How to Manage Pests
Pests of Homes, Structures, People, and Pets
Many people fear or dislike spiders but for the most part, spiders are beneficial because of their role as predators of insects and other arthropods, and most cannot harm people. Spiders that might injure people—for example, black widows—generally spend most of their time hidden outside homes in woodpiles or in clutter in the garage. The spiders commonly seen out in the open during the day are unlikely to bite people.
Spiders resemble insects and sometimes are confused with them, but they are arachnids, not insects. Spiders have 8 legs and 2 body parts—a head region (cephalothorax) and an abdomen. They lack wings and antennae. Common spider families are described in Table 1.
Most spiders have venom which they use to kill their prey. However, only those spiders whose venom typically causes a serious reaction in humans are called “toxic” spiders. See Spider Bites.
Black Widow Spider
The black widow spider, Latrodectus hesperus, is the most common harmful spider in California. Venom from its bite can cause reactions ranging from mild to painful and serious, but death is very unlikely and many symptoms can be alleviated if medical treatment is obtained. Anyone bitten by this spider should remain calm and promptly seek medical advice. It is helpful if the offending spider can be caught and saved for identification by an arachnologist.
The typical adult female black widow has a shiny black body, slender black legs, and a red or orange mark in the shape of an hourglass on the underside of the large, round abdomen. In contrast, immature and male western black widows have white stripes on a tan abdomen and look very different from the adult females. Only the larger immature female and adult female spiders are able to bite through a person’s skin and inject enough venom to cause a painful reaction.
The adult male black widow is one-half to two-thirds the length of the female, has a small abdomen, and is seldom noticed. The male black widow does possess venom, but its fangs are too small to break human skin. Contrary to popular belief, the female black widow rarely eats the male after mating but may do so if hungry.
The web of the black widow is an irregular, tough-stranded, sticky cobweb mesh in which the spider hangs with its underside up, showing the red hourglass. The egg sacs are mostly spherical, about 1/2 inch long and 1/2 inch in diameter, creamy yellow to light tan in color, opaque, and tough and paperlike on the surface.
Black widow spiders occur in most parts of California. They and their associated webs usually are found in dark, dry, sheltered, relatively undisturbed places. People are most likely to be bitten when they disturb the spider while they are cleaning out or picking up items in such places. The symptoms of a black widow bite are largely internal; little more than local redness and swelling may develop at the bite site. The internal effects may range from mild to severe. Small children, the elderly, and persons with health problems are likely to suffer some of the more severe consequences of the bite. Black widow bites are fairly common in California.
For more information about black widows, see Pest Notes: Widow Spiders and Their Relatives.
Yellow Sac Spider
The common, house-dwelling yellow sac spider, Cheiracanthium mildei, is a small spider that spins a silken sac web in the corners of ceilings and walls, and behind shelves and pictures; it is also commonly found outdoors in shrubbery.
This spider is light yellow and has a slightly darker stripe on the upper middle of the abdomen.The 8 eyes of this spider are all about equal in size and arranged in 2 horizontal rows.
Yellow sac spiders can be seen running on walls and ceilings at night and quickly drop to the floor to escape if they are disturbed. Bites usually occur when the spider becomes trapped against a person’s skin in clothing or bedding. Typical symptoms of a bite include initial pain similar to a bee sting, redness, and sometimes swelling.
Recluse spiders of the genus Loxosceles include the well-known brown recluse spider, L. reclusa, which does not occur in California. While the brown recluse has occasionally been brought into California in household furnishings, firewood, and motor vehicles, there are no breeding populations in the state. Related native recluse species are found in California's southeastern deserts.
Recluse spiders can have a violin-shaped mark (with the neck of the violin pointing backward) on the top side of the head region (cephalothorax). However, the mark is not always distinct, so it should not be used as an identifying character. A more diagnostic feature of recluse spiders is their 6 eyes, arranged in pairs in a semicircle, which can be seen with the use of a good hand lens. Most other spiders have 8 eyes.
More detailed information on these spiders is available in Pest Notes: Brown Recluse and Other Recluse Spiders.
In addition to the species mentioned above, there are only a few other species of spiders in California that may on occasion bite humans. (Remember, if the bite of any spider causes an unusual or severe reaction, contact a physician).
