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Environment & Science

Invasive brown widow spiders are pushing out black widows

The somewhat less-poisonous but invasive brown widow spider.
The somewhat less-poisonous but invasive brown widow spider.
Rick Vetter
The somewhat less-poisonous but invasive brown widow spider.
The poisonous black widow spider, under pressure in SoCal from the brown widow, which hails from Florida.
Rick Vetter

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Spider expert Rick Vetter at UC Riverside reports a suprising finding: Black widows, the legendary poisonous spider that lives in your water-heater shed or in the garage by the light switch, are being pushed out by their less-venomous cousin, the brown widow spider.

There's good reason to believe they're being pushed out from the city to the country by brown widows, an African native that came to us via Florida, probably in landscaping pots, Vetter says. He's getting anecdotal evidence that people are seeing far fewer black widows and many more brown widows.

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When they meet, there's no brown on black violence. "They're not physically attacking them," Vetter says of the invasive brown widow spiders. "They just get in, flood an area, and the next generation (of black widows) can't establish."

(Not a black widow spider in Cypress Park in 2011. John Rabe)

Is this hurting the ecosystem? "The brown widows are probably doing the same ecological task as the black widows," Vetter says, "so I don't think you're going to see a great ecological Armageddon happening. But we never like to see a native displaced by a non-native."

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But that does not mean black widows are through in SoCal, and that the brown widows have totally taken over. The black widow population, Vetter says, "has probably taken a hit from the brown widows, but the black widows are still very common in agricultural areas, where the brown widow does not seem to establish."

The good news for humans? While to their tiny prey, the difference between black widow and brown widow venom is moot, for humans, the brown widow's bite stings, then goes away; but the black widow's bite can be serious for humans.