East river and west river are different spider country
South Dakota is often treated as one place for pest purposes, and for spiders that is a mistake. The eastern half is prairie and farmland, with deep soil, shelterbelts, and towns built around full basement housing. Spider habitat there is largely structural: basements, window wells, grain and machine buildings, and the insect populations that gather at yard lights. West of the Missouri the picture changes. The ground is rockier, the rainfall lighter, and the Black Hills add stone outcrops, rubble, and south facing slopes that hold heat. That kind of terrain suits sun loving, dry adapted spiders in a way the eastern prairie does not, and the CDC's description of black widows being most common in southern and western states is worth keeping in mind when you are turning over rocks near Rapid City. Practically, the advice diverges only a little. In the east, spend your effort on the basement and the outbuildings. In the west, add gloves and a habit of looking before reaching into rock piles, woodpiles, and stone walls.
Web builders and hunters look different for a reason
Nearly every South Dakota spider falls into one of two groups, and being able to sort them on sight removes a lot of anxiety. Web builders sit still and wait. Cellar spiders hang in loose tangles under joists with long thin legs that make them look bigger than they are. Cobweb spiders build small messy sheets in corners and window wells. If you find a spider sitting in a web, it is almost certainly one of these, it is not going anywhere, and it is harmless. Hunters do not build webs at all. Wolf spiders and their relatives are stocky, fast, and often striped brown, and they run across floors at night hunting insects. They are the ones that startle people, and they are also harmless. They are indoors because prey is indoors, not because they nested in your wall. The practical difference matters. Web builders respond to vacuuming and clutter removal. Hunters respond to door sweeps and to reducing the insects at your exterior lights.
The August afternoon that decides your September
There is a single high value window for spider work in South Dakota, and most people miss it by about four weeks. It falls in the last week of August, right before spiders reach adult size and start moving. Work it in a set order. Start in the basement with a shop vacuum: joists, sill plate, stairwell, and the corners behind the furnace and water heater. Then the window wells, which by August hold a season of leaves, dead insects, and silk. Then the garage ceiling and door tracks. Finally the shop or shed if you have one, sweeping ceiling corners and the tops of door frames. One pass at this point removes the population and, more importantly, the egg sacs, so you are subtracting from next year at the same time. Doing the identical work in late September removes fewer spiders and no egg sacs that have already hatched. Finish by walking the outside with a caulk gun and a new door sweep. That combination handles most of what people call about in October.
Rock piles, shelterbelts, and the yard side of the problem
Indoor spider control in South Dakota fails when the yard keeps restocking it, and the yard usually does. Stacked scrap lumber, unused pallets, piles of fieldstone, old tires, and rows of stored equipment all sit against the ground, hold heat and moisture underneath, and shelter insects, which is everything a spider needs. Shelterbelts play a similar role. They are valuable windbreaks and they also concentrate insects, which concentrates spiders along the tree line and then along the nearest wall. Nobody should remove a shelterbelt over spiders, but knowing it is a source explains why the north side of a house is always the busiest. The items worth moving are the ones close to the foundation. Get firewood on a rack away from the wall, move rock and lumber piles out to a corner of the lot, and keep a strip of bare ground or short grass against the siding. In the drier western part of the state, this same cleanup is also the best protection against finding a widow with a bare hand.
Keeping bite fears in proportion
The gap between how frightening spiders feel and what they actually do is wide, and the research is straightforward about it. Start with the misattribution problem. UC Riverside found that more than 95 percent of purported brown recluse bites were reported from urban areas outside the spider's range entirely, meaning the wound had another cause. In neighboring states and beyond, roughly 20,000 spiders submitted by concerned California residents were examined and none were brown recluses. People are very confident and very often wrong. Then consider the one spider that does bite meaningfully. University of Florida entomologists recorded 2,168 self-reported black widow bites across the United States in 2010, with 892 treated in health care facilities. That is the whole country in a year, concentrated in the states where widows are common. So the sensible posture in South Dakota is calm. Wear gloves when you move rock, wood, or long stored boxes, keep a light with you in a crawlspace or shop, and take any worsening wound to a clinician rather than to the internet.