BUG OF THE MONTH: February 1996

Sowbugs Revisited

Isopod Crustaceans, superfamily Oniscoidea

Copyright © 1996 by  Louise Kulzer

This article originally appeared in Scarabogram, Feb. 1996, New Series No. 190, pp. 2-3.

I trust you've had a chance to observe a few sowbugs since December's article and have grown accordingly in grace and wisdom? Remember, if it rolls up in a ball it's a pillbug. If not, it's a sowbug or potato bug, depending, respectively, on whether or not you live in Lynnwood. Last time we found out some interesting stuff about sowbugs. But there's more!

pillbug drawings and diagram
Armadillidium vulgare: bitter pill?

The common garden pillbug Armadillidium vulgare may live up to four years. Not bad for a crustacean, huh? And during those four years, a basic challenge faced by these formerly aquatic critters is the threat of desiccation. When not foraging at the surface, isopods live in the soil. And in the soil, the relative humidity is quite high, typically above 70% in the chaparral of California (Paris 1963). You can see why they might like it here!

Paris did a magnificent study of many aspects of pillbug behavior and environment in California coastal grassland (San Francisco Bay area). He found that during the rainy season, pillbugs were almost all in the upper 4 cm (1.7 inches) of the soil. As the soil dried out in late summer and fall, the isopods descended deeper into the soil and were more often found in aggregations. During their time at the surface, chaparral isopods would tend to aggregate around plants known to collect and condense water. This population also was much more active at the surface at night. These measures all serve to conserve water and minimize the potential for desiccation.

OK, so you're probably thinking these guys must love rain? Well, that depends. Remember, sowbugs respire through special appendages on the underside of the abdomen called pleopods. If the pleopods become waterlogged, the sowbug dies. So, although desiccation is always a threat, drowning by waterlogging is also a concern. Paris also did a laboratory study of pillbug response to short periods of waterlogging. He found that waterlogging for an hour or less was not lethal. However, only two of ten individuals survived waterlogging for 7 hours. Waterlogging for 2 and 4 hours caused lesser mortality. Based on these results and field observation, Paris concluded that soil flooding from winter storms, which could last one to three or more days, was a mortality factory in natural populations. He observed that during heavy rains, isopods would come to the surface and seek refugia: things like leaf shelters, rock overhangs, and the sides of culverts. It's not hard to believe waterlogged soil in Seattle gardens is a very real environmental challenge sowbugs face.

It would be an interesting backyard research project to keep records of sowbug numbers and weather statistics. One could correlate population size with a number of weather related variables and see how important each was in explaining changes in population size. (Remember, population size can be easily estimated using capture/recapture techniques. More on that in a future article.)

What about natural enemies? Well, recall that most of our isopods are introduced. Carabid beetles will eat them, but not many other critters will bother. Apparently centipedes and spiders don't like them, perhaps due to secretions of special glands (Gorvett 1956). [On the other hand, one abundant basement and woodpile spider, Steatoda grossa, loves isopods and virtually specializes on them in some situations. --editor]  All the more promise for our weather/population study, since fewer confounding variables will exist. (Maybe we could get a grant, one of the two the National Science Foundation is able to give out next year?)

There are two more kinky things about isopods I just have to mention. The first is the phenomenon of sperm-mixing. Females often carry sperm from more than one male. Therefore broods of one female (from those marsupial pouches, remember?) can be multi-paternal (Sassaman 1978).

On the other hand, as mentioned by Scarab Crawford in his species key, some species reproduce parthenogenically (the virgin-birth thing). Eggs develop without the benefit of fertilization. Now that's what I call playing all the odds. With few predators and a "fast, fluid and flexible" sex life, it's no wonder sowbugs are so successful. Indeed, aside from people, the weather may be their only significant survival challenge!

Show some respect the next time you pick up a flowerpot and see sowbugs lumbering away.


Gorvett, H. 1956. Tegumental glands and terrestrial life in woodlice. Proceedings of the Zoological Society of London, 126: 291-314.

Paris, Oscar. 1963. The ecology of Armadillidium vulgare (Isopoda: Oniscoidea) in California grassland: food, enemies and weather. Ecological Monographs, 33(1): 1-22.

Sassaman, C. 1978. Mating systems in porcellionid isopods: multipaternity and sperm mixing in Porcellio scaber Latr. Heredity, 41: 385-397.

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