A. From your description, it sounds like you have been the victim of a millipede migration. Millipedes, sometimes called “thousand-leggers” are normally found outside, often around building foundations that provide lots of mulch, ground cover, or other damp shelter. Millipedes can build up to high numbers in heavily watered lawns or wooded yards with grass piles or accumulations of rotting leaves.

Millipedes like secluded damp areas where they feed on decaying plant material. Sometimes though, when conditions change outside, millipedes migrate in large numbers looking for better conditions. If they can find openings, they will enter garages, basements, crawl spaces, and ground floor living areas. They often end up in the dampest areas indoors, near washing machines or sump pumps. No one knows for sure what causes these millipede migrations but it seems to be a combination of temperature changes and too much or too little humidity.

Millipedes don’t bite. When disturbed or threatened, some millipedes give off a foul-smelling defensive fluid. They don’t damage furnishings and they don’t reproduce indoors. In fact, as you’ve seen, they don’t live long indoors. What killed them is no doubt dryness. Even though you might think that your basement is damp, it’s not nearly as damp as the outside areas where millipedes are normally found. Millipedes require a high level of humidity and usually die shortly after they have found their way inside. You can speed this along by using fans or heat to dry out basements, crawl spaces and other areas where you find millipedes.

If millipedes have become an indoor problem, there are steps you can take to reduce the numbers of millipedes in your yard. Keep outside conditions drier by closely mowing lawns, dethatching lawns, watering early in the day so grass dries out before nightfall, removing rotting wood, piles of leaves or grass clippings near the house, and pulling mulch back from the house 12 inches. A pest control professional can also apply an outside pesticide barrier treatment around the foundation and treat around doorways and other ground level openings where millipedes are entering.