Thursday, October 1, 2015

Spiders in the House by Donna Duffy

Photo courtesy notyourhomepage.com
You’ve probably noticed an increase in spiders in the house. I know I have – I’m greeted most mornings by a spider trapped on the shower floor or in the sink. Spiders start wandering indoors in the early fall when cooler outdoor temperatures force them to find shelter. Before you panic, remember that most Colorado spiders are harmless.
There are two potentially dangerous spiders in Colorado, the Widow spiders and the Brown Recluse. 
Photo courtesy healthcare.utah.edu

Widow spiders have characteristic red or red-orange markings on the underside of the abdomen. Bites from the widow spider are painful and potentially dangerous because they contain a nerve venom. Fortunately, widow spiders are non-aggressive and rarely bite. When bites do occur they happen when the female is provoked, for example, when an unwitting person presses down on a spider that is resting beneath a log or rock. 

Photo courtesy webmd.com
Brown Recluse spiders are rare in Colorado because of our cold winters and dry climate. The brown recluse lives within a loose, messy web in dark corners of buildings. Brown recluse spiders are pale brown or buckskin colored with long, dark brown legs. A violin shaped dark marking is present behind the head, and the abdomen is uniformly colored. The venom of the brown recluse is damaging to human cells. In susceptible individuals a slow-healing, ulcerous wound may form at the bite site. Oftentimes the original bite is not noted, but after a few hours a blister will form and pain develops. 

From a biological standpoint, it is rarely necessary to control spiders. However, if it is desirable to get rid of spiders in the home, a combination of sanitation and pesticides should be effective. Pesticides alone, without some effort to remove or modify favorable spider habitats, will not be effective.

Remove rocks, wood piles, compost piles, old boards, and other sheltering sites adjacent to the home. Eliminate migration of spiders into homes by caulking cracks and crevices around the foundation. Make sure all screens and doors are sealed tight. Keep crawl spaces free of debris and limit boxes and other potential hiding places from basements and other dark storage areas. Regularly vacuum or brush spider webs. 

Residual insecticides can be used to control spiders when applied to corners and other sites where spiders tend to breed. Household insecticide products containing various pyrethroids are commonly available for this purpose and must be applied in accordance with the label’s instructions. 

For more information and photos of common Colorado spiders, check out CSU Extension’s Fact Sheet 5.512, Spiders in the Home.