The spiders, which were first spotted in North America in 1935, didn't arrive in Southern California until 2003. They now outnumber the native black widow in some parts of the region, the Los Angeles Times reported Monday.
"The brown widow is really taking over," Richard Vetter, a staff research associate at the University of California, Riverside, told the newspaper.
Research by Vetter and his colleagues is published in the July issue of the 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," Vetter said in a release.
"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."
Researchers found 20 times as many brown widows as black widows outside homes, especially under outdoor tables and chairs, and in tiny spaces in walls, fences and other objects.
Neither spider was found in the living space of houses.
Vetter said the good news is brown widow spider bites are less toxic than those of native western black widow spiders.