One kind of red and black jumping spider, Phidippus johnsoni, may bite if it is disturbed, but the bites are usually not serious. The female spiders are black with red on the top side of the abdomen whereas the males have a black cephalothorax with an all red abdomen. These spiders range in size from ¼ to ½ inch long.
Tarantulas are long-lived spiders that occupy burrows in the ground during the day but wait at at their burrow entrance at night to prey on insects passing near the burrow. They commonly are feared because of their large size and hairy appearance. These spiders range from 1 to 2 inches in body size. Some tarantulas that occur in tropical parts of the world have bites toxic for humans, but the bites of California tarantulas are not likely to be serious—at worst, they are similar to a bee sting. However, because of the variety of tarantulas sold in the pet trade industry, there is a spectrum of venom potencies among these creatures.
The hobo spider, Eratigena agrestis, is a common spider in the northwestern United States and British Columbia. It builds webs in dark, moist areas such as basements, window wells, wood piles, under driftwood, and around the perimeter of homes. It is a large (1 to 1 3/4 inch, including legs), fast-running brown spider with a herringbone or multiple chevron pattern on the top of the abdomen. The hobo spider has not been documented in California, but it has expanded its range from the Pacific Northwest to northern Utah, Wyoming, and Colorado. Although it has been stated as being a toxic spider, recent research is challenging the original data that elevated this spider to medical importance. See Pest Notes: Hobo Spider, listed in the References section.
One spider frequently found indoors is the common house spider, Parasteatoda tepidariorum, which makes a cobweb in corners of rooms, in windows, and in similar places. Another is the marbled cellar spider, Holocnemus pluchei, which was introduced into the state in the 1970s and has since displaced the once common longbodied cellar spider, Pholcus phalangioides, a long-legged spider that resembles a daddy-longlegs. These spiders are only marginally capable of biting humans because their fangs are too short to pierce people’s skin in most cases; they primarily cause problems by producing messy cobwebs.
Various kinds of small hunting spiders may wander indoors and occasionally, rather large, hunting-type spiders are discovered in homes or garages. Often these are fully grown wolf spider or tarantula males that have reached maturity and are searching for females. When these spiders are wandering, one or more may accidentally get indoors. New houses and other structures in developments may be invaded by wolf spiders that have lost their usual outdoor living places. The more insects that exist inside a building, the more likely it is to have spiders living there. Immature and adult female burrow-living spiders sometimes wander for a time during the rainy season if they have had to abandon wet burrows.
Spiders may enter houses and other structures through cracks and other openings. They also may be carried in on items like plants, firewood, and boxes. To prevent spiders from coming indoors, seal cracks in the foundation and other parts of the structure and gaps around windows and doors. Good screening not only will keep out many spiders but also will discourage them by keeping out insects that they prey upon.
Regular vacuuming or sweeping of windows, corners of rooms, storage areas, basements, and other seldom used areas helps remove spiders and their webs. Vacuuming spiders can be an effective control technique because their soft bodies usually do not survive the tumbling process through the hose. Indoors, a web on which dust has gathered is an old web that is no longer being used by a spider because occupied spider webs are maintained for cleanliness to enhance prey capture.
Individual spiders can also be removed from indoor areas by placing a jar over them and slipping a piece of paper under the jar that then seals off the opening of the jar when it is lifted up. They can then be released outdoors. Watch this video to learn more: How to catch a spider.
In indoor storage areas, place boxes off the floor and away from walls, whenever possible, to help reduce their usefulness as a harborage for spiders. Sealing the boxes with tape will prevent spiders from taking up residence within. Clean up clutter in garages, sheds, basements, and other storage areas. Be sure to wear gloves to avoid accidental bites.
Outdoors, eliminate places for spiders to hide and build their webs by keeping the area next to the foundation free of trash, leaf litter, heavy vegetation, and other accumulations of materials. Trimming plant growth away from the house and other structures will discourage spiders from first taking up residence near the structure and then moving indoors. Outdoor lighting attracts insects, which in turn attracts spiders. If possible, keep lighting fixtures off structures and away from windows and doorways. Sweep, mop, hose, or vacuum webs and spiders off buildings regularly.
Insecticides will not provide long-term control and should not generally be used against spiders outdoors. Typically, controlling spiders with pesticides is difficult unless you actually see the spider and are able to spray it. There are various insecticides available in retail outlets labeled for spider control, including pyrethrins and pyrethroids (synthetic versions of pyrethrins) such as resmethrin, allethrin, or combinations of these products. Control by spraying is only temporary unless accompanied by housekeeping. It is just as easy and much less toxic to crush the spider with a rolled up newspaper or your shoe or to vacuum it up. Sticky traps offer a noninsecticidal way to remove spiders from your home as long as you can place the traps where pets and children can’t tamper with them.
