European Sowbug

2024 September 15

Here is a European Sowbug that we found on July 4, 2024. These are not pillbugs, because they can’t roll up into a ball. They are distinguishable from the Common Rough Woodlouse by the fact that European Sowbugs are (a) shinier, and (b) the edges of their armor are pale.

European Sowbugs are Oniscus asellus, and as you might guess from the common name, they are an unintentional import from Europe. It isn’t quite clear when they arrived in North America, but they were probably carried over in things like ship ballast.

They like to hide under things that are in contact with the ground, so it is pretty common to pick up a piece of wood, or a flowerpot, or a rock and find a few hundred of them under there.

They aren’t insects, they are a specie of crustacean, which is wh they have so many legs. They are mainly evolved to squeeze into crevices of various sorts, which is their primary defense against being eaten.

What they eat themselves, is a variety of rotting things. They are decomposers, not crop pests, and they generally don’t get into foodstuffs until they have started rotting enough that humans don’t want to eat them anyway.

We have several types of woodlice around here, all of which arrived from other places fairly recently (within the last few hundred years). Which one is dominant at any time varies a great deal from year to year. When we first moved out here in 1998, we had mostly the Common Rough Woodlouse. Over the next few years, they were gradually superceded by pillbugs, which dominated until probably 2020 or so. Since then, the number of pillbugs has been dropping, and most of what we have now are these sowbugs. I suspect that they will keep cycling from one to the other, depending on which ones are recovering from disease outbreaks or variations in humidity and temperature conditions. As crustaceans, woodlice still have what are essentially gills, not lungs. They need to keep these moist to breathe properly, and so they are very sensitive to humidity and prone to drying out.

2 Responses leave one →
  1. September 17, 2024

    Here in the coastal desert that is San Diego, our pillbug populations fluctuate wildly with the time of year as our humidity changes with the seasons. In our relatively short rainy months, you’ll see them everywhere, but in times like now, when there’s been no rain for months, you see very few of them.

    It’s always sad to think of them slowly drying up like that. I’ve always liked the little guys.

  2. September 18, 2024

    I’m actually not sure what happens to them when it gets dry. While they do disappear, I don’t ever seem to see little piles of their dessicated corpses. I wonder if they just find really inaccessible places to shelter until the rains come again?

Leave a Reply

Note: You can use basic XHTML in your comments. Your email address will never be published.

Subscribe to this comment feed via RSS