Richard Vetter/UC Riverside Non-native brown widow spiders are taking over Southern California habitat from more poisonous native black widows.
Can the invasion of a non-native species ever be a good thing?
In one case, while arachnologists might say no, homeowners in Southern California are likely to say yes.
The brown widow spider, a less-poisonous species than its cousin, the black widow spider, is making its claim in the dark recesses of Southern California trash can lids, plant pot lips and wood piles – and pushing its deadly cousin out of the region.
The new spider was first spotted in Torrance in 2003, though it’s been recorded in the southern U.S. since the 1930s. Researchers think the species originated in South Africa.
A new study by researchers at UC Riverside, Fullerton College and Humboldt State University found 20 times more brown widows in the Los Angeles Basin than black widows during a recent census. The research appears in this week's Journal of Medical Entomology.
"The brown widows really burst onto the scene in a very short time, and we found brown widows in many habitats where we expected to find black widows," said Richard Vetter, an entomologist at UC Riverside and author of the study. "There may be some competition where brown widows are displacing black widows because there is some habitat overlap."
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Vetter said the new species is likely to reduce the number and frequency of black widow spider bites in the region and reduce the overall venom levels in the area.
There are roughly 2,500 reported black widow bites every year in the United States. Bites generally cause severe pain and muscle cramping.
In this most recent analysis, researchers combed Southern California looking for spiders in typical black widow areas and found a predominance of brown widows. The researchers found that around homes in urban areas, brown widows were extremely common, but in agricultural settings, such as nurseries and orchards, black widows still seemed to predominate.
Vetter said the less toxic bite of a brown widow is a good thing. But he says the black widow is native to the area and has evolved over tens of thousands of years with the other animals and plants in the region. Brown widows "don’t belong here,” he said.
Vetter said he kills the brown widow, the eggs and specimens he finds, while releasing black widows back into the wild.
He said that after working with black widows for 30 years, handling them without gloves, he’s felt their fangs only six times and has never been bitten deeply or long enough to experience a reaction.
Not many brown widow spiders have been found in the northern part of the state. Vetter said that as far as he knows, none have been identified in the San Francisco Bay Area. He has, however, received three positive samples from the Sacramento area and one from Redding.
If you think you've seen a brown widow spider where you live, check out Vetter's website and get in touch with him.