Hobo Spider — The Reputation and the Evidence

Start with the correction

The hobo spider is the most misrepresented animal in this county, and the misrepresentation runs in the direction that sells treatments. So the correction goes at the top rather than buried under a description.

The Washington State Department of Health states that hobo spiders are not considered dangerous as previously thought. It describes them as shy, retiring and reluctant biters, and it says directly that there is no evidence hobo spider venom has caused skin necrosis or necrotic wounds in people.

That is the state health agency for the state you are standing in. It is not a contrarian blog, it is not a minority reading, and it is not a claim we are making on our own authority.

The regional extension handbook that WSU, Oregon State and the University of Idaho publish jointly goes further and uses the word exonerated. Its entry records that recent research indicates bites from this spider are no more dangerous than bites from most other spiders, and that the CDC has removed it from its list. We checked the CDC’s current venomous-spiders guidance on the day this page was written: it names the black widow and the brown recluse, and nothing else.

There is no softer version of this to offer. The necrotic-lesion reputation was built on evidence that did not hold, and the agencies that once carried it have withdrawn it.

How the reputation was built

The claim has a specific origin, and knowing it helps explain why it stuck.

In the late 1980s and early 1990s a small body of work proposed that this European import had become the Pacific Northwest’s answer to the brown recluse — a spider whose bite produced slowly expanding dead tissue. The idea was plausible on its face. The region had a wound problem nobody could explain, it had no brown recluse to blame, and here was a newly arrived spider that lived in exactly the basements and window wells where people were being bitten by something.

The proposal traveled fast because it filled a gap. It reached public health material, it reached regional newspapers, and within a decade it was simply common knowledge in Washington, Oregon and Idaho. Nobody had to be persuaded of it, because everybody already believed it.

What did not travel was the follow-up.

What the follow-up found

Three separate lines of work went after the claim, and none of them supported it.

The animal work. UC IPM’s pest note on the species, written by the arachnologist who did much of the reassessment, records a 2001 study in which venom was tested against rabbits — the standard model for dermonecrotic envenomation, and the model the original claim leaned on. Neither venom tested produced necrotic wounds in the animals.

The case review. A 2004 editorial in Annals of Emergency Medicine — Vetter and Isbister, volume 44, pages 605 to 607 — asked whether hobo spider bites cause dermonecrotic injuries and went back through the literature to see what the answer rested on. UC IPM summarizes the result: across everything published, there was a single case of a verified bite by a hobo spider that resulted in a necrotic skin lesion. One. A verified bite means the spider was caught in the act, kept, and identified by somebody competent to identify it, and that standard turns out to disqualify almost every case in the file.

The laboratory work, done here. In 2011 a team at Washington State University published in the Journal of Medical Entomology what is probably the most thorough single test of the hypothesis. They looked at three things at once: what bacteria the spiders carry, whether the spiders can pick up and transfer a dangerous pathogen, and what the venom actually does to blood cells.

The bacteria they found were ordinary environmental organisms, not pathogens. When the spiders were run across surfaces contaminated with MRSA, they neither acquired nor transferred it. And the venom’s hemolytic activity — its capacity to destroy red blood cells, the mechanism behind a genuine necrotic bite — came in between 0.62 and 0.93 percent, against more than 37 percent for brown recluse venom run alongside it as a control. The authors concluded that their findings could not substantiate the spider’s involvement in human necrotic tissue lesions.

A later Oregon study, also cited by UC IPM, turned up one verified bite, with minor symptoms and no lasting damage.

That is the whole evidentiary picture: a failed animal model, a case file that collapses under a verification standard, venom that does not do the thing, and almost no verified bites to examine in the first place.

Why the correction has not reached most people

Because retractions do not travel the way alarms do.

Everything published between roughly 1990 and 2010 that repeated the necrosis claim is still online, still indexed, still being read as current, and carries no mark saying it was superseded. Nothing about a fifteen-year-old article announces its age to somebody who found it through a search.

The commercial incentive runs one way as well. A spider that is dangerous justifies a recurring treatment. A spider that is harmless justifies a door sweep. It is not hard to work out which one a pest control company would rather you believe, and we are including this paragraph on our own website with our own phone number at the top of it precisely because the incentive is obvious and we would rather name it than benefit quietly from it.

And there is the name. This species has been called the “aggressive house spider” for decades, which sounds like a warning and is in fact a translation error. Utah State University Extension traces it back to a misreading of the species name agrestis, which means “of the field.” The animal was named for where it lives, not for how it behaves, and every reference to an aggressive house spider carries a mistake forward.

What the animal actually is

Now the biology, which is genuinely interesting once the alarm is off the table.

Eratigena agrestis is a funnel-weaving spider in the family Agelenidae, introduced to the Pacific Northwest from Europe. In Europe, where it has lived alongside people for a great deal longer than it has here, it has never carried a medical reputation of any kind.

