Why Did Thousands of Venomous Spiders Swarm a House?

Spiders literally creeping out of your walls may sound like a Halloween movie, but it was reality for one Missouri family: They were forced to move after more than 4,000 venomous spiders infested their home.

For several years the Trost family of Weldon Spring had seen brown recluse spiders coming out of the fireplace, the blinds, the pantry ceiling, even the shower, according to the St. Louis Post Dispatch. At least two pest-control companies failed to eradicate the plague. (See “7 Bug and Spider Myths Squashed.”)

Brown recluse spiders rarely bite people. Photograph by Robert Noonan, Getty

Why do spiders congregate like this—and could it happen to us? National Geographic spoke with Joel Ledford, who studies spiders at the University of California, Davis.

Where does this kind of mass gathering happen? 

I study spiders that are in caves, and I’ve been in caves where there have been many hundreds [of spiders] that belong to the same genus.

The species [Loxosceles reclusa] is focused in the [U.S.] Midwest, [including] Missouri, Alabama, large parts of Texas, Louisiana, and Arkansas. So in someplace like Missouri, where the conditions are just right, they can be very abundant. (Also see “Spiderwebs Blanket Countryside After Australian Floods (Pictures).”)

Is this common? How often do you see huge numbers of spiders in one place?

There are lots of spiders everywhere. I’ve heard anecdotes that you’re never more than ten feet [three meters] from a spider, so spiders are abundant and common—even in urban environments.

But spiders don’t get together and form packs. Most spiders live very secretive lives.

There are some spiders that are social, and they form little colonies that contain many hundreds—or a thousand—of individuals.

I’m not surprised that there were lots, but certainly not seeping through the walls. That seems out of character for this kind of spider. They tend to spin a web and hang out there and not go very far. (See pictures: “World’s Biggest, Strongest Spiderwebs Found.”)

So why do you think this happened?

Four thousand spiders seeping out of the walls? I can’t even conceive of a situation that would be like that. It must’ve been very unique!

This spider is very secretive and not aggressive, and it doesn’t really do anything but hang out and eat harmful insects. It’s very unlikely that they’d start coming out of the walls—unless they’re males. (See “Strength in Numbers: 5 Amazing Animal Swarms.”)

When the males mature and they’re reproductive, they leave the safety of their web and go looking for love. That could have something to do with it.

How dangerous is this situation? Thousands of venomous brown recluse spiders?

Brown recluse spiders have a very special kind of venom that causes necrotic lesions—your skin starts seeping and dying and flaking off. It’s a nasty kind of thing. You don’t want to trivialize something like that. (Also see “Ask Your Weird Animal Questions: What Happens If You Swallow a Spider?“)

But the truth of the matter is that they rarely bite people. They don’t stalk people or jump at you.

Spider bites tend to get overdiagnosed and sensationalized by doctors.

 

Related Video: World’s Largest Spider

 

What do you say to those of us who are concerned about a spider infestation in our own homes?

Spiders live around you already—you’re surrounded by them all the time. But none of them hunt people.

They’re really important part of any ecosystem, even an urban ecosystem. They’re good to have around for insect control.

People can just relax. Worry more about your neighbors than the spiders around you.

This interview has been edited and condensed.

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Meet the Author
Mollie Bloudoff-Indelicato is a science journalist who loves em dashes, ’80s music and parasites. She has a master’s degree from the Columbia University Graduate School of Journalism with concentrations in science journalism, photography, and radio reporting. Contact her at news@mbloudoff.com, and follow her on Twitter at @mbloudoff.