & Pest Control Specialists, we see an influx of overwintering pest calls each fall and early winter as these creatures slip indoors through cracks, vents, and siding. Understanding which pests are most common in Colorado and how to keep them out can save homeowners from frustration, property damage, and even health risks.
Overwintering pests are insects or animals that enter a state of dormancy during the colder months. Unlike rodents that remain active all winter, overwintering insects typically huddle in hidden spaces like attics, wall voids, and basements until temperatures rise again. While they don’t usually reproduce indoors, they can still become a nuisance and emerge in large numbers during warm winter days or in early spring.
In Colorado, overwintering pests are especially problematic because of the state’s fluctuating winter temperatures. Sudden warm days can wake these insects prematurely, leading to unexpected indoor infestations. For Denver homeowners, this means that pest problems aren’t limited to summer and fall—you may see activity well into January or February when temperatures bounce above freezing for a few days.
Boxelder bugs are among Denver’s most recognizable overwintering pests. They gather in huge numbers on sunny sides of buildings in the fall before migrating indoors through cracks and gaps. While harmless to humans, their droppings can stain curtains, walls, and furniture.
Boxelder bugs are often mistaken for other beetles, but their black bodies with distinct red markings make them easy to spot. In Denver, they are especially common in neighborhoods with boxelder, ash, or maple trees nearby. They cluster on exterior walls to soak up warmth before squeezing inside through siding, vents, or poorly sealed windows.
Prevention tips:
Brown marmorated stink bugs have spread across Colorado in recent years, and Denver homeowners now see them regularly in the fall. These insects release a pungent odor when disturbed, making infestations unpleasant. They tend to cluster in attics, basements, and wall spaces.
One challenge with stink bugs is their attraction to light. Porch lights and window illumination can draw them toward homes at night, increasing the chances of entry. Outdoors, they damage crops and gardens by feeding on fruits and vegetables, which is why many homeowners see them in higher numbers near urban gardens or farms.
Prevention tips:
Cluster flies resemble house flies but behave differently. In fall, they gather in attics and upper wall voids to overwinter, often emerging in large groups on warm winter days. While they don’t spread disease like house flies, their buzzing and sheer numbers are a nuisance.
Cluster flies typically originate outdoors in soil, where they lay eggs that parasitize earthworms. As temperatures drop, they instinctively search for warm overwintering spots. Once inside, they can be nearly impossible to eliminate without professional exclusion, since they prefer small cracks in upper stories, gables, and attics.
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Unlike native ladybugs, Asian lady beetles often invade homes in large swarms during the fall. Once inside, they can stain surfaces with their yellowish secretions and may even bite when handled.
Asian lady beetles were originally introduced in the U.S. for agricultural pest control, but in Colorado, they’ve become one of the most common nuisance overwintering insects. They tend to gather on south- and west-facing walls, entering through siding gaps or window frames. Some homeowners even report allergic reactions to their secretions, making infestations more than just a cosmetic concern.
Prevention tips:
Though not technically “overwintering” in the same way as insects, rodents are among the most serious pests to invade Denver homes in fall and winter. They remain active year-round and can cause significant damage by chewing wires, insulation, and food packaging. According to the Centers for Disease Control and Prevention (CDC), rodents can spread more than 35 diseases directly or indirectly.
Rodent infestations in Denver rise sharply in late fall as temperatures dip below freezing. Mice only need a hole the size of a dime to squeeze through, while rats can chew through wood, plastic, and even thin metal to gain entry. Once inside, they multiply quickly, turning a small problem into a major infestation within weeks.
Prevention tips:
While DIY sealing and cleaning help, overwintering pests often exploit hidden vulnerabilities homeowners miss. At Animal & Pest Control Specialist, our team uses detailed inspection methods to find even the smallest entry points. Professional exclusion involves sealing cracks, securing vents, and applying targeted treatments that discourage pests from settling indoors.
Unlike one-time chemical sprays, professional exclusion is based on integrated pest management (IPM)—a long-term strategy that combines structural repairs, sanitation, and targeted treatments. This approach not only removes pests but also prevents future invasions. Professionals use advanced tools like thermal imaging to find hidden entry points, rodent-proof mesh to block openings, and safe treatments that deter insects without endangering pets or children.
Industry data shows that professional pest exclusion reduces overwintering infestations by up to 70% compared to DIY methods, largely because of specialized tools and expertise. Beyond exclusion, experts also provide safe removal of insects or rodents already inside and sanitation to prevent secondary issues like odors or contamination.
Denver homeowners can minimize pest risks this winter with a proactive approach:
By creating a seasonal checklist and completing these tasks in the fall, you can significantly lower the chances of pests settling in for the winter. It’s also wise to monitor throughout the colder months—watch for droppings, odors, or sudden insect appearances after warm days.
Investing in professional pest exclusion offers more than short-term relief. Over time, homeowners benefit from:
Taking these steps now ensures a pest-free home when temperatures plunge. Instead of dealing with boxelder bugs or rodents in the middle of winter, you’ll enjoy peace of mind knowing your home is protected.
At Animal & Pest Control Specialist, we’re committed to keeping Denver families safe and comfortable year-round. With professional exclusion and removal services, your home can stay secure against overwintering pests this season. For expert help, contact Animal & Pest Control Specialists today.
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