Royal Alberta Museum: Invertebrate Zoology - Bug Facts

[Caterpillars and Other Immature Insects]

Fact Sheets: Commonly Observed and Asked About Insects and Spiders Found in Alberta

European Sowbug (Oniscus asellus)

European Sowbug
Credit: Terry Thormin

INTRODUCTION

Sowbugs are not insects, as is evident by the number of legs, seven pairs as opposed to the three pairs that insects have. Sowbugs are actually crustaceans, so their closest relatives include things like fresh water shrimp, crabs, crayfish and lobsters. Sowbugs are often common in gardens, hiding in damp places under logs and rocks.

IDENTIFICATION

The oval shape, greyish colour and fourteen legs should separate the Sowbug from any other creature. Adults may be up to 15 mm long.

DISTRIBUTION

This is not a native species, although it is found throughout much of North America. It was introduced from Europe. In Alberta it is presently restricted to urban areas and is probably found in most larger communities.

TIME OF YEAR

Although they are dormant in the winter, they are present once the ground thaws in the spring and remain active until the first hard frost.

HABITAT AND HABITS

Sowbugs love dark, damp places and are most common in yards under wood or rocks. The more tangled an area one has, the more likelihood they are present. Compost heaps are a great retreat for this animal. They also often crawl through spaces in the foundation or through basement windows and end up in the basement where they can survive quite well if the basement is unfinished. They feed primarily on detritus, dead plant and animal matter, including rotting wood.

SIMILAR SPECIES

There is only one species of sowbug in Alberta and nothing else that looks like them, although people sometimes mistake them for millipedes because of the many legs. Millipedes have a long, very cylindrical body. In other areas of North America the very similar pill bug can be found. The pill bug has the capability of rolling itself up into a ball, something the sowbug cannot do.

COMMENTS

People often find these creatures in their basements, and are concerned about the possibility that they may do damage to the house or that they may be dangerous in other ways. Older houses with dry rot might suffer some damage as a result of Sowbugs, but this is likely to be minimal. They cannot bite humans and are not poisonous. In some areas some species of Sowbugs can be pests on cultivated plants. They are also very effective decomposers and are often an integral part of a compost heap.

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Last update: November 22 2004