Sorptive dusts containing amorphous silica gel (silica aerogel) and pyrethrins, which can be applied by professional pest control applicators only, may be useful in certain indoor situations. Sorptive dust particles affect the outer covering of spiders (and also insects) that have crawled over a treated surface, causing them to dry out and die. When applied as a dustlike film and left in place, a sorptive dust provides permanent protection against spiders. The dust is most safely and effectively used in cracks and crevices and in attics, wall voids, and other enclosed or unused places.
Unlike mosquitoes, spiders do not seek people in order to bite them. Generally, a spider doesn’t try to bite a person unless it has been squeezed, lain on, or similarly provoked to defend itself. Moreover, the jaws of most spiders are so small that the fangs cannot penetrate the skin of an adult person. Sometimes when a spider is disturbed in its web, it may bite instinctively because it mistakenly senses that an insect has been caught.
The severity of a spider bite depends on factors such as the kind of spider, the amount of venom injected, and the age and health of the person bitten. A spider bite might cause no reaction at all, or it might result in varying amounts of itching, redness, stiffness, swelling, and pain—at worst, usually no more severe than a bee sting. Typically, the symptoms persist from a few minutes to a few hours. Like reactions to bee stings, however, people vary in their responses to spider bites, so if the bite of any spider causes an unusual or severe reaction, such as increasing pain or extreme swelling, contact a physician, hospital, or poison control center (in California, the number is 1-800-876-4766 or 1-800-8-POISON). One must also keep in mind that although it is impossible to be certain, people and their physicians often blame spiders as the cause of skin lesions with no proof of spider involvement when there are many similar-appearing, non-spider maladies; therefore, care must be exerted in the diagnostic process.
Sometimes a person may not be aware of having been bitten until pain and other symptoms begin to develop. Other species of arthropods whose bites or stings may be mistaken for that of a spider include ticks, fleas, bees, wasps, bedbugs, mosquitoes, the conenose (kissing) bug (Triatoma protracta), deer flies, horse flies, and water bugs (Lethocerus spp.).
For first aid treatment of a spider bite, wash the bite, and use ice, ice water or cold compresses to reduce swelling and discomfort. Bites or stings from a variety of arthropods can result in an itching wound. Rather than scratching, if necessary try to relieve the itch with medication. Scratching can break the skin and introduce bacterial infection, which you or even a physician may mistake for an arachnid bite. If you receive a bite that causes an unusual or severe reaction, contact a physician. If you catch the critter in the act, save it for identification, preserve it (or whatever parts of it remain), and take it to your county UC Cooperative Extension office. If no one there can identify it, ask that it be forwarded to a qualified arachnologist.
Hedges SA, Vetter RS. 2012. Pest Control Technology: Field Guide for the Management of Urban Spiders 2nd Ed. Cleveland: GIE Media.
O’Connor-Marer P. 2006. Residential, Industrial, and Institutional Pest Control. UCANR Publ. 3334. Oakland, CA.
Vetter RS. 2015. Pest Notes: Hobo Spiders. UC ANR Publ. 7488. Oakland, CA.
Vetter RS. 2015. The Brown Recluse Spider. Cornell University Press. Ithaca, NY.
Vetter RS. 2017. Pest Notes: Widow Spiders and Their Relatives. UC ANR Publ. 74149. Oakland, CA.
Vetter RS, Isbister GK, Bush SP, Boutin LJ. 2006. Verified bites by yellow sac spiders (genus Cheiracanthium) in the United States and Australia: Where is the necrosis? Amer. J. Trop. Med. Hyg. 74 (6): 1043-1048.
Vetter RS, Ubick D. 2014. Pest Notes: Zoropsis spinimana, A Mediterranean Spider In California. UC ANR Publ. 74143. Oakland, CA.
Pest Notes: Spiders
AUTHOR: Richard S. Vetter, Entomology, UC Riverside
Originally compiled from Barr BA et al. 1984. Spiders. UC ANR Leaflet 2531. Oakland, CA. Out of print.
Technical Editor: K Windbiel-Rojas
Produced by University of California Statewide IPM Program
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