Is The Venomous Brown Widow Spider Moving North?


Figure 1. Adult Female, underside view showing the classic, orange hourglass shaped marking

I first learned about this little beauty about two years ago.  She hails from South Africa and has adapted well to tropical and subtropical climates around the world.  The brown widow Spider,Latrodectus geometricus, was first introduced into the United States by way of Florida, where it has remained for decades.  Recently however, it has expanded its range, moving first into Texas, Georgia and North and South Carolina. It has also been reported in Colorado, Arkansas, Mississippi and Alabama.  It entered southern California sometime around 2000 and the first specimens were collected in Torrance in 2003.  It is now established in Orange, Los Angeles, Riverside, San Diego, Ventura, Santa Barbara and San Bernardino counties.  In 2009 specimens were collected much further north.  According to UC Riverside scientists the University received 3 females from Sacramento and another three females from the state of Washington.  It is not known whether there are established populations in those locations.  No further specimens have been seen in either place.  According to entomologists at the University of Florida and the Sarasota county extension office, this spider loves to hide out and lay its eggs in cars, RV’s, trucks and trailers, so it is readily transported long distances.

As with the black widow, it is the female of the species that is dangerous.  The brown widow’s venom is reported to be twice as potent as the venom its cousin, the black widow produces.  This fact alone has a lot of folks very concerned but, considering the behavioral characteristics of this spider it appears that the danger is over stated.  The brown widow lives in all the placed you’d expect to find a black widow, but will also build its web out in more open areas such as under eaves, in children’s play equipment, empty plant pots, near wood piles etc.  They tend to occur in larger numbers too.  When disturbed a brown widow tends to retract into a ball shape and avoid the threat, unlike the black widow.  It seems that most bites occur when the spider is squeezed between a person and some object such as furniture or clothing.  Although this spider has the more powerful venom its bite is seldom serious.  Generally, the brown widow injects less venom when it does bite.  According to UC Riverside’s Department of Entomology brown widow bites are immediately painful and usually produce two marks, with redness and swelling of the tissues, much like the bites of other common spiders.  According to their entomologists there was a documented case in Africa where a bite was more serious, producing classic black widow bite symptoms requiring hospitalization of the victim.  The potential for serious injury from brown widow bites is real, but only one in twenty bites proves fatal.

Identification of the brown widow female is difficult because she so closely mimics the male black widow in appearance.  Identification of specimens can be difficult and is best made by trained entomologists or pest control technicians.  However, if the female is found with her egg sac identification becomes much easier because the sac is unique.  Like other spiders  egg sacs it is round with a cotton-like appearance, but has one major difference: It is covered with spike-like protrusions that make it look much like the old harbor mines used against ships during World War II.  (See Figure 2.)  These spiders are very prolific.  Each female can produce up to about 20 egg sacs during her lifetime, with each sac containing up to roughly 80 eggs.  Incubation is only 10 days.  The young spiderlings disperse by ballooning, the practice of spinning a web into the air and floating on the wind to new locations.  They will mature in 135 to 240 days.

Females live up to 950 days, mating with several males in her lifetime.  Like

Figure 2. Adult Female and Egg Sac

 the black widow, the brown widow often devours the male.  In the case of the black widow, the male will try to flee, but the brown widow male must be a fatalist.  He actually presents his abdomen to the female during or after mating, in a behavior called somersaulting and allows her to kill him.  It’s a strange way to ensure monogamy of the species.

If there is one up side to the spread of this spider it may be that, due to sheer numbers, it may out-compete its black cousins.  Brown and black widows share the same food sources, the same predators and some of the same habitat so, what the long-term impact on the local ecology will be is uncertain.

If you believe you have sighted a brown widow female in central California or locations further north the University of California, Riverside would like you to mail it to them.  Contact Rick Vetter:  Email: rick.vetter@ucr.edu