Colorado homes deal with a predictable rotation of uninvited insects throughout the year. The specific bug in your house likely depends on the season: miller moths swarm indoors during late spring migration, boxelder bugs and western conifer seed bugs appear in fall as they seek winter shelter, and spiders turn up year-round in undisturbed corners. Here’s how to identify what you’re seeing and what to do about each one.
Miller Moths in Late Spring
If you’re finding dozens (or hundreds) of grayish-brown moths fluttering around your lights between May and early July, you’re dealing with miller moths. These are the adult form of the army cutworm, and every year they migrate from Colorado’s eastern plains up to the mountains to feed on wildflowers. Your house just happens to be in their flight path.
During the day, miller moths look for dark, tight spaces to rest. Small cracks around doorways, garages, and window frames make perfect hiding spots. The problem is that cracks leading to the outside of your home often continue into the living space, so a moth looking for a daytime shelter can take a wrong turn and end up in your kitchen. At night they emerge to resume flying and feeding, which is when they mob porch lights and lamps.
The good news: miller moths in your home do not feed, do not lay eggs, and do not damage furnishings or food. They will eventually find their way back outside or die without reproducing. The only real nuisance, beyond the annoyance, is that when large numbers die indoors the fat in their bodies can turn rancid and produce a noticeable smell. Reducing outdoor lighting at night during the migration period is the single most effective step to keep them from congregating near entry points. The return flight from the mountains happens in late September and is usually much shorter and less dramatic.
Boxelder Bugs and Conifer Seed Bugs in Fall
Once temperatures drop in September and October, a different crew shows up. Boxelder bugs are the flat, black-and-orange insects you’ll find clustered on south-facing walls soaking up afternoon sun. Western conifer seed bugs are longer, brownish, and shaped a bit like a shield, with a distinctive leaf-shaped flare on their back legs. Both species are looking for the same thing: a warm, protected space to survive the winter so they can reproduce in spring.
Neither one bites, stings, or damages your home. Boxelder bugs feed on the seeds of boxelder and maple trees outdoors, and conifer seed bugs feed on pinecones. Once inside, they’re essentially hibernating. They don’t breed indoors or infest food. The main complaints are their sheer numbers and, in the case of conifer seed bugs, a mildly unpleasant smell if you crush them. A vacuum cleaner is the easiest removal method for both.
You’ll often notice them on warm winter days when sunlight heats an interior wall and tricks them into thinking spring has arrived. They emerge disoriented, bumble around a room, and then disappear again. This cycle can repeat from October through March.
Spiders You’ll Actually Find Indoors
Most spiders in Colorado homes are harmless. Common house spiders, funnel weavers, and cellar spiders (the delicate, long-legged ones in basement corners) are the usual residents. They eat other insects and generally stay out of your way.
The one medically significant spider established in Colorado is the western black widow. Females are shiny black with the characteristic red or orange hourglass marking on the underside of the abdomen, though this shape varies considerably. It can appear as two separate dots, a rectangle, or a faded marking that’s hard to see. Males are much smaller, only about an eighth of an inch long, and are light brown or gray with banded legs. You’re most likely to encounter black widows in undisturbed areas: garages, window wells, woodpiles, and storage boxes close to the ground. Bites are almost always from females, since males are smaller and their fangs are less likely to penetrate skin. Symptoms include muscle pain, chest tightness, abdominal cramping, nausea, and difficulty breathing. Fatal bites are extremely rare but possible in young children and the elderly.
A common concern in Colorado is the brown recluse, but this spider is extremely rare in the state. Colorado State University’s records include only one confirmed specimen collected in the state, from Boulder County in 1996. The few sightings that do occur are almost always hitchhikers transported from infested areas in the South or Midwest. Only two locations in Colorado, one in Pueblo County and one in Prowers County, have had semi-credible reports of an established population, and both were isolated to a single building. If someone tells you they found a brown recluse in their Colorado home, it’s far more likely a woodlouse spider, a yellow sac spider, or another brown look-alike.
How Bugs Get Inside
Colorado’s dry climate and wide daily temperature swings create the perfect incentive for insects to seek shelter, and most homes have more entry points than you’d expect. A gap of just 1/16 of an inch is enough for insects and spiders to squeeze through. That’s thinner than a credit card. A 1/4-inch gap, roughly the diameter of a pencil, lets mice in too.
The most common entry points are the gaps under exterior doors, cracks where siding meets the foundation, openings around utility pipes and wires, foundation vents, and unscreened weep holes in brick. Garage doors are especially leaky. Older Colorado homes with original weatherstripping tend to have larger gaps than newer construction, but even new builds develop settling cracks within a few years.
Keeping Bugs Out
Sealing your home is the most permanent solution. Start with door sweeps or thresholds on every exterior door. Lie on the floor and look for daylight under the door. If you can see light, insects can get through. Apply caulk along the bottom outside edge and sides of door thresholds, around foundation vents, and where utility lines enter the house. Silicone-based caulk lasts longer than latex in Colorado’s UV-intense sun.
For windows, check the seal where the frame meets the siding, not just the weatherstripping between the sash and frame. Expanding foam works well for larger gaps around pipes and conduit, but trim the excess flush so it doesn’t degrade in sunlight.
If sealing alone isn’t enough, an exterior barrier treatment with a liquid insecticide can supplement your efforts. Apply it at the base of all exterior doors, around garage and crawl space entrances, foundation vents, and utility openings. Treating a two-to-six-foot band along the ground around the foundation perimeter, and two to three feet up the foundation wall, creates a chemical barrier that reduces the number of insects reaching your entry points. This works best when applied in late summer before fall invaders start looking for shelter, and again in spring before miller moth season.
For miller moths specifically, the most effective prevention is simply turning off or dimming exterior lights during the May-June migration. Switching to yellow or amber bulbs reduces attraction significantly compared to white or blue-toned lighting. If moths are already inside, catching them in a bowl of soapy water placed near a lamp overnight is more efficient than chasing them one at a time.