Montana State University

MSU Extension’s Schutter Diagnostic Lab answers questions about Montana spiders

October 14, 2014 -- MSU Extension

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Tel: (406) 994-4571
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BOZEMAN – From identifications to cataloging species present in Montana, experts with Montana State University Extension often address questions about spiders.

Schutter Diagnosic Laboratory on the MSU campus offers information and help to the public in identifying spiders and other insects.

While there is conflicting information on the Internet and in the media, the brown recluse is not native to and cannot survive in Montana’s cold, dry climate. Its native range is from southeastern Kansas south to Texas and east to western North Carolina and south to southern Georgia.

“It is extremely unlikely that any spider bite from this area is from a brown recluse,” said Lauren Kerzicnik, insect diagnostician at the Schutter Diagnostic Laboratory. “While we often receive submissions from people wondering if a spider is a brown recluse, we have never positively identified one in Montana.”

Identifying a brown recluse is difficult because it has a violin pattern that is common to many spiders. Brown recluse spiders have six eyes that are arranged in three pairs of two behind its head. To be sure, identification of a brown recluse must be done by a trained arachnologist or entomologist.

Brown recluse bites are consistently misdiagnosed in areas where the spider is not present, including Wyoming, Colorado and Montana. The venom of brown recluse spiders contains a component called sphingomyelinase D, which creates mild to severe necrotic lesions in the immediate area of the bite.

Reactions to the toxin in the venom delivered from the brown recluse bite mimic several other types of medical issues, including bacterial infections, chemical and allergic reactions, lymphoma and other conditions. The spider is often erroneously blamed for bacterial-caused rashes and lesions that have nothing to do with spiders or spider bites but, rather, are caused by some other wound or puncture that allows bacteria to enter the body.

Bites from brown recluse spiders do not typically cause body-wide or systemic reactions. The venom itself does not cause infection. Instead, the open wound creates an entry point into the body for bacteria. The only way to confirm that a spider or insect is responsible is if the specimen is captured and identified.

The only spider commonly found in Montana with venom harmful to humans is the black widow. Its venom causes latrodectism, which results in persistent sweating, muscle cramping, and other neurological responses. Bites from black widows are very rare. 

The hobo spider, which has also been called the aggressive house spider, is present in most of central and western Montana. It does not cause necrotic lesions and is not directly harmful, despite misinformation on the internet. There has been significant research on this subject over the last decade and any suggestions that hobo spider bites or lesions are dangerous has been discredited. Nevertheless, if a wound becomes inflamed or soreness persists, medical care should be sought as secondary infection can enter the body through the wound.

To minimize the risk of spider bites, take caution when working in crawl spaces, garages, the laundry room, and in areas that are not often encountered.  In general, bites are rare from spiders because they are small, their fangs are small, and they lack the musculature to pierce the human skin. If think you have a spider bite, see a dermatologist if your symptoms persist.

Facts about the brown recluse and other spiders in Montana:

  • The brown recluse has never been positively identified in Montana
  • The brown recluse bite causes localized necrotic lesions on the skin due to a toxin in its venom
  • Necrotic lesions can be caused by several factors, including some spider bites or secondary infections in the bite area
  • The most common cause for such necrotic lesions in areas of the country where brown recluse spiders are not found (such as Montana) is MRSA (Methicillin Resistant Staphlococcus aureus infection)
  • A brown recluse spider must be identified by an experienced professional
  • The black widow is the only spider harmful to humans that has been identified in Montana
  • Hobo spiders are present in Montana but are not aggressive and do not cause necrotic lesions

If you find a spider or insect of concern, please place it in a leak-proof container and either freeze it or preserve it in rubbing alcohol. Bring the sample to your local county Extension office or mail it to the Schutter Diagnostic Lab at 119 Plant BioScience Bldg, PO Box 173150, Bozeman, MT, 59717-3150.

Contact: Lauren Kerzicnik, insect diagnostician, (406) 994-5704, lauren.kerzicnik@montana.edu.