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Brown Widow Spiders Pushing Out Black Widows In California

Aired 7/9/12 on KPBS News.

The notorious black widow spider is being displaced by its brown cousin. The non-native brown widow outnumbers the black widow 20-to-1 in some parts of California, according to a new report.

Brown widow spider
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Above: Brown widow spider

The notorious black widow spider is being displaced by its brown cousin. The non-native brown widow outnumbers the black widow 20-to-1 in some parts of California, according to a new study by researchers at UC Riverside, Fullerton College and Humboldt State University.

The brown widow spider was discovered in downtown San Diego in 2003. Since then it has multiplied exponentially and taken over the black widow’s habitat – from the coast to the inland valleys.

Brown widows don’t have any known predators to interfere with their reproductive success, said Jim Berrian, field entomologist at the San Diego Natural History Museum.

“The black widow’s greatest threat is from parasitic wasps, and the wasps might not be clued into what the brown widows are yet,“ said Berrian.

Berrian said the invasive creature could have a negative impact on the ecosystem.

"If the black widows go by the way. If the wasps are specifically adapted to going after the black widows, the wasp can be threatened with extinction if they don’t have a host anymore. And what ever is feeding on the wasp could have a problem and so on," Berrian explained.

The brown widow is venomous just like the black, but it only injects about half the amount of venom. Berrian said browns are also less aggressive.

"They’re very shy animals, they tend not to bite. Just about every bench at Balboa Park has brown widows under them but we don’t have people getting bit, so they’re just not very aggressive spiders," he said.

Berrian said the other good news for people is that brown widows tend to favor outside plastic patio furniture and aren’t found very often inside of people’s homes.

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