Monday, May 2, 2011

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. 


Anonymous said...

Mike, what's a Webster mop?

Mike Merchant, PhD said...

Websters are ingenious cleaning mops for reaching high spider webs and getting into corners, etc. I think they should be essential equipment for any residential pest control technician. To see a video click here. Google webster or cobweb duster to find commercial sources.

Ted Snyder said...

I've even found some brown widows up here in Wisconsin. No outdoor breeding populations - just some indoors, imported from Florida.

Here's an article I wrote on the Webster many years ago that your readers might find useful. Sometimes nonchemical control trumps chemical use.

Anonymous said...

tons of brown widows in our backyard here in Los Angeles. I'm not sending a sample, but each has been tucked in a little webby hidey-hole next to one or more white, spiky egg sacs. I found and killed about fifteen of them yesterday, and another two today (9/17/12). It must be egg-laying season because they were all over and each had eggs next to them.

Anonymous said...

I'm not sure if I have brown widows, but I am finding egg sacs similar to your description on my trash cans. They are white balls with little projections. The eggs are very difficult to sweep away and are attached to webs. I have also found webs like this in my mailbox.

I live in Riverside CA.

Anonymous said...

We just killed about 15 of them in our backyard in North San Diego county in our backyard, Nov. 2013. They were under the picnic table and benches, and lawn chairs. Lots of spikey egg sacs.

Anonymous said...

They are very widespread in the canyons and parks of San Diego, CA. I have seen them everywhere I've gone hiking and while doing yard work.

Anonymous said...

I just found and killed one yesterday in our back yard. I live in Riverside, CA as well. There were 3 spiky eggs and the mother had a web hiding hole like someone described above.