It’s a twice-a-year chore, lugging the big pots of houseplants from their summer quarters in the garden to their winter homes in the basement or by a large window upstairs. Then for weeks afterwards, there’s an additional chore: vacuuming up the brittle corpses of pill bugs and sowbugs littering the floor.
When you bring your plants in as cool weather arrives, lots of other things can come in, too: crickets, millipedes, spiders, earthworms, occasionally even small frogs or lizards. Those little grey scurriers we commonly call sowbugs and roly-polies — the British call them woodlice, and scientists use the more scientific name “isopods” — are probably the most abundant.
The isopods in our gardens and woodlands are crustaceans — terrestrial relatives of crabs, lobsters, shrimp and crayfish. All of these species have the same crunchy external skeletons; in isopods, this exoskeleton is divided into a series of plate-like segments that cover the entire dorsal surface. Unlike their larger aquatic cousins, however, isopods lack legs specialized with claws or for swimming. Their seven pairs of legs are all pretty much alike; indeed, isopod in Greek roughly means “equal foot.”
There are many species of aquatic isopods, too. Of the 10,000 or so species of isopods in the world, about half are terrestrial and half are aquatic. All of them have gill-like structures called pleopods that allow them to absorb oxygen; they only work in water or moist environments, so this limits our land isopods to moist environments like under the pots you’ve left sitting on the ground all summer. Here they feed mostly on decaying plant litter and fungal threads, although they aren’t above snacking on lower leaves of houseplants where they touch the soil. Lift the pot in the fall and you’re likely to send a dozen or more of these gray, half-inch denizens of the damp scuttering into the mulch.
Some of these isopods likely made their way up into the soil of your pots, and as the dirt dries out in your house, they venture out in search of moisture. If they don’t find any, their pleopods stop working, and the isopods literally suffocate.
Isopods come in two flavors. The ones we commonly call pill bugs or roly-polies can roll themselves into tight little balls like armadillos. This behavior — which goes by the tongue-twisting term “conglobation” — protects them from many predators and also serves to conserve moisture when their environment dries out.
Most of our land isopods can’t curl up this way, though. They’re the ones we call sowbugs and woodlice (although the latter term is sometimes used for pill bugs, too). They have the same segmented body plans, but the butts of sowbugs sport two very obvious antenna-like appendages called uropods that seem to perform some sensory functions and secrete chemicals used in defense and communication with each other. Pill bugs, in contrast, have short uropods — so short that they’re hard to see — but easy to tuck when the pill bug rolls up.
Here in the DMV we have very few native pill bugs or sowbugs. Of Maryland’s 22 or so recorded species, 17 are immigrants to North America, mostly from Europe, including all the ones you’re likely to see in local urban and suburban habitats.
The roly-poly species you will likely encounter are in the aptly named family Armadillididae, from the animal namesake armadillo. These include the common pill bug and the nosy pill bug, the latter named not because it is especially inquisitive but because it has a nose- or horn-like bump on its head. The common pill bug is mostly dark gray with scattered yellow splotches and can roll into a very tight ball. The nosy pill bug can’t quite make a perfect sphere when it conglobates and is typically lighter colored with distinct bands or stripes of yellow or cream running down its upper side.
We have many more kinds of sowbugs to choose from, but most are difficult for casual observers to distinguish. Two you are likely to spot are the common rough woodlouse and common striped woodlouse. The rough woodlouse is usually pale grey with indistinct lighter mottling, while the striped woodlouse is named for the distinct dark stripe running down the center of the back.
The life histories of pill bugs and sowbugs are very similar: Adults overwinter and mate in the spring. The females have special appendages called a marsupium on their undersides to carry their eggs, and baby isopods look for all intents and purposes like adults. When you uncover a colony of isopods under a rock or log, it’s normal to find a range of sizes and ages together.
If your plants stay consistently moist over the winter in the house, pill bugs and sowbugs may just hunker down inside the pots until you put the whole enterprise back outside in the spring. You can toss any living isopods you find out into the garden. Or you can keep them as pets.
Indeed, isopods are increasingly popular in the pet trade: They’re easy to take care of, needing only a moist terrarium with pieces of bark or rock to hide under. They do fine on a diet of leaf litter supplemented from time to time with fish flakes or dried shrimp meal for added protein.
Isopod enthusiasts have discovered in the wild or selectively bred some very colorful land isopods with fanciful names like Powder Blue, Powder Orange, Zebra, Dairy Cow and Panda King.
Even these exotic-looking isopods prefer lurking under cover, so don’t expect them to come out to play. On the other hand, they don’t need a litter box or daily walks.
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Rick Borchelt is a science and natural history writer, field naturalist, and garden and botany enthusiast. Reach him with questions about this column at rborchelt@gmail.com.