WSU’s fact sheet puts it at roughly one to one and three-quarter inches including the legs, which makes it a medium-sized spider by local standards and noticeably smaller than the giant house spider it is confused with. It is brown, hairy-legged rather than glossy, and drab. Coloration varies enough that WSU Extension warns directly against using it for identification: most non-arachnologists try to use color as a diagnostic feature, and it is one of the least reliable characters available.

The web is a flat horizontal sheet with a tube or funnel at one edge. The spider sits in the tube. The sheet is not sticky in the way an orb web is — it works by tangling an insect long enough for a very fast animal to cross the sheet and grab it. In this county those webs turn up in window wells, against foundations, in the corners of unfinished basements, under stored material in a garage, in the gaps behind a woodpile and in low vegetation. Dark, undisturbed and slightly damp is the pattern.

They are poor climbers. The species is a ground-level animal, which is why the useful measures against it are all at floor height.

How common it actually is here

Less common than the name recognition suggests, and this matters.

WSU Extension’s identification factsheet reports that in the Pacific region west of the Cascade Mountains, the giant house spider outnumbers the hobo spider by about three to one. UC IPM goes further, noting that the larger species has competitively displaced hobo spiders across parts of the Pacific Northwest.

So the base rate in a Bellingham basement already favors the harmless relative heavily before anybody looks at the spider. Add the barn funnel weaver Tegenaria domestica, which WSU also lists for this state and which is extremely common indoors, and a large brown funnel weaver in a Whatcom County house is several times more likely to be something other than a hobo spider than to be one.

This is the part that gets lost. The species has enormous name recognition and modest actual abundance, which is close to the worst possible combination for accurate identification. People find a spider, reach for the only name they know, and the name arrives with a reputation attached that was never true.

Why you cannot identify one, and neither can we

We get asked to confirm hobo spiders constantly, usually from a photograph, and the honest answer is almost always that it cannot be done.

WSU Extension states the position plainly: unless you are able to examine the very small reproductive structures, you cannot definitively identify a hobo spider. UC IPM says the same in different words — distinguishing funnel-weaving species requires examining the reproductive structures, and that is work for a qualified arachnologist.

What that means in practice is a dead or anesthetized specimen, a stereo microscope, and a mature animal. On a male it is the pedipalp, specifically the projection on the tibia, which in this species is single and squared off on top. On a female it is the epigynum, where the hobo spider carries a large central cavity ringed by thick raised ridges. Juveniles do not have developed structures and cannot be determined at all.

What does not work, in the order people try it:

Color and pattern. Explicitly ruled out by WSU as among the least reliable characters. These spiders are variable and they are drab.

Size. Overlapping, and the giant house spider is generally the larger animal anyway, so a big frightening spider argues against the hobo rather than for it.

The chevrons. The herringbone marking on the abdomen is the test that circulates most widely online, and it is worth noticing that neither WSU’s identification key nor UC IPM’s pest note uses it at all. Both skip it and go to the genitalia. A character that the two authorities on identifying this species do not bother to mention is not a character you can identify it with in a kitchen.

Leg banding, used forwards. This one needs care, because there is a real character underneath a bad folk test. WSU’s key uses banding only to rule the species out: if you can see dark rings around the legs, it is not a hobo spider, because hobo legs are uniformly colored. That is a valid exclusion and it is worth knowing. What people actually do with it is run it backwards — uniform legs, therefore a hobo — and that inference is worthless, because plenty of other funnel weavers have unremarkable legs too. Ruling one species out is not the same as ruling one species in.

There is one field character in WSU’s key that does some work, and it needs the spider on its back. The sternum is the plate on the underside of the cephalothorax, between the leg bases. Hobo spiders carry a light stripe running down the middle of it. Giant house spiders and barn funnel weavers carry three or four pairs of light spots along the sides instead. WSU’s rule of thumb comes down to this: spots mean it is not a hobo spider. On a live spider moving at the speed these animals move, in a jar, this is harder than it sounds — but it is the one thing short of a microscope that is actually diagnostic in the useful direction.

What a bite is and is not

Nobody has much data, because verified bites are rare.

Where a bite is documented, the reported effect is minor and local: some pain, some redness, resolving. USU Extension’s summary is that hobo spiders will bite in defense, causing mild pain and redness, and are neither aggressive nor of significant health concern. The health department’s description — shy, retiring, reluctant biters — matches.

These are not animals that seek out people. A spider that hunts by sitting in a funnel waiting for something small enough to eat has no reason to approach something the size of a person, and the bites that do happen are almost all compression bites: the spider was against skin, inside clothing, in a glove, under a body in bed, and it responded the way anything responds when it is being crushed.

The more useful thing to know is about wounds rather than about spiders. The Washington State Department of Health warns that unexplained wounds are often misdiagnosed as spider bites, and that this can result in a condition not being treated properly. That is a real harm and it runs in the opposite direction from the one people worry about — not a spider bite mistaken for an infection, but an infection mistaken for a spider bite while it goes untreated.

The 2011 WSU work is relevant here too. Part of what it tested was the suggestion that even if the venom did nothing, the spider might be delivering bacteria. It could not pick up MRSA from a contaminated surface and it could not pass it on. The bacteria it does carry are the ordinary organisms any animal walking around a basement would carry.

