Arizona species
False black widow in Arizona: the black widow look-alike
Steatoda grossa
A glossy, dark cobweb spider often mistaken for a black widow or common house spider. No red hourglass, and its bite is much milder.
Source: Penn State Extension: False Black Widow Spider
Source: Montana State University Extension: Cob-web Weavers (Steatoda)
How to identify
- Glossy purplish brown to black, round abdomen.
- No red hourglass under the abdomen.
- Faint pale markings on top that are often hard to see.
- Red brown front body with slightly darker legs.
- Size
- Adult females about 1/4 to 3/8 inch long, about the size of a true black widow, according to Penn State Extension.
- Habitat
- Dark, sheltered spots in and around buildings: under furniture, low corners, garages, sheds and wood piles.
- Where in Arizona
- Not native. Penn State Extension calls it a common spider in homes in the southern and western states. Research grade iNaturalist records in Arizona come mostly from Flagstaff, Prescott and Mayer, with some from Tucson, Sierra Vista and Gila County.
Look-alikes
Prevention
- Seal gaps around doors, windows and utility lines.
- Add door sweeps and repair screens.
- Reduce clutter in garages, closets and low corners.
- Vacuum webs and egg sacs, and move wood piles away from the house.
Call Hydra if you can't tell a false black widow from a true black widow, or if cobwebs keep coming back indoors. Our Service Pros will confirm the ID before treating.
When to call Hydra
Call or text us, we answer both. Or send us your details and we'll come back with a free custom quote.
Frequently asked questions
Is the false black widow dangerous?
Not usually. Penn State Extension says its bite causes symptoms similar to a black widow bite but much less severe, such as pain, occasional blistering and sometimes nausea or headache. If a bite causes pain or a reaction, call Poison Control at 1-800-222-1222. This is general information, not medical advice.
How do I tell it from a black widow?
Check the underside. A western black widow female has a red hourglass. The false black widow has none, and its glossy abdomen is often purplish brown rather than jet black.
Is it the same as the common house spider?
No. Both are cobweb spiders and are often confused, but the false black widow has a darker, glossy, nearly solid abdomen, while the common house spider is tan and mottled.