Sowbugs, Pillbugs, and Millipedes: The Harmless Bugs You'll See After It Rains

The morning after the first real rain of the season, you slide open the patio door and there they are: a scatter of little gray armored bugs across the concrete, a few rolled up tight like ball bearings, and something long, brown and segmented curled into a neat spiral on the doormat. By the second storm, some of them have made it into the garage. By the third, someone in the house has found one in the bathtub and wants to know whether this is the beginning of something.

These three get lumped together because they show up together, in the same places, for the same reason. They're quite different animals, and only two of them are closely related.

Pillbugs are the ones kids pick up. They are small, gray to slate-blue, oval and armored, with a body made of overlapping plates, seven pairs of legs and short antennae. Touch one and it rolls into a near-perfect ball, which is where the names roly-poly and pill bug come from. They aren't insects at all. They are crustaceans, land-living relatives of shrimp and crabs, and like their relatives they breathe through gill-like structures that have to stay moist.

Sowbugs look almost the same at first glance, and most people never realize there are two different animals. The difference is that a sowbug can't roll into a ball. It is a little flatter, often more brownish or mottled, and has two small tail-like points sticking out from its back end. Disturb one and it runs instead of rolling. Biologically it belongs to the same group as the pillbug, with the same need for moisture and the same harmless habits.

Millipedes are long, slow and cylindrical, brown to nearly black, with a hard, shiny, segmented body and a fringe of tiny legs underneath. Look closely and you will see two pairs of legs on most body segments, one of the quickest ways to tell them from centipedes. When threatened, they curl into a tight, flat spiral and stay put. Many species also release a defensive fluid that smells unpleasant and can stain or irritate skin, so move them with a paper towel or a dustpan, and wash your hands if you do handle one.

Centipedes get confused with millipedes, but they are a different animal with a different temperament. They are flattened, fast and long-legged, with one pair of legs per segment, and they are predators that hunt other small creatures. Larger ones can give a painful bite if handled. They like damp places too, so you may see them in the same spots, but they move very differently: a millipede trundles, a centipede sprints.

All three live in the top layer of soil and in the damp material on top of it: mulch, leaf litter, compost, and the undersides of stones, boards, pots and pavers. They need moisture, but they need air too, and fully saturated ground is more water than they can handle. When a storm soaks the soil, they move up and out, looking for somewhere damp but not drowned.

In practice, that often means toward your house. A slab foundation, a concrete patio, a garage floor and a door threshold are all a little higher and drier than the soaked bed beside them, and they are the first hard surfaces a pillbug crawling out of the mulch is likely to reach. Humid nights after rain also let all three travel farther than usual without drying out, which is why you tend to find them in the morning rather than watching them arrive.

Millipedes add a twist. In some years they move in large numbers in the fall, occasionally in the hundreds, and those wanderings often get going with the first rains. Nobody has a tidy explanation for why some years are so much bigger than others, but heavy moisture after a long dry spell seems to be part of it.

In Southern California, irrigation does the same job for most of the year. An overwatered bed against the house, a sprinkler head that soaks the wall, a drip line that runs too long or a leaky hose bib can keep a strip of soil wet enough that these three never leave it. If you see them in numbers in the middle of a dry summer, go looking for the water source.

To your house, no. None of these three chew wood, eat insulation, damage wiring, undermine foundations or harm drywall. They have no interest in the structure at all. They eat decaying plant material, which is why they live in mulch and leaf litter and why gardeners tend to think of them as part of the cleanup crew.

To your garden, occasionally, a little. Pillbugs and sowbugs will nibble seedlings, tender new growth and soft fruit lying on damp soil, like strawberries or squash resting on the ground. Millipedes can do the same in large numbers. It is usually minor and limited to things in direct contact with moist soil.

To your health, not a concern. They don't bite, sting or carry disease. The only practical issue is the millipede's defensive fluid, which is unpleasant to touch.

Sweep them up, vacuum them, or carry them back outside. That's all there is to it. There is no reason to spray a pillbug. A retail spray doesn't fix the damp soil that sent them your way, the next storm will send more, and you would be adding pesticide to your house for an animal that was going to dry out on its own.

None of this needs to be perfect. The goal isn't a yard without pillbugs, which would be neither possible nor desirable. It is a dry buffer around the house, so the ones flushed out by the next storm stop at the edge of the patio instead of the kitchen.

A burst of pillbugs and millipedes after the first storm, and a trickle after the next few, is normal, and not something you need a pest control company for. What deserves a closer look is a pattern: live ones turning up indoors week after week, in the same room every time, or numbers that don't drop off once the weather dries out.

In those cases, the bugs are pointing at something. It could be a leak under a sink or behind a shower, a slab that wicks moisture, a crawl space that never dries out, or drainage that sends water toward the house instead of away from it. Older raised-foundation homes in particular can have damp subareas nobody has looked at in years.

The same damp conditions attract the pests that do matter: subterranean termites need moist soil, Argentine ants nest in it, and earwigs, silverfish and crickets all gather where it's wet. So when pillbugs are the only thing you've noticed so far, think of them as the early warning. A general pest inspection looks at the exterior perimeter and the places water collects, and recurring perimeter service, usually quarterly, is how we keep the moisture-loving insects that come along with it in check.