[Focus On Spiders] Hobo Spiders: Are They Everywhere

In the late 1980s, the hobo spider, Tegenaria agrestis, acquired a reputation as a medically important spider in the Pacific Northwest, capable of causing rotting skin lesions similar to a brown recluse spider bite. Prior to that, necrotic wounds were blamed on the brown recluse spider even though no populations of any recluse species could be verified there. (And despite the potential for education of the medical community, brown recluse bites are still being diagnosed in the Pacific Northwest in juxtaposition to a lack of recluse spiders.) The hobo spider has therefore joined the widow and recluse spiders to become a member of the triumvirate of poisonous spiders with which the general public is familiar.

The hobo spider is a European immigrant that became established in the Puget Sound port area in the 1930s. Like many non-native organisms, once freed from native environmental constraints that kept populations in check, hobo spiders put down roots and spread from the initial focal point into British Columbia and east and south in the United States. It has been known by several lesser-used names including the "aggressive house spider" which is somewhat of a misnomer because the spiders are not overly aggressive. In order to create name stability, the American Arachnological Society has chosen "hobo spider" as the spider’s official common name and was similarly accepted by the Entomological Society of America.

Despite the hoopla surrounding the hobo spider as an emerging medical agent of dermatologic damage, there is little life history information regarding it. Most of the published distribution reports are in the form of technical bulletins, are local in scope and are not easily available to a general audience. Therefore, in order to get a better idea of where the hobo spider lives, a fairly comprehensive study was conducted regarding its distribution. The results have been published in the Journal of Medical Entomology (Vetter et al. 2003, volume 40:159-164).

HOBO SPIDER DISTRIBUTION. Several collaborators assembled to provide data from each of their states or province and should be given proper credit for their efforts. These included Robb Bennett (British Columbia), Art Antonelli (Washington), Lynn Royce (Oregon), Craig Baird (Idaho), Alan Roe (Utah) and Will Lanier (Montana). In California, I sought spiders from a multitude of sources (county agricultural commissioner offices, museums, arachnologists, vector control personnel, pest management professionals, etc.). In Colorado, Paula Cushing of the Denver Museum is conducting a comprehensive statewide spider survey and was able to provide data from her state. Data from Wyoming was rather sparse because a collaborator could not be found during the course of the study although some information was provided after the study was being readied for publication.

The current known range of the hobo spider is from southernmost British Columbia to the middle of Montana, south through west-central Wyoming into the northern portion of Utah (but only east of the Great Salt Lake) and into southern Oregon. All of Idaho and Washington are considered to be colonized by the hobo spider. In Colorado, out of the 24,000 spiders collected for the statewide survey, only 11 were hobo spiders, most found associated with two houses so they were probably transported there and may not spread further. In Wyoming, hobo spiders are found in the western portion of the state, are common in Casper and have been found in one northeastern town but it is not know whether it is well-established statewide. In southernmost Oregon, hobos were common in Klamath Falls which is high elevation (4,600 feet) and on the dry side of the Cascade Mountains. Few hobos were submitted from the southern coastal region of Oregon. No hobo spiders have been found yet in California. One of the limitations of the distribution study is that extensive areas of the northwestern U.S. are sparsely populated by humans; it is difficult to get an accurate handle on exactly where the hobo spider is because submissions are concentrated in metropolitan areas.

The seasonal abundance of mature spiders was fairly consistent in that males were first seen around July as they leave their trampoline-like funnel webs in search of mates, reach peak abundance in August, followed by a drop in September, and they are typically gone by October. Females lagged about a month behind the males, being found first in August, peaking in September and then petering out but still showing up occasionally in November. Considering the size of the spiders, several researchers feel that the hobo spider takes two seasons to reach maturity.

The hobo spider will probably continue to spread east in Montana and Wyoming. In Colorado, the hobo spider may either just be establishing a foothold or may never expand further than its current location. The Utah data set was extensive and was generated over decades so hobo spiders will probably not expand further there. There isn’t enough data from Oregon to determine if it exists in the southern coastal regions and therefore, no predictions can be offered as to whether the spider will get to California.

