It’s a dark, overcast night as the horde emerges from the nearby woods. There’s no real coordination, but thousands of them—perhaps tens or even hundreds of thousands—seem to wander aimlessly through the yard.  Some approach the darkened farmhouse and a few even manage to make it inside…

If this were and episode of The Walking Dead, the protagonists would be in a tough spot, but we’re not talking about zombies in this case.  Instead, the topic is millipedes, which have been surprisingly abundant this summer in parts of the Upper Midwest.

Most everyone is familiar with millipedes.  They technically aren’t insects, but they are related as demonstrated by their segmented legs and “crunchy” exoskeleton (both are types of arthropods).  These multi-segmented, worm-like creatures can be common in damp areas and are perhaps most recognizable by their slow walk and their habit of curling into a spiral when disturbed.

Unlike the zombies portrayed in on TV, millipedes are really quite harmless.  Some millipede species have been documented as minor crop pests, but in the grand scheme of things, I mostly think of millipedes as being beneficial detritivores.  Millipedes feed on decaying plant materials and they return nutrients to the soil.  Their feeding also breaks down plant materials into smaller pieces, allowing microbes to more easily assist in the decomposition process.  Millipedes can be especially common in damp locations with abundant plant materials: compost piles, rich soil with high organic content, mulch beds, wooded or prairie areas, CRP land, lawns with a heavy thatch layer, and similar.

While mostly beneficial, millipedes can occur in very high numbers under the right conditions and can be a nuisance when they seem to suddenly appear in yards and homes.  Hopkin and Read’s The Biology of Millipedes (1992) describes situations where massive millipede hordes have covered acre after acre of farmland or stopped trains, quite literally, in their tracks.  The Midwest does see large masses of millipedes on occasion and it was a particularly busy year at the UW Insect Diagnostic Lab for calls about these creatures.

The reasons behind millipede mass migrations aren’t fully understood, but moisture is often noted as a common factor.  Other potential reasons range from general weather patterns to habitat disruption, competition, and reproduction.  When millipedes do move about, many species shun the sun and prefer to move at night or during very overcast days.  When they encounter a building, millipedes can sneak inside, although this is really accidental—it’s too dry for them to survive indoors and they typically die within a day or two.

Millipedes can be frustrating when mass migrations occur as there’s not much that can be done to completely stop them.  It’s not uncommon to have cases where hundreds or thousands of of millipedes crawl onto the foundation or siding of a home every night.  If they mostly stay outside, that’s one thing, but this summer I’ve had multiple cases where large numbers of millipedes (hundreds) had snuck under a building’s siding and then rained down through ceiling light fixtures.  This sounds like something out of a sci-fi film, but if you were trying to sell your home it could be a real-life nightmare scenario.  In such cases, there simply isn’t any way to make the millipedes magically disappear.  Insecticides may be tempting but only help to a certain extent because more millipedes can simply show up the next day.

If you’re staring down a millipede horde, one of the most important approaches is physical exclusion.  Inspecting the exterior of a home and physically sealing up cracks, crevices, and other potential entrance points with caulk, expanding foam, or new weather stripping can be a chemical free, long-term solution to at least keep millipedes outdoors.  Because millipedes prefer damp areas with decaying plant material, keeping landscape pants, fallen leaves, and thick layers of mulch away from the foundation of a home could also help reduce hiding areas for millipedes.

Luckily, millipede mass migrations eventually run their course and quiet down on their own.   This year, I saw a spike in millipede reports starting in mid-June and running into early August before subsiding.