If you have a lesion that concerns you, the useful move is a physician, promptly, regardless of what you think caused it. We identify spiders. We do not read wounds, and a pest control company that offers to is going well past what it knows.

Where they turn up in this county’s housing

If you want to know whether funnel weavers are living around your building, the places to look are consistent and none of them are in a bedroom.

Window wells are the classic one, particularly on older Bellingham stock where a basement window sits in a concrete or corrugated well that has filled with leaf litter over a few autumns. That litter is a habitat in its own right — damp, sheltered, full of the small arthropods a funnel weaver eats — and the window at the back of it is usually the least well-sealed opening in the building.

Unfinished crawlspaces and the underside of decks come next. Undisturbed, humid, dark, and in this county rarely dry. On a house with an unsealed crawlspace the space below the floor supports a standing population that restocks the level above it continuously, which is why crawlspace moisture work does more for spider numbers than anything applied to the siding.

Then garages, sheds and equipment stores, where stacked material creates exactly the arrangement of narrow gaps a sheet-and-funnel web needs. Rural properties around Lynden, Everson and Nooksack carry substantial populations in outbuildings, and in most of those buildings the spiders are earning their keep on flies.

Outdoors, look along the foundation line, under the lip of a deck, in the gap behind stacked firewood, in retaining walls, and in low dense vegetation against the building. WSU’s guidance on this species includes checking firewood before it comes indoors for a reason.

What all of those have in common is that they are ground-level and undisturbed. Funnel weavers are poor climbers, which is the single most useful fact about where they live and where the work to reduce them belongs.

When you actually see them

The seasonal pattern here is the same one that drives every spider call in this county, and it is worth being precise about what it is.

From late August through October, mature males of several local species leave their webs and go looking for females on foot. They travel at night, they follow edges and surfaces, and a proportion of them go under doors. That is the wave.

The Burke Museum — the state museum, whose arachnology collection is the reference for this region — makes a point about it that the industry generally skips: the increase in autumn sightings coincides with the mating season of the species involved, and what people are seeing is sexually mature males wandering in search of mates. Not spiders fleeing the cold. Not spiders moving in for the winter. Males on a schedule.

Which is why the season ends by itself. By November the males have found females or they have not, and either way they are done walking. A treatment applied at the end of September collects the credit for a calendar.

The rest of the year, a hobo spider is a sedentary animal in a funnel web somewhere dark, and most households never see one at all.

Why we say this warrants no treatment

The verdict at the top of this page is that treating for hobo spiders is rarely warranted, and it follows from everything above.

If the reason to treat was the danger, the danger is not there. The health department has withdrawn it, the federal listing has dropped it, the venom chemistry does not support it, and the case file cannot produce the injuries. Take that away and what remains is an ordinary funnel weaver that you would rather not have in the basement — a preference, and a legitimate one, but a preference rather than a hazard.

If the reason to treat is that preference, then the honest advice is that spraying is the weakest tool available for it. WSU’s own management guidance for this species leads with sealing entry points at door thresholds and window screens, checking firewood before it comes indoors, and mechanical removal with a vacuum or a broom, and it notes that chemical treatments are temporary without those measures. That is a land-grant extension service, with no product to sell, recommending carpentry and a vacuum cleaner ahead of pesticide.

We agree with it, and we will say so on a visit. The work that changes how many funnel weavers live in a Whatcom County basement is the door sweep on the garage side door, the window well that has become a leaf-filled habitat, the crawlspace hatch that does not seat, the utility penetrations nobody sealed, and the moisture underneath all of it that grows the insects the spiders are eating. Spider numbers track their prey. A building with a lot of spiders has a lot of insects, and the insects are the lever.

Where we do treat, it is because somebody has a heavy population in a garage or crawlspace they use daily, or because a household includes someone whose fear of spiders is genuinely affecting their life. That second one is a real reason and we do not treat it as a silly one. Neither of those reasons is this spider is dangerous, because it is not.

If you want one identified anyway

Catch it under a glass with a piece of card slid beneath, and put it into a small jar of rubbing alcohol if you want it kept. Alcohol preserves the structures that matter; a spider flattened on a tissue usually cannot be worked on at all.

Photographs are worth taking even though they rarely settle it. Straight down from above, from the side, and if you can manage it through the base of a clear container, from underneath — the underside shot is the only one with any chance of showing the sternum. Put a coin in the frame.

Send it over and we will tell you what we think, and we will tell you when the answer is that it cannot be resolved without a microscope. That answer is more useful than a confident guess, and considerably more common.

What you will not hear from us

We will not tell you that the spider in your window well is a medical threat, because the state health department says it is not.

We will not sell a quarterly program on the strength of a reputation that the CDC dropped and that the researchers who examined it could not reproduce.

And we will not repeat the necrosis story with a hedge in front of it. “Some experts believe” is how a retracted claim stays alive for another decade, and this one has done enough damage already.