Finally, I have been contacted by people throughout the southern and eastern portions of the United States who think that they have populations of hobo spiders. Of the specimens that have been sent to me for verification, none yet have been hobo spiders. Most have been similar looking funnel weaving spiders but others have been the uniformly colored yellow sac spider (from eastern U.S.) and a massive mygalomorph (related to tarantulas) from California. Several submissions were "identified" by pest management professionals who see a funnel-web and tell their clients that they have hobo spiders when in fact the spiders were non-hobos. There is an overzealous tendency by non-arachnologists to identify their spiders as poisonous beasts (either recluses or hobo spiders) when in reality, the spider they have in hand is harmless and shares little resemblance to a dangerous arachnid.

IDENTIFYING A HOBO SPIDER. One of the biggest problems that occurs in arachnology is the misidentification of spiders. Non-arachnologists try to identify spiders from coloration, like matching the plumage of birds to the illustrations in ornithology field guides. Except for the rare and uniquely colored spider, coloration is one of the least useful, diagnostic characteristics that arachnologists use when trying to properly identify a spider. This is especially true in regard to the hobo spider. Well-meaning Web sites will flash a picture of a hobo spider with the question "Have you seen this spider?" Well, sure, of course, everyone has because the hobo spider is a generic-looking brown spider and there are dozens of species that look just like that to the non-arachnologist. The hobo spider is a funnel-weaving spider, and there are about 70 additional species of closely related funnel-weaving spiders in the United States with similar drab brown markings. There are also another 150 or so wolf spider species that can also be mistaken for them.

After seeing several non-arachnologists in government agencies misidentify spiders as hobos using the currently available guides, I wrote a guide that has been getting much positive feedback for its clarity and ease of use (http://pep.wsu.edu/pdf/PLS116_1.pdf) Although it was written specifically for a Washington state audience, it will have application in much of the United States.

Basically, in order to absolutely identify a hobo spider, one must examine the reproductive structures of the spiders and match them to the images presented in the aforementioned paper (like the figure presented on page 48).

If you continue to attempt to identify hobo spiders by coloration or size, you will be inaccurate. The guide also offers an elimination process and explains what is not a hobo spider. If folks were able to master this latter process, which is often possible without a magnifying aid, many spiders could be identified as non-hobos.

HOBO VENOM TOXICITY. Although the hobo spider has been linked to necrotic wounds in the northwestern United States, venom toxicologists are starting to question the veracity of this concept (Binford 2001). The hobo spider is not considered to be a medically important arachnid in its native Europe and, in preliminary, unpublished research, venom scientists were unable to replicate the original results showing that hobo spider can cause necrosis.

Much of the corroborating information presented to implicate the hobo was made using incomplete or circumstantial case history studies. It may be that hobo bites do occasionally cause necrosis, but not because of the toxicity of the venom alone. Instead, it may be through a bacterial agent in concert with the venom, digestive regurgitants or some other mechanism. Hobo spider venom should undergo an interesting re-examination in the next few years, which may shed some light on the mechanism of necrosis and the degree to which the hobo spider is responsible. Until then, the hobo spider should still be considered of potential medical importance but, it may not be the horrible monster that many claim it to be. Nonetheless, the hobo spider is well entrenched in the minds of the general public as a dangerous spider so whether it is truly poisonous or not, it will still be of concern to homeowners and pest management professionals will still be called in for control issues.

References

Binford, G. J. 2001. An analysis of geographic and intersexual chemical variation in venoms of the spider Tegenaria agrestis (Agelenidae). Toxicon 39: 955-968.

Vetter, R. S., and A. L. Antonelli. 2002. How to identify (and misidentify) a hobo spider. Washington St. Univ. Coop. Ext. Pest Leaflet Series #116, 10pp. (http://pep.wsu.edu/pdf/PLS116 _1.pdf)

Vetter, R. S., A. H. Roe, R. G. Bennett, C. R. Baird, L. A. Royce, W. T. Lanier, A. L. Antonelli and P.E. Cushing. 2003. Distribution of the medically-implicated hobo spider (Araneae: Agelenidae) and its harmless congener, Tegenaria duellica, in the United States and Canada. J. Med. Entomol. 40:159-164.

Photos are courtesy of the author.

The author is a staff research associate in the department of entomology at the University of California-Riverside and can be reached at rvetter@pctonline.com.