This hits home for me because I am having this problem. Those who live next to woods will typically have more because they come in large numbers from the duff in the woods.
Millipedes are just a nuisance, even when they come in your home. They do not bite, sting or transmit diseases, nor do they infest food, clothing or dry, structurally-sound wood.
There are several species of millipedes that vary in size and color, but the most common species that invades buildings is the “garden millipede,” which is brownish-black and about 1 inch long.
Each body segment has two pairs of very short legs. When disturbed, millipedes often curl up into a “C” and remain motionless. They crawl slowly and protect themselves by secreting a cyanidelike compound that has an unpleasant odor. Some people confuse millipedes with centipedes, which look somewhat similar. Centipedes have only one pair of legs per body segment, and the legs are longer than those on millipedes. Centipedes also move quicker than millipedes.
Millipedes pass through the winter primarily as adults and lay eggs in the soil in the spring. Millipedes are attracted to dark, cool, moist environments that are rich in organic matter such as compost piles, heavily mulched shrub or flower beds, rotting logs, the soil under logs and stones or rotting leaf litter in the woods.
Millipedes are scavengers, feeding primarily on decomposing vegetation. Major nuisance problems typically occur when conditions become too hot and dry and the millipedes move to find moisture, or in this case, when it’s too wet and saturated soils force them to the surface and higher ground.
Millipedes can also migrate in the fall, presumably in the search of overwintering sites. All of these activities result in millipedes invading buildings. Common points of entry include door thresholds (especially at the base of sliding-glass doors and garage doors), expansion joints and through the voids of concrete block walls. Frequent indoor sightings usually mean there are large numbers breeding outdoors.
Fortunately, millipedes don’t survive indoors more than a couple of days (more likely just a few hours) because they can’t find suitable moist conditions. So most of the ones you find indoors already might be curled up and dead.
Your main emphasis for control should be first placed on reducing conditions and access points favorable to millipede invasions.
If you live by the woods, there is not a lot you can do. If we ever return to drier conditions, you can do a few things around your house to discourage them. Eliminate trash piles, rocks, boards, leaf piles and similar materials from around the house. This does not mean you can’t have mulch around the foundation, just keep it a few inches away and don’t let it get too thick.
Prevent water from accumulating near the foundation, in basement walls or in the crawl space.
Keep gutters and downspouts free of debris and use either splash guards or perforated pipe to reduce puddling.
Homes with poor drainage might need to have foundation drains installed or at least the surrounding ground contoured or sloped to redirect surface water away from the foundation.
Reduce the humidity in crawl spaces and basements by providing adequate ventilation, sump pumps, polyethylene soil covers or dehumidifiers.
If it ever quits raining, much of the problem will go away on its own, but until then, keep fighting!