Spiders are the subject of great interest and not a little fear in Montana. Just over a third of the specimens submitted to the Schutter Diagnostic Lab for identification over the last few years have been spiders. Of these, well over half have been Tegenaria, the genus that contains the hobo spider, and almost three-fourths of those have been the hobo spider, Tegenaria agrestis. Recent research has shown that the fear of hobo spiders is likely unfounded, however, or at best exaggerated greatly beyond the actual risk that these spiders pose.
Hobo spiders are very common in Montana. The most typical presentation is a mature male found wandering in the basement or ground floor of a house in late summer. (Hobo spiders are poor climbers.) Females are captured far less often than males because they tend to sit in their webs and wait for prey (and males) to come to them. Males do not build webs and thus do not feed while they wander about looking for females, so most of them die soon after mating or exhaust themselves in their search. As the year progresses, therefore, we tend to get fewer spiders sent in overall, but a higher proportion of them will be females.
Like all living things, spiders are classified into progressively more closely related categories. Tegenaria is one of many genera in the family Agelenidae, the “grass spiders” or “funnel web spiders.” (Despite the common name, they are not related to the very dangerous and definitively aggressive funnel web spiders of Australia.) There are several species of Tegenaria in the United States, all but one of them (the cave-dwelling T. chiricahua of the desert Southwest) unintentionally imported from Europe. Four species of Tegeneria occur in Montana, all of which look very similar except for their size and the details of their anatomy. The largest and least common is T. atrica, the “dust spider.” The next largest is T. duellica, formerly known as T. gigantea. Its common name, the “giant house spider” tells us both that it is very large, even though it is smaller than T. atrica, and that it is often found in houses. There is also T. domestica, the “house spider” or the “lesser house spider,” and T. agrestis, the “hobo spider.” All of them are yellowish brown medium-to-large spiders with long legs. Under a microscope they differ in the number of teeth on their chelicerae (the structures that hold the fangs), and the fine details of their reproductive structures.
Some years ago the hobo spider was called the “aggressive house spider” because of a misunderstanding of its specific name “agrestis.” The Latin word agrestis does not mean “aggressive.” It is the root of our word “agriculture,” making T. agrestis the “agricultural spider” or the “spider from the fields.” (The hobo spider is no more aggressive than a host of other spiders, and far less aggressive than some.) In Europe T. duellica and T. domestica are found indoors, and T. agrestis is found, as one would expect by its name, outside in the fields. A similar situation occurs in North America in areas where the two house spiders and the hobo spider have been introduced; it is rare to find a hobo spider indoors where the other two occur. The mechanism by which T. domestica and T. duellica limit the presence of the hobo spider in dwellings is unknown. Perhaps they take over the best hiding places, perhaps they kill and eat the smaller hobo spiders, or perhaps there is some chemical cue that persuades the hobo to seek its fortunes in the out of doors.
As with its supposed aggressiveness, the evidence for the hobo spider actually being medically important (specifically, causing the kinds of necrotizing or “rotting flesh” wounds that are a well-established result of bites from the brown recluse spider) has not stood up to scrutiny. Researchers at the University of California at Riverside have identified at least 90 other causes of necrotizing wounds initially presented as “spider bites,” ranging from antibiotic-resistant Staphylococcus infections (MRSA or the “flesh-eating bacteria”) to autoimmune disorders and some strains of herpes viruses. There is exactly one case in the medical literature of a necrotizing wound developing from a proven hobo spider bite. (That patient had phlebitis and diabetes, conditions that predisposed her to develop necrotizing infections.) Despite the popular myth, numerous internet sites, and the fact that everybody in Montana seems to have heard of someone who knows someone who developed a necrotizing wound after a hobo spider bite, it is extraordinarily rare to encounter someone who has actually been bitten by a hobo spider. Where the brown recluse occurs, by contrast, the actual spider is captured and presented in about 20% of “spider bite” cases.
One very small study by an amateur arachnologist some 25 years ago showed necrotizing wounds developing in about half of a sample of rabbits which hobo spiders were forced to bite. At least two studies some years later, including one in which venom was extracted from hobo spiders and injected into rabbits, failed to reproduce these results. Comparison of the venom of male and female hobo spiders, and of hobo spiders from the US with their relatives in Europe, found no differences in the composition of the venom. Finally, Europeans do not consider the hobo spider dangerous, despite having lived with it for generations, and there is no special abundance of spider bites in areas where the hobo naturally occurs. Despite a lot of looking, in other words, there does not seem to be much to find where hobo spiders and dangerous necrotizing wounds are concerned.
All spiders are poisonous; that’s how they eat. Only a very few have fangs strong enough to penetrate human skin. Many of the agelenids can bite, as can some of the larger wolf spiders, some clubionids, and a handful of others. Usually spiders bite only when they are roughly handled or smashed. No spiders seek out humans in order to bite them, any more than we bite each other’s cars. Can we imagine a scenario where such a thing might happen? Of course: people in extreme pain in accidents exhibit all sorts of bizarre and counter-productive behavior. Most spider bites occur in similar situations, when someone sits on or rolls over on a spider, or when one gets trapped inside clothing. As with snakebites and wasp stings, a certain number of individuals get bitten while handling or “messing with” large spiders, or when they carelessly put their hands into areas where they are likely to occur. It is prudent to wear gloves, for example, when cleaning or removing clutter from areas that have not been disturbed for a while, and when moving rock piles or handling firewood.
All that said, most spiders are neither cuddly nor cute, and most of us do not want to share our houses with them any more than the absolute inevitable minimum. (A few are going to get in every now and then, despite our best efforts. Even research stations in Antarctica have a few spiders.) Minimizing the number of spiders in the house is best accomplished by reducing clutter around and inside the house to deprive the spiders of a hiding place, cleaning and vacuuming regularly to pick up any stragglers that wander inside, and making sure that the doors and windows and utility entrances are well-sealed so that the spiders can’t get in. Keeping the window screens intact, caulking or foaming any openings or cracks, especially utility penetrations in the basement or crawlspace, and making sure that all the door thresholds fit well and are in good condition is an excellent start.
It is difficult to exclude spiders from homes using insecticides alone. They don’t groom themselves nearly to the extent that insects do, and most of them do not move around a great deal so they aren’t exposed to treated surfaces to the extent that insects are. Glue boards will remove a surprising number of spiders (it may be more accurate to say that most people are surprised by the number of spiders they find out are in their houses when they put out sticky traps or glue boards), but should be carefully placed where children and pets will not get stuck on them. (Hell hath no fury like a cat on a glue board.) As you might expect, glue boards will capture mostly the wandering males. Simply vacuuming spiders up when you see them will cut down on their numbers, too. Very few spiders will survive the trip through the vacuum.
Although the internet changes constantly, and can be as much a source of bad information as good, the references below were current at the time of this writing, and were very helpful in preparing this article.
Cam Lay, Entomologist, Montana Dept. Agriculture