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

Western Black Widow (Latrodectus hesperus)

Western Black Widow
Photo CD 0023 3291 1601, Image #25 - Credit: Terry Thormin

INTRODUCTION

The "dreaded" Western Black Widow, although it is one of the most poisonous spiders in the world, is also a very timid spider that will do everything it can to avoid a confrontation.

IDENTIFICATION

This is a fairly large spider (the abdomen can be about as large as the tip of your little finger) that is generally all black except for the red hourglass mark on the underside of the abdomen. It is a hairless species with a very round abdomen. Immature individuals usually have striking pink or white markings on the abdomen, and in some individuals some of these markings may last through to adulthood. This is a cobweb spider which means its web is a tangle of lines with no apparent structure.

DISTRIBUTION

This western species is found from southern Saskatchewan, Alberta and British Columbia south into Mexico. In Alberta it is restricted to the prairies. Individuals that turn up in central and northern Alberta have been brought in from elsewhere. On occasion individuals come in on grapes from California.

TIME OF YEAR

Because this species can overwinter as adults, they can be seen as soon as temperatures are warm enough and they will remain active until the first good frost.

HABITAT AND HABITS

Black Widows prefer dark shaded areas and avoid full sun. They can often be seen in abandoned ground squirrel and badger burrows. They also frequent wood piles and, when given an opportunity, will invade houses and out-buildings. In houses they are usually found in unfinished basements.

SIMILAR SPECIES

Although there are many species of cobweb spiders in Alberta, there is only one that is this large and this black. The one species that is occasionally mistaken for a Black Widow is the Boreal Cobweb Spider, Steatoda borealis. This species never gets as large as the Black Widow, and does not have the extensive light markings that an immature Black Widow would have.

For more information on widow spiders see the article Potentially Medically Significant Spiders of North America on our web site.

COMMENTS

Although Black Widows have a notorious reputation, it is largely undeserved. As stated earlier, they are very timid spiders that will do everything they can to avoid confrontation. There has never been a recorded death from a Black Widow bite in Alberta, and the last death in North America north of Mexico was over ten years ago. Where they occur, some simple precautions, like not grabbing a piece of wood from a wood pile without first checking it, will greatly reduce the chances of being bitten. Anyone in the Edmonton area who finds a Black Widow can bring it in to the museum and we will keep it alive, and possibly exhibit it.

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Last update: November 25 2004