The Hobo Spider Was Never Dangerous

For about twenty years the hobo spider appeared on official lists of medically significant North American spiders, and Tegenaria agrestis was the reason people in the Pacific Northwest were afraid of the spiders in their garages.

That listing rested on a small body of work from the late 1980s and early 1990s proposing that hobo bites caused necrotic lesions similar to a recluse.

It has not held up.

What happened to the claim

The original studies were small, and the key experimental work involved injecting rabbits rather than documenting bites in people. Later attempts to reproduce necrosis in animal models did not succeed.

Reviews of the case records found the same problem that runs through the whole subject of necrotic arachnidism: almost none of the reports involved a spider caught in the act and identified by an arachnologist. Lesions were attributed to hobo spiders because hobo spiders were present in the region, which is the reasoning rather than the evidence.

In Europe, where Tegenaria agrestis is native and abundant across a huge range, it has never had a reputation for causing anything. That absence is difficult to reconcile with the North American claim.

The CDC removed the hobo spider from its venomous spiders page in 2017. Current reviews treat it as not medically significant.

Why the story took hold

The timing and the geography lined up.

The Pacific Northwest has no brown recluse population, and it never did. But necrotic skin lesions occur everywhere, because bacterial skin infections occur everywhere — and the late 1990s and 2000s saw a substantial rise in community-acquired MRSA, which produces exactly the lesion people were attributing to spiders.

A newly-introduced spider that lived in houses and could not be easily identified filled the explanatory gap. Something had to be causing these wounds, and this was the candidate.

The same logic produces tens of thousands of recluse reports every year from places recluses do not live — the pattern documented in do brown recluses live where you do.

Identifying one, honestly

You mostly cannot.

Eratigena agrestis — it was reclassified — is separated from its close relatives, including the giant house spider, by microscopic examination of the genitalia. Every field character it has been credited with has turned out to be unreliable:

• "No dark bands on the legs." Real, but shared with several other species, and lighting makes it hard to judge.
• "A herringbone pattern on the abdomen." Present on many funnel weavers.
• "Runs fast." So do they all.
• Size. Overlaps heavily with the giant house spider, which is common in the same buildings.

Which means almost every confident hobo spider identification made by a member of the public — and a good share of those made by pest control operators — is a guess.

What you can say is the family. Funnel weavers build a flat sheet web with a tube retreat at one corner, in a wall gap, a woodpile, or the corner of a window frame. That structure is distinctive and is one of the four in the web guide.

What it means practically

If you are in the Pacific Northwest and something bit you, the hobo spider is no longer a reasonable first explanation. A skin infection is, and it is worth not delaying on — the comparison is set out in spider bite or skin infection, and the signs that matter are spreading redness, fever and a wound still enlarging after 48 hours.

If you are trying to identify a large fast brown spider in a garage in September, the most likely answer is a mature male funnel weaver of some species out looking for a mate, which is what almost every large spider in the open at that time of year is — why there are suddenly spiders everywhere.

The general lesson

A species got a reputation from a small number of unreplicated studies, the reputation outlived the evidence by two decades, and it is still repeated by pest control marketing that has every reason not to update.

Which is worth remembering the next time a spider is described as dangerous — including the ones where the description is accurate, like the widows, whose actual bite course is a good deal more mundane than the reputation as well.