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Spider Bites - Hobo, Widow & Recluse
- Most spiders are harmless, a very few are poisonous, all are beneficial predators -
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Summary: While most spiders are harmless and even beneficial, a few have a poisonous bite. In the US the most commonly encountered poisonous species are widow spiders, brown recluse spiders (south-central states) and hobo spiders in the northwest. Learn to recognize and avoid these species.
Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)
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 "Most spiders are harmless." No matter how often this is repeated many people's innate fears take over and they recoil at the sight of even small spiders. Most spiders are harmless!. Having repeated that, however, there are a few that deserve to be taken seriously and their bites should be avoided.
All spiders are predators and all use venom to subdue their prey. A few have venom in their bites that is dangerous to humans. The poisonous species vary depending on where you live.
Australia has an especially poisonous species called the Sidney funnel web spider and the tropics have a number of poisonous ones as well. In the US three spiders get the most attention -- hobo spider (aka aggressive house spider), black widow spider and the brown recluse spider. This page will help you identify and rationally deal with these spiders.
Different spider venoms cause different types of injury. Some cause necrotic, or "tissue killing", wounds (hemolytic venom) which are very slow to heal and can become seriously infected. Some spider venoms attack nerve tissue (neurotoxic venom) which can lead to trembling, convulsions, respiratory failure and even death.
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Black widow spider - note red "hourglass" on underside of abdomen.
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Hobo spider
Three related spiders commonly enter homes. The house spider (Tegenaria domestica), the giant house spider (Tegenaria duellica) and the hobo spider (Tegenaria agrestis) [pictures]. These species are very difficult to tell apart except for fully grown, female giant house spiders which are considerably larger than the other two. Of the three only the hobo spider is thought to have a dangerous bite and these reports may be exaggerated.
The house spider (T. domestica) is found throughout North America while the giant house spider (T. duellica) is found mainly in the Pacific Northwest and western Canada. The hobo spider (T. agrestis) is currently expanding its range from the Pacific Northwest, east and south.
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There are many reports of serious, localized necrotic (open, weeping) wounds resulting from the bites of the hobo spider. Some researchers, however, are starting to doubt that this spider is poisonous at all.
While the scientific "jury" may still be out, care should be taken with this common spider where it occurs. Use the suggestions below for reducing the chances of bites. Especially take care when working around woodpiles and bringing firewood indoors.
Encounters with all house spiders are far more common during fall when amorous male spiders are wandering in search of female spiders. During the rest of the year these spiders tend to be fairly secretive. All three house spiders originated in Europe where, interestingly, the hobo spider is not considered to be poisonous.
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Widow spiders (Latrodectus spp.)
Widow spiders (pictures) are very secretive. They occur in undisturbed areas and under debris. They rarely bite but bites can be very dangerous. They possess a neurotoxic venom that can result in serious medical complications, even death. Fortunately antivenom is now widely available. As with hobo spiders be cautious when working around woodpiles or undisturbed debris in areas where widow spiders occur. Widow spiders tend to be more common in dry and warm climates but occur throughout North America.
Brown recluse spider (Loxosceles reclusa)
The distribution of the brown recluse spider is much more limited than many people believe [range map]. It occurs only in the south-central US. Bites sometimes result in serious necrotic wounds that can develop into dangerous medical conditions especially in those with a compromised immune system. Brown recluse spiders have a very distinctive "violin" shape mark on the spider's upper surface (cephalothorax) (picture).
Unfortunately, there are many myths about spiders and spider bites -- check this Burke Museum site for details.
Treating household spider infestations
Spiders that you find outdoors can be left alone. They do not generally pose a threat as long as you exercise some reasonable caution. Wear workgloves when working around woodpiles or debris piles. Widow spiders tend to occur in out-of-the-way places in outbuildings and crawlspaces so be cautious there as well.
Sticky board traps are usually the best solution for spiders that wander indoors like male hobo and recluse spiders. Widow spiders only rarely enter livingspaces. If they are found indoors they can be individually removed.
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