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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Brown Recluse Spider in Iowa

This article was published originally on 7/26/2013

 A recent review of reports of brown recluse spider specimens in Iowa confirms what we have said for some time:  brown recluse spiders are rare in Iowa but MIGHT be found in the southern portion of the state.  According to Rick Vetter, Department of Entomology, University of California, Riverside, all available evidence indicates Iowa remains on the northern edge of brown recluse range.  The spiders are absent in the northern portions of the state and though they might be found in isolated incidences in southern Iowa are still scarce, especially compared to southern Illinois, Missouri and other states to the south within the expected range of brown recluse.  See the map below.
 
The occasional discovery and confirmation of a brown recluse spider in central or southern Iowa does not indicate the spiders are common, widespread or found throughout the county.  Specimens could be an individual hitch-hiker intercepted upon arrival or from an isolated and localized infestation.  

  •  The brown recluse is a small spider with long spindly legs. The body is only about 3/8 inch long and tan to straw-brown.
  • The touted "violin-shaped marking" on the top of the cephalothorax varies in color and prominence. Several common spiders have dark markings on the cephalothorax that can be misconstrued as violin-shaped.  See photo below.
  • The brown recluse has only 6 eyes rather than the typical 8. The eyes are arranged in 3 pairs.
  • As the name implies, the brown recluse is reclusive. That is, they are secretive, rarely seen and difficult to find in their hiding places (cracks and gaps under and behind boxes, shelves, appliances and other items). Brown recluse spiders are nocturnal.
  • Those rare brown recluse spiders found in Iowa are usually individual itinerants transported here from their normal range of the south central USA.
  • The brown recluse is found outdoors in the southern USA but in northern areas would only survive indoors.
  • The brown recluse is not aggressive. Bites are a defensive reaction used only when escape is not possible (such as when the spider is trapped against a person's skin).
  • Brown recluse spider bites are usually painless or accompanied by a slight stinging sensation. A localized burning sensation may last for an hour. Ulceration of the skin around a severe bite develops slowly but may persist for several weeks and require intensive medical attention.
  • Not all skin ulcers are from spider bites and overdiagnosis of "spider bite" is widespread.  See Spider Research from UC-Riverside.  
  • Sanitation, especially vacuuming of harborages to remove spiders, webbing and egg sacs is the initial treatment technique. Insecticides are notoriously ineffective and should be limited to spot and void treatments of spider harborages.

 
See the following websites for more information on brown recluse spiders.  
Spider Research, UC-Riverside
The Spider Myths Site.  
 
 
Distribution of Recluse spiders in the U.S.  The distribution of the brown recluse spider is in red.  Map from R. Vetter, UC-R.Distribution of Recluse spiders in the U.S. The distribution of the brown recluse spider is in red. Map from R. Vetter, UC-R. Brown recluse spider showing small body, long legs and violin-shaped marking on the cephalothorax.Brown recluse spider showing small body, long legs and violin-shaped marking on the cephalothorax.