Poisonous Spiders

Spiders attempt to bite humans only as a last resort when threatened, injured, or trapped in clothing. They prefer to retreat rather than attack, and will generally avoid contact with humans. A large portion of the spiders commonly encountered cannot penetrate human skin with their tiny fangs. The majority of those capable of biting people produce a venom that has either a relatively low toxicity to humans or is injected in insufficient quantities to cause a serious reaction. The typical reaction to a spider bite consists of localized inflammation, swelling, and pain similar in intensity and duration to that resulting from a wasp or bee sting. Symptoms usually disappear in 12 to 24 hours. Only a few people suffer an allergic reaction to bites from spiders not considered poisonous to man. In general, children are more sensitive to spider bites than adults.

Of the hundreds of species of spiders that occur in North Carolina, only two are known to cause a significant toxic or poisonous reaction in humans–the black widow and the brown recluse. Persons most likely to suffer severe or prolonged effects from toxic bites are the very young, the aged, or people in poor health. Anyone who believes they have been bitten by either of these two spiders should see a physician immediately. If possible, capture and/or kill the spider (try not to crush it) and take it to the physician for a positive identification. If the medical personnel are unable to identify the specimen, take it to your local North Carolina Dept. of Agriculture Extension office or place it in a crushproof container with a preservative, such as alcohol or vinegar.

Black Widow: Black widows belong to a group of spiders known as “cobweb weavers,” named for the erratic and sparse placement of silk strands in their snares. The name “black widow” refers to the female’s color and her habit of occasionally eating the male after mating. Years ago, black widows were notorious residents of outdoor toilets and storm cellars. As the number of these facilities decreased, so did the incidence of black widow bites. This spider’s preferred habitats now include dumps, trash piles, and isolated areas in and around houses.

Adult females have shiny, black, rounded abdomens, and are approximately 1/2 inch long, not including the legs. Most individuals have a reddish-orange hourglass-shaped mark on the underside of the abdomen (Figure 1), but there is considerable variation in this marking. Some specimens have a complete hourglass figure, while in others the mark is reduced to two spots or, in some cases, no markings at all. Coloration on the back of mature females ranges from completely black to black with red, white, or even yellowish markings. Adult males have smaller bodies, longer legs, generally more red and white markings, and do not feed or bite. Newly hatched spiderlings are predominantly white or yellowish-white, and gradually acquire more black and varying amounts of red and white with each molt.

Black widow venom is a nerve poison that is reported to be as much as 15 times more toxic to humans (by volume) than that of the prairie rattlesnake. Fortunately, the amount of venom injected by the spider is relatively small. While there may be little initial pain associated with a black widow bite, severe localized pain usually occurs within 30 minutes and gradually spreads throughout the body. The most pronounced symptoms are severe abdominal and muscular pains. Other possible symptoms include profuse sweating, swollen eyelids, alternate salivation and dryness of the mouth, lack of localized swelling at the site of the bite, nausea, vomiting, development of a rash, and coma. Additional complications can occur due to infection of the bite. Symptoms usually diminish in several hours and are gone after several days.

A specific antivenom and other medications are available to reduce the pain. It should be emphasized that black widow bites are uncommon and that serious long-term complications or death resulting from them are rare. Only four deaths were officially attributed to black widow bites in the United States for the period of 1960-69.

Brown Recluse: As the name implies, this spider is brown and rather shy. It weaves an irregular sheet-like web in sheltered locations outdoors and indoors in undisturbed dry areas such as basements, closets, farm buildings, and storage facilities. Clothing left hanging in such areas for long periods should be examined and shaken prior to wearing to dislodge any spiders that might be hiding inside.

The recluse, also known as the “brown” or “fiddle-back” spider, has body dimensions of approximately 3/8 inch long by 3/16 inch wide. Including its fully extended legs, a mature brown recluse would be slightly smaller than a fifty-cent piece. General body color varies from light gray-brown to reddish-brown. The most predominant marking is a dark brown violin- or fiddle-shaped mark located on the back, with the neck of the fiddle pointing toward the abdomen (rear). Other spiders that may occur in and around structures can be confused with the brown recluse. Fortunately, the fiddle-shaped mark and the presence of only six eyes (arranged in three pairs on the front of the head region) should be sufficient to differentiate this spider from the harmless species that have no fiddle-shaped mark and eight eyes.

Individual susceptibility to brown recluse venom varies considerably and may range from almost no reaction to severe complications or even death. However, as with the black widow, fatalities are extremely rare (six deaths nationwide from 1960-69). Typically there is little or no initial pain associated with a recluse bite, and more than an hour may pass before the individual realizes something is wrong. An intense localized pain generally develops, followed by inflammation of the area and the appearance of a white or mottled blister at the site of the wound. The bite area becomes hard to the touch and often necrosis (tissue death) becomes evident within a few days as the tissue begins to loosen or slough off. The toxin is more active on the underlying tissues than on the skin, which causes the center of the bite area to sink and the surrounding area to raise slightly. An ulcerating sore develops which may continue to increase in size and, as the toxin remains in the underlying tissues for a considerable period of time, may take several months to heal. Various systemic reactions may also occur while the ulcer is present.

No specific antivenom is available for brown recluse toxin but various other treatments are used with some success. Early diagnosis and treatment are essential to alleviate pain, speed healing of ulcerated tissue, reduce the probability of systemic reaction, and to prevent infection.

Spider Control

The consistent presence of spiders in structures is often a sign of insect infestation as the spiders could not survive long periods without food. If this is the case, control efforts directed at the insects should eventually decrease or eliminate the spiders as well.

Removing spider webs and egg sacs along with frequent cleaning of closets, attics, basements, and other storage areas will discourage the establishment of spiders.

Insecticides applied directly to spiders, webs, and areas frequented by spiders should control existing infestations. Insecticides available for spider control in and around structures include chlorpyrifos, diazinon, malathion, propoxur, pyrethrins, and resmethrin. These insecticides are registered under various brand names and in different formulations. Reinvasion from outdoors or the hatching of hidden eggs will make additional treatments necessary. Peeler Environmental is the exterminator in Salisbury, NC to assist you with all of your spider control needs.