I have them too, on my white colored vinly siding. In the midwest, they come from the fields once the crops are harvested, thier natural habitat having been removed. I tried caulking around vents & pipes, which works to some degree. Also try adding weatherstripping along window screens, which often don't fit tightly to the window frame. The best thing for me was spraying the exterior of my house & windows with a mix of cypermethrin insecticide (powder) & water. This is residual for weeks and may not appear to work instantly, but the number of dead bugs you will see outside is proof it works. What bugs do get in, will die shortly after since they passed through the insecticide on the way in. Also stopped beetles, flies, & spiders effectively (the dead bugs are the proof). You can buy cypermethrin powder from a licensed pest controller or online.