Brown widow spiders spreading in California

Brown widow spider with her spiky egg cases.
Photo by Martyn Hafley.

The black widow spider is well known to most pest management professionals. Less familiar is the brown widow, Latrodectes geometricus.  The brown widow is more common in areas where it occurs, though it is reported to be more shy and less prone to bite than its black cousin. Nevertheless, the venom of this species is thought to be at least as toxic as the black widow.

Rick Vetter at the University of California - Riverside reports that the brown widow is showing up at more locations in southern California, and is soliciting samples from that state to document the spread.  In Texas, the brown widow is reported from the Gulf coastal areas from Houston to Corpus Christi.

One of the most distinctive features of the brown widow is its spiky egg case.  The black widow spider makes a tough, spherical egg case without any ornamentation.  The brown widow egg sac is covered with spiky projections.  The spider itself is brown with a yellow or orange hourglass marking on the underside of the abdomen.  The Latin name for the species, geometricus, comes from the geometrically shaped patterns on the sides of the abdomen, patterns that are mostly lacking on the black widow.

The current range of the brown widow is thought to extend in the southeast from Florida to Georgia, South Carolina, Alabama, Mississippi, Louisiana, Texas and Arkansas. It has also been reported from southern California, Nevada and Colorado.

The brown widow is a spider to be respected, if not feared. Vetter calls them "not very dangerous". Dr. G.B. Edwards, an arachnologist with the Florida State Collection of Arthropods in Gainesville, says that the brown widow venom is twice as potent as black widow venom; however, he notes they do not inject as much venom as a black widow, are very timid, and do not defend their web. On the other hand, where they become established, they are likely to be more common than the black widow spider.

I am a hardy promoter of vacuums when it comes to spider control.  Because of their large size and tendency to remain on their cobwebs, Latrodectus spiders may be difficult to control with residual insecticides.  Residual and contact sprays may still be useful in control; but use of vacuums or "Webster" mops to physically remove the spiders and their egg cases is likely to be superior to insecticide use alone. 

If you encounter brown widow spiders in Texas, I would be happy to receive specimens or images of your find.  This will help us have a better understanding of the spread and frequency of occurrence in this state.  Images can be emailed to m-merchant at tamu dot edu.  Or specimens can be mailed in a small amount of alcohol to Extension Urban Entomologist, Texas AgriLife Research and Extension Center, 17360 Coit Road, Dallas, TX  75252-6599.