Most spiders you’ll spot inside a Michigan home are harmless. The state is home to over 500 spider species, but only a handful regularly show up indoors, and just one, the northern black widow, poses any real medical concern. Here’s what you’re likely sharing your house with and what to do about it.
Yellow Sac Spiders
Yellow sac spiders are probably the most common spider found inside Michigan homes, and they’re the ones most likely to bite you. Two species live in the state. One is a light yellow to cream color, while the other has a faint greenish tinge to its abdomen. Both are small, with females measuring 5 to 10 millimeters and males slightly smaller. Look for a slightly darker stripe running lengthwise down the abdomen and dark brown tips on the legs and jaws.
These spiders don’t build webs to catch prey. They’re active hunters that roam around at night looking for insects. During the day, they hide inside small silken tubes or sacs they spin in the corners of walls and ceilings, under window sills, or behind picture frames. One of the two species found in Michigan almost exclusively lays its eggs indoors, so it’s truly a year-round resident of your house.
Bites happen when a sac spider gets trapped between your skin and bedsheets, clothing, or shoes during its nighttime wandering. The bite stings and can leave a red, irritated mark, but it’s not dangerous. Applying ice and keeping the area clean is enough for most people.
Cellar Spiders
If you’ve ever walked into a basement or garage corner and run face-first into a messy, irregular web with a pale, spindly spider dangling upside down, you’ve met a cellar spider. Often called “daddy longlegs” (not to be confused with harvestmen, which aren’t spiders at all), cellar spiders have extremely long, thin legs relative to their small bodies. They thrive in the dark, undisturbed corners of basements, crawl spaces, and garages where humidity is higher. Their webs accumulate quickly and look untidy, but these spiders are completely harmless to people and actually prey on other spiders and insects.
Wolf Spiders
Wolf spiders are the ones that startle people the most. They’re large, fast, hairy, and tend to sprint across the floor right when you’re walking barefoot at night. Michigan has many wolf spider species, and they range from about the size of a nickel to the size of a half-dollar including their legs. They’re typically brown or gray with darker markings that help them blend into leaf litter and soil.
Like yellow sac spiders, wolf spiders hunt without webs. They live primarily outdoors in leaf litter, under stones, and in garden beds. They don’t want to be in your house. Most indoor sightings happen in fall when dropping temperatures push them toward warm foundations, and they wander inside through gaps under doors or cracks in the foundation. A wolf spider can bite if cornered, but the bite is comparable to a bee sting and carries no lasting harm.
Cobweb Spiders
The classic “house spider” in Michigan is the American house spider, a member of the cobweb spider family. These are small, brownish, round-bodied spiders that build the tangled, irregular webs you find in window corners, behind furniture, and along ceiling edges. They’re timid, rarely bite, and spend most of their lives sitting motionless in their webs waiting for small flies and gnats. If you disturb their web, they’ll usually play dead or drop to the ground and curl up. Multiple cobweb spider species live in Michigan, and they’re among the least concerning spiders you’ll encounter.
Northern Black Widows
Michigan does have black widows, specifically the northern black widow. They appear to be more common in the western Lower Peninsula. Unlike the classic image of a solid red hourglass, the northern black widow’s hourglass marking on the underside of the abdomen is incomplete or split in the middle. These spiders also have a series of red spots running along the top of the abdomen, and many display lateral white stripes.
Northern widows prefer to stay outdoors in old stumps, hollow logs, under fallen fence posts, in abandoned animal burrows, or in brush piles. They sometimes turn up in the corners of sheds and crawl spaces. Their webs are irregular meshes of strands, and the spider hangs upside down within them. Finding one inside a living space is uncommon. A black widow bite causes muscle cramping, pain and burning at the site, headache, nausea, and increased sweating, typically within an hour. Children and older adults are at higher risk of serious reactions. If you suspect a black widow bite, get medical attention promptly.
Brown Recluse: Rare but Present
Many Michigan residents worry about brown recluse spiders, but the state is outside the species’ native range. According to Michigan State University Extension, there are rare, isolated populations, and the vast majority of Michiganders will never come within a half-mile of one. Brown recluses have been found in 10 Michigan counties, including Genesee, Lenawee, Washtenaw, Oakland, Wayne, and a handful of others. None of these spiders walked to Michigan on their own. Every confirmed find arrived in suitcases, boxes, or other transported goods.
At least one reproducing population, found in a single-family home in Tecumseh in Lenawee County, has been eradicated. The point is that while a brown recluse sighting is possible, it’s genuinely unusual. The spider you found in your Michigan basement is almost certainly something else.
Why You See More Spiders in Fall
Spider sightings inside Michigan homes spike from late August through October. Cooler temperatures and shorter days push spiders toward warm structures, and many species are at their largest by late summer after feeding all season. The bigger spiders that survive into the cooler months are simply more noticeable. You’re also spending more time indoors yourself, which increases the odds of crossing paths. It’s not that your home suddenly has a spider problem. You’re just seeing spiders that were nearby all along.
Keeping Spiders Out of Your Home
The most effective long-term approach is sealing entry points. Spiders follow insects inside, and both get in through the same gaps. Start with exterior doors: install door sweeps and check for light filtering under the door while lying on the floor. Even gaps of 1/16 inch are enough for spiders and insects to squeeze through. Pay special attention to the bottom corners of door frames, which are the most common entry points.
Caulk cracks around windows, doors, and fascia boards with silicone or acrylic latex caulk. Seal utility openings where pipes and wires enter the foundation, around outdoor faucets, gas meters, dryer vents, and cable lines. These can be plugged with caulk, expandable foam, steel wool, or copper mesh. For garage doors, fit a rubber bottom seal (vinyl seals don’t hold up well in Michigan winters). Repair any torn window or door screens, and install quarter-inch wire mesh over attic, roof, and crawl space vents.
Inside the home, reduce clutter in basements, garages, and storage areas. Spiders set up in undisturbed spaces, so regular cleaning and vacuuming corners and behind furniture disrupts their habitat. Outdoor lighting near doors attracts the insects that spiders feed on, so switching to yellow or sodium vapor bulbs near entry points can reduce the food supply that draws spiders close to your house in the first place.
If sealing alone isn’t enough, an exterior barrier treatment with a long-lasting insecticide applied around the base of doors, foundation vents, utility openings, and along a two-to-six-foot band around the foundation perimeter can supplement your efforts. Spray two to three feet up the foundation wall and underneath siding for best coverage.