This article was published originally on 9/12/2007
Spiders come in all shapes, sizes, and colors. Some live indoors, most live outdoors and a few become accidental invaders that wander indoors by mistake, especially in the fall of the year. Spiders are ecologically beneficial as they feed solely on insects and other arthropods, but that doesn't stop people from wondering, worrying about or hating spiders, especially the few that wander inside in early fall. Typically, the caller wants to know if the spider they've seen is dangerous, and what, if anything, should be done in terms of control.
First, remember that nearly all spiders are harmless. Spiders are timid and will retreat if they can. They will not attempt to bite humans unless held or accidentally trapped and nearly all common Iowa spiders have fangs too small or weak to puncture human skin. Only two of the hundreds of species found in Iowa may be considered dangerous. Fortunately, both the black widow and brown recluse are very rare. For more on these spiders see ISU Extension bulletin PM-1721, "Potentially Dangerous Spiders" (not available online) and the Horticulture and Home Pest newsletter May 4, 2002.
Hunting spiders such as the common wolf spider occasionally enter buildings through gaps or cracks around windows and doors or through other openings in the course of their active hunting. Other spiders such as the grass spiders (funnel weaver spiders) that build horizontal sheet or funnel-shaped webs on the lawn or low shrubs also wander in by mistake while searching for new web sites. Invader, outdoor spiders usually live for less than a day or two and can not reproduce in the house.
Effective management of home-invading spiders starts with exclusionary techniques: Keep outdoor spiders outside by sealing cracks, gaps and other openings, and by repairing windows and doors (especially the door sweep) so they fit tight. Remove potential harborages from around the outside foundation of the building. Trim back shrubs and vines and move firewood and debris away from the foundation.
Insecticides applied outside the structure have little if any benefit, though residual barrier treatments to door and window thresholds, garage and crawlspace entrances and likely entrance points such as between the foundation and siding may provide a temporary reduction. Insecticide fogs and sprays are not recommended inside for the management of spiders. These treatments are generally ineffective in eliminating existing spider problems and will not provide long-term prevention. Vacuum or sweep up occasional invaders and discard. For management of household spiders use a vacuum or broom to remove spiders, their webs and eggs sacs regularly and frequently. For more information, see bulletin Pm-1722, "Common Spiders in and Around Homes" (not available online).
Grass Spiders, AKA Sheet-web or funnel weaver spiders
IC-497(22) -- September 12, 2007