Fall is the busiest time of year for pest calls in Illinois, and it is not close. From September into November, box elder bugs, Asian lady beetles, cluster flies, brown marmorated stink bugs, and rodents all start looking for a warm place to wait out the cold. Your house is a good candidate.
The good news is that most of this is preventable with an afternoon of work before the first hard frost.
Seal the obvious gaps
Walk the outside of the house and look at every spot where something passes through the wall: pipe penetrations, the dryer vent, the gas line, the spot where the cable comes in. A mouse fits through a gap the width of a pencil. Seal these with steel wool packed tight and then caulk or an exterior sealant over it.
Check the weatherstripping on every exterior door, including the garage. Hold your hand near the bottom corner of a door on a windy day. If you feel air, a box elder bug can get in there too.
Cut the moisture
Damp basements and crawl spaces draw pests in fall the same way they do in summer. Clear the gutters so water moves away from the foundation. Fix the downspout that dumps next to the wall. If the crawl space smells musty, that is a sign worth following up on.
Know what is coming
Cluster flies and box elder bugs are nuisance pests. They do not damage the house and they do not breed indoors, but they collect in wall voids and show up on warm windows all winter. Rodents are the bigger concern. A mouse that gets in during October will not leave on its own, and it will chew and contaminate while it stays.
If you are already seeing activity, sealing alone will not solve it once pests are inside the walls. That is the point to bring in a licensed exterminator who can treat the harborage and close the entry points properly.