PEST NOTES Publication 74149University of CaliforniaStatewide Integrated Pest Management ProgramAgriculture and Natural Resources

October 2009

There are two species of widow spiders in California, the western black widow and the brown widow. Both are in the genus Latrodectus and are characterized by a similar body shape, reclusive habit, and haphazardly constructed cobwebs.

The western black widow spider—a native species—is widespread and is the spider posing the greatest threat to humans in the Western United States. It is well known in many localities, and nonprofessionals can identify it easily.

In the first decade of the 21st century, the non-native brown widow became established in Southern California, and although it isn’t nearly as dangerous as the black widow, it causes alarm be-cause of its potentially deadly relative.

BLACK WIDOW SPIDERSSeveral species of black widow spiders are common in North America, but in the Western United States, the only species is the western black widow, Latrodectus hesperus. Its habitat ranges from British Columbia to Mexico and throughout the Rocky Mountains to the western portions of the Great Plains.

In California, it is a common desert spi-der that can survive very hot, dry condi-tions. However, black widows also can be found in mountainous terrains above the 5,000-foot elevation in Southern Cali-fornia where snow covers the ground every winter. Outside of California, they are common in urban Colorado and in Central and Eastern Washington state.

Because the holes, cracks, crevices, trash, and clutter associated with human struc-tures attract the western black widow, these spiders are often very common around homes, barns, outbuildings, and rock walls. In supportive habitats, a ma-ture female can be found every few feet.

IdentificationThe mature female western black widow spider (Fig. 1) is about 1/2-inch long, not including the legs, and has a rounded abdomen and very characteristic color-ation. She is shiny jet black all over her body and legs except for a red pattern on the underside of the abdomen, which looks, in perfect specimens, like an hour-glass. Some specimens have a brownish or plum-colored tinge, but usually these are females that are so well fed the black pigment on the abdomen has expanded until it looks brown instead of black.

The red hourglass can vary from two perfect triangles whose points merge to make a perfect hourglass to two tri-angles separated by a space, a triangle and small bar, or just minimal almost imperceptible red coloration. The false black widow, which is discussed below, is chocolate brown and never has red coloration, although many people fre-quently mistake it for a black widow.

As easy as it is to identify an adult female black widow, the immatures look noth-ing like the mother (Fig. 2). When baby black widow spiderlings emerge from their egg sac (Fig. 3), they have tan legs and tan cephalothorax, the body part to which the legs attach, while the abdomen is mostly white with a few black spots.

As the spider grows, the background coloration of the abdomen becomes olive gray, and there is a longitudinal white stripe on the top of the abdo-men and three diagonal stripes on the flanks with a small black dot at the up-permost portion of each diagonal stripe.

Like all spiders, as the spiderlings grow larger, they molt in order to shed their restrictive exoskeleton. With succes-sive molts in females, the white stripes become thinner, the olive gray darkens

toward black, and eventually the spider acquires its well-known black color-ation. Some mature females retain one or two conspicuous, indented white lines on the front surface of the abdo-men that look like a corporal’s chevrons.

In the youngest spiders, the space where the hourglass develops starts off being a whitish shield. As the spider

Integrated Pest Management for Home Gardeners and Landscape Professionals

Black WidoW and other WidoW SpiderS

Figure 1. Mature female western black widow spider.

Figure 3. Western black widow spider-lings and egg sac.

Figure 2. Immature female western black widow spider.

October 2009 Black Widow and Other Widow Spiders

◆ 2 of 5 ◆

grows and goes through several molts, the color of this shield turns from white to yellow to orange to red and changes from a shield with thick middle to a hourglass with a thin, tapered middle.

In contrast to the female, the male black widow (Fig. 4) retains the coloration of the juvenile. After it matures, it stops eating, and its abdomen shrinks, because its only task at that point is to mate. The male still retains its one longitudinal stripe and set of three diagonal flank stripes on each side the abdomen. The males are much smaller than the females in body length al-though sometimes their legs are almost as long as the adult female.

One more variation involves the longi-tudinal stripe that runs up the middle of the top surface of the abdomen in im-mature black widows. Sometimes it has a vivid red stripe within the confines of the white border. This coloration can cause anxiety for anyone who isn’t fa-miliar with widow immatures, because they might incorrectly identify it as the Australian redback widow spider. This widow has red markings on the top but otherwise is uniformly black rather than mottled as an adult. Redback widow spiders aren’t found in North America.

Egg SacThe egg sac of the western black widow is a yellowish, tear-drop shape, tapered at the top and bulbous at the bottom. The margins of the sac are well de-fined, as opposed to some spider egg sacs, which look like fluffy cotton balls, making it difficult to determine exactly where the egg sac starts. The egg sac is very tough and difficult to rip apart.

A female western black widow typical-ly lays about 300 eggs per sac. Because they can store sperm from their first mating, they can produce more than 10 egg sacs without subsequent matings without a decrease in the number of eggs or a reduction in the percentage of eggs that will hatch into spiderlings.

HabitatThe western black widow is found al-most everywhere in California where people live. Although they can be found in homes, black widows typically are outside, around the home and in clut-ter. In garages, they usually make webs by doors, near vents, and in other places where they are guaranteed lots of insects for food. Because most people don’t toler-ate large numbers of insects in their liv-ing spaces, widow spiders usually won’t find sufficient prey to survive in homes.

Black widows are shy spiders that seek retreats such as a hole between two bricks or a pipe hole in a wall where they can hide during the day and then come out at night. In natural settings, you’ll of-ten find them in rodent burrows and rock faces. The spider makes a web of tangled silk extending from this retreat hole.

The web doesn’t have a very recogniz-able pattern although it does have verti-cal support threads above and below the central areas where the spider sits while it waits for prey at night. The lower sup-port threads also alert the widow to the presence of a prey item blundering into the web. In most cases the widow spider will seek a retreat near the ground as the home base for her web, which con-nects to the retreat, allowing the spider to emerge to catch both flying and crawling prey at night. However, some spiders will make a retreat well above ground level, such as in the eave of a house, and then drop down 10 or more feet before building their web.

Widow spiders come out at dusk. After making improvements to their exist-ing web, they take up a position in the middle, their underside facing upward, to wait for prey. Any large disturbance of the web that indicates something larger than a prey item causes the spi-der to quickly move toward the safety of its retreat.

The silk of a mature black widow is very strong; running a finger through the web that a large spider has made re-sults in an audible ripping sound. Dur-ing World War II, black widow silk was used to make crosshairs for gun sights.

Medical AspectsThe black widow bite itself is painless or may feel like a little pinprick. Almost all medically important black widow bites are from the adult female, which is much larger than the male; the female also has stronger biting muscles and a larger venom reserve. At the site of the bite, you might see a little red mark or red streaking away from the bite. With-in an hour, symptoms start to appear.

Bite victims might suffer from some but not all of the following symptoms: rigid stomach muscles, which some medical professionals have misdiagnosed as ap-pendicitis; sweating, sometimes of just the bitten body part, such as a bite to the hand that results in only the arm sweat-ing profusely; pain that can be local, radiating, or regional; urine retention; and—less commonly—numbness, agita-tion, fever, and patchy paralysis. An-other symptom is bite victims will move or rock back and forth incessantly to try to lessen the pain from the venom injec-tion process. However, these symptoms are the most severe manifestation; many black widow bite symptoms merely resemble the flu. Black widow bites don’t cause conspicuous swelling, necrosis, or deterioration of tissue around the bite.

As a neurotoxin, the venom of a black widow affects the nerve-muscle junction in the body. Normally the body’s neurons work like a light switch; they make the muscle, or “light,” turn on and then off again, so that the muscle can relax and be ready to contract again if needed. The venom causes the muscle to repeatedly contract. It would be like flipping on a light switch and not being able to turn it off again.

Figure 4. Mature male western black widow spider.

October 2009 Black Widow and Other Widow Spiders

◆ 3 of 5 ◆

If bitten, seek medical attention im-mediately. You can place a cold pack on the bite to relieve the pain. An antivenom for black widow bites is available that works for all species that have been tested, worldwide. Response is fast, and bite victims can go from intense pain back to normal in 30 min-utes. The antivenom is based on horse serum, so physicians need to monitor for anaphylactic shock. American phy-sicians are somewhat reluctant to use antivenom for this reason and might prefer to have the bite victim simply endure the symptoms, which can be similar to a bad flu episode and that usually dissipate in a few days.

BROWN WIDOW SPIDERSThe brown widow spider, Latrodec-tus geometricus, is found worldwide in subtropical habitats. It probably originated from Africa although there is some conjecture that it also could have been native to South America. In North America, for many decades, it was found only in Florida, where it was rather common. However, in the first decade of the 21st century, the spider began appearing from Texas through-out the Gulf Coast states and up the Atlantic Coast into South Carolina.

While it was expanding in the South-eastern United States, it was being col-lected in great numbers in Southern California. Initial news reports exag-gerated the impact of the brown widow. However, unlike the black widow, this spider isn’t much more toxic to humans than the typical spider.

IdentificationThe brown widow spider is a mottled collection of tan, brown, and gray (Fig. 5). It has the longitudinal stripe on the abdomen and the three diagonal stripes on the side similar to immature western black widow spiders. However, the coloration is more of a tan whereas the western black widow is more olive gray.

Also in the brown widow, the black marks at the top end of the 3 diago-nal abdominal stripes are large and

squarish whereas in the western black widow, the black mark is more round and much smaller. Additionally, the hourglass of the brown widow spider is more of an orangish color or orange in the middle with a yellowish border.

In many ways, the mature brown widow female looks very similar to an immature western black widow, so you need to have some skill to identify the two accurately. However, the brown widow egg sac, as described in the next paragraph, is a sure way to confirm which species you’ve found.

Egg SacThe egg sac (Fig. 6) of the brown widow is so characteristic, the spiders them-selves need not be seen in order to verify their presence in the home. The sac has protuberances of silk all over its surface, and some say it looks like a large pollen grain.

Brown widows produce about 80 eggs per egg sac and are able to make 20 or more egg sacs in a lifetime, several in a short time period. It is not uncommon to collect a female brown widow with several simultaneously developing egg sacs.

HabitatThe habitat of the brown widow is similar in many respects to that of the black widow. They generally reside in cluttered areas outside such as wood-piles or in cluttered areas of garages, but you’ll also find them in more exposed areas such as on chain-link fences where black widows normally wouldn’t be found. Their webs also are cobwebby.

The brown widow appears to be dis-placing the black widow in many of its habitats. Brown widows are common in Los Angeles, Orange, and San Diego counties. Because this spider has been found in Southern California only since about 2002, it probably still is expanding its range and might eventu-ally spread up the coast toward Santa Barbara or into the Central Valley.

Medical AspectsThe bite of the brown widow is much milder than the western black widow. In one study in Africa, where the brown widow might have originated, the most common symptoms in 15 veri-fied bites were that the bite hurt when it happened, and it left a red mark; none of these patients developed the typical dynamic symptoms of black widow envenomation. However, there is one American record of a verified bite where the patient developed more severe symptoms and required hospi-talization.

Although its venom is as potent as black widow venom, drop for drop, the brown widow probably doesn’t inject very much venom during a bite, mak-ing it much less dangerous than the native black widow spider.

The recent arrival of the brown widow in Southern California is not cause for alarm. In fact, if the brown widow is indeed displacing the more toxic black widow, there actually might be a reduc-tion in spider bite risk compared to previous decades.

Figure 5. Mature female brown widow spider.

Figure 6. Brown widow spider egg sacs.

October 2009 Black Widow and Other Widow Spiders

◆ 4 of 5 ◆

FALSE BLACK WIDOWThe false black widow, Steatoda grossa, (Fig. 7) is not a true widow spider; however, it is in the same family, Theri-diidae, as the widow spiders and easily can be confused with them. It shares the same rounded-abdomen body form and web-making traits. It is slightly smaller than a mature western black widow spider, is chocolate brown, and never has red coloration on its belly.

The false black widow is a European immigrant that has become extremely common in Pacific Coast homes from San Diego to British Columbia. It doesn’t seem to need as much food as the black widow, so the spiders are more likely found inside cupboards and underneath refrigerators or cabinets.

False black widows make an egg sac (Fig. 8) that looks like a cotton ball with indistinct margins. Unlike black widow spiders, baby false black widow spider-lings are dark like their mothers when they emerge.

Because they are more common in homes, they frequently are involved in bite incidents. They have mild venom and cause symptoms similar to a mild black widow spider bite. When black widow antivenom mistakenly has been used on false black widow bites, it ap-peared to eliminate the venom effects.

MANAGEMENTOne of the easiest ways to minimize encounters with widow spiders is to reduce clutter around your home, which deprives them of places to make retreats. However, it is impractical to eliminate them completely by removing all clutter.

Regularly vacuuming or sweeping 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 don’t survive this process.

In the garage, keep items such as gardening clothes and gloves in bags

closed with zipper locks or twist ties. Store seasonal items such as winter clothes or Christmas decorations in boxes that you can tape shut and can place off the floor and away from walls in order to exclude spiders. When cleaning up clutter in garages and oth-er storage areas, be sure to wear gloves to avoid accidental bites.

Areas of concern include children’s pedal-powered toy vehicles made of molded plastic that have open spaces facing downward where spiders can crawl in. Picnic tables and other large pieces of furniture where you place your fingers underneath to lift also can be a source of exposure.

Spiders can enter houses and other structures through cracks and other openings. To prevent spiders from com-ing indoors, seal cracks in the founda-tion and other parts of the building and gaps around windows and doors. Good screening not only will keep out many spiders but also will discourage them by keeping out the insects they eat. However, baby black widows have no problem crawling through regular window screen mesh.

Be careful that you don’t carry spiders indoors on items such as plants, fire-wood, and boxes. Stack woodpiles away from your house, and never pick up pieces of wood unless you are wear-ing gloves.

Eliminate places for spiders to hide and build their webs by keeping the area next to the foundation free of trash, leaf litter, and accumulations of other ma-terials. Removing ivy and other heavy vegetation growing around founda-tions and trimming plant growth away from your home and other structures will discourage spiders, in general, from taking up residence near the structure and then moving indoors.

Outdoor lighting attracts insects, which in turn attracts spiders. If possi-ble, keep lighting fixtures off structures and away from windows and door-ways. Sweep, mop, hose, or vacuum webs and spiders off buildings regular-

ly. Insecticides won’t provide long-term control, so generally you shouldn’t use them against spiders outdoors.

Because widow spiders are nocturnal, a nonchemical method of eradication is to search for them at night with a flash-light and kill them with a shoe or rolled up newspaper. If you are concerned about wildlife and feel comfortable doing so, you can remove individual spiders from indoor areas by placing