Two spiders found in Oregon, the western black widow (Latrodectus hesperus) and the yellow sac (Cheiracanthium inclusum), have venom that is poisonous. Another Oregon spider, the hobo spider (Tegenaria agrestis), may have toxic venom, but its effect on humans has not been verified. Gardeners may encounter some of these spiders.
Western Black Widow Spider
The western black widow spider is probably the most poisonous spider in Oregon. Although its venom is 15 times as toxic as the venom of the prairie rattlesnake, the amount of poisonous venom in a black widow's bite is typically not enough to cause serious harm in most people.
Black widows are most often found in eastern and southwestern Oregon. They are uncommon in the Willamette Valley. They usually hang out in dark places, including crawlspaces, basements and garages, spinning messy webs underneath or behind furniture and clutter, in crevices and in tight corners.
Adult female black widows have poisonous venom, not the males or juvenile females. The poisonous adult females are glossy black and usually have a bright-red hourglass shape on the underside of their round abdomen. They are about 1/2 inch long and roughly 1 1/2 inch wide with their legs spread. Females start out yellowish white or white, turning more black and acquiring red as they molt their exterior. Males often have yellow and red spots and bands on their backs.
Black widow venom damages the nervous system, leading to sweating, fever and difficulty in breathing, restlessness, nausea, muscle cramps and tremors. The symptoms of a black widow bite last several days. Less than 1 percent of people bitten by a black widow die.
Yellow Sac Spider
Whether you live in Ashland in the south, Portland to the north or Bend in the east, you may spot yellow sac spiders under boards, stones and leaf litter in your garden and yard as well as see them on your home's walls and ceilings and under its windowsills and siding. If you encounter a yellow sac spider, it may bite you. More people are likely to be bitten by yellow sac spiders than any other spider species.
Yellow sac spiders are a cream to light-yellow color with dark-brown tips on their legs and their palpus, which are appendages near the mouth. Males are 1/6 to 1/3 inch long. Females are 1/5 to 2/5 inch long.
A yellow sac spider's bite is typically painful with burning that lasts up to one hour followed by skin rashes and blisters that may last one to 10 hours. Severe symptoms include muscle cramps, malaise, nausea and fever. A bite may cause rotting flesh that does not ordinarily leave scars.
The hobo spider found throughout Oregon was imported from Europe, where its venom is not considered toxic. A brown spider from 1/4- to 5/8 -inch long, the hobo spider builds its funnel-shaped web outdoors in vegetation, under wood and rocks, and in cracks of bricks and blocks.
A hobo spider bite reportedly can lead to rotting flesh that progresses for several days, and another reported symptom is a headache that lingers for several days, according to the May 2006 edition of Pest Notes by the University of California Agriculture and Natural Resources. Research on rabbits suggested that a hobo spider's venom is potentially poisonous to humans, but later research showed the spider's venom didn't cause rotting wounds in rabbits.
Treatment for a Bite
The Mayo Clinic recommends that someone bitten by a spider should wash the bite with mild soap and water, and then put cold packs on the bite to lessen swelling and pain. If a spider bites your leg or arm, elevate it. Take an over-the-counter pain reliever if you need it, following directions on the product's label. Watch for signs of infection.
As a precaution, call your physician or the poison control center hotline at 1-800-222-1222. The hotline is open 24/7. Its operators can direct your call to the poison control center nearest you.
If a black widow spider bites you and you experience unbearable pain or what appear to be life-threatening symptoms, then a doctor can inject an anti-venom into your thigh or vein; however, the injection must be given cautiously because it can cause serious allergic reactions.