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

Boreal Cobweb Spider (Steatoda borealis)

Boreal Cobweb Spider
Credit: Terry Thormin

INTRODUCTION

This is a spider that is often found in outbuildings and unfinished basements. People often confuse it with the Western Black Widow Spider, but the Boreal Cobweb Spider never gets as large and is considered harmless.

IDENTIFICATION

The Boreal Cobweb Spider is a smaller cobweb spider, with the females reaching a maximum size of about 7 mm. Although at a quick glance this spider may look black, a closer look will show an abdomen that is reddish brown with a lighter "T" shaped mark at the front end. The underside of the abdomen is also considerably lighter than the upper side.

DISTRIBUTION

Northern United States and Canada north to the tree line. This species is found throughout Alberta.

TIME OF YEAR

This spider, like most spiders in Alberta, is present only during the summer months and dies in the fall after producing an egg case. Most people find this spider in the latter part of the summer or the fall when the females are full-grown.

HABITAT AND HABITS

This spider can be found in low herbaceous vegetation but is most likely to be found under bark and stones, in rock crevices, on bridges and in outbuildings and unfinished basements.

SIMILAR SPECIES

Because the Boreal Cobweb Spider tends to look blackish, and builds a web of tangled lines, it is often mistaken for a Black Widow. It can be distinguished from the Black Widow by its smaller size and by the light coloured "T" mark on the front of its abdomen. A good look at this species also reveals a rather pale colour to the underside of the abdomen. In the Black Widow this should be black with a red hourglass mark in the middle.

COMMENTS

Most people who bring this spider in to us think that it may be a Black Widow. The information presented above should clarify this. This is not considered to be a dangerous spider, and may possibly out-compete spiders like the Hobo Spider and thus prevent them from establishing territories in the house. You may want to look at the write-ups for the Western Black Widow and the European House Spider on this web site.

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Last update: August 29 2008