How to Spot a Brown Widow Spider

Southern California’s native black widow spiders have an enemy that may be threatening their existence, according to a new study.

Monday, the Entomological Society of America touted the study, “The Prevalence of Brown Widow and Black Widow Spiders (Araneae: Theridiidae) in Urban Southern California.”

The study appears in the Society’s most recent edition of Journal of Medical Entomology.

"The brown widows really burst on to the scene in a very short time, and we found brown widows in many habitats where we expected to find black widows," said corresponding author Richard Vetter of the University of California, Riverside.

"There may be some competition where brown widows are displacing black widows because there is some habitat overlap. There are also places where only brown widows were able to make homes, but in other habitats the black widows still predominate."

According to researchers, brown widow spiders are relatively new to North America. They were first documented in Florida in 1935 and were discovered in Southern California in 2003.

Locally, the newcomers have been very successful at displacing native black widow spiders.

After collecting data at 72 sites throughout Southern California, the study’s authors found 20 times as many brown widows than black widows outside homes, especially under outdoor tables and chairs, and in tiny spaces in walls, fences and other objects.

"Homeowners would benefit to know about the hiding places of brown widows, displaying care when placing their hands in nooks and crannies," the authors conclude.

But there is some good news for homeowners: Even if there are more brown widows crawling around, the dangers are lessened because the brown widow bite is less toxic than that of the black widow.

How to spot a brown widow spider. The following is a description of the brown widow spider from UC Riverside's Center for Invasive Species Research:  

Unlike its starkly black-and-red colored relative, the black widow, the coloration of a brown widow consists of a mottling of tan and brown with black accent marking. In mature females, there is usually a dorsal longitudinal abdominal stripe and three diagonal stripes on each flank.  At the top of each diagonal stripe, there is a black mark, which is rather conspicuous and square-ish. The Brown Widow Spider does have an hourglass but it is typically an orange shade rather than the vivid red of a black widow. The brown widow looks similar to immatures of the western black widow spider, the latter of which has smaller black spots on the top of the diagonal abdominal stripes and more olive grey background coloration. Being able to discern brown widows from immature black widows is therefore difficult and requires some experience.  However, a more diagnostic feature of a brown widow is its egg sac. Most spider egg sacs that are free (i.e., are not attached to flat surfaces) look like a lemon drop candy or a little cotton ball with indistinct edges. The egg sac of a brown widow has multiple silk spicules projecting out from the surface. The egg sac has been described as looking like a large pollen grain or a World War II harbor mine designed to blow up ships. The egg sac of the Brown Widow Spider is so distinctive that it is readily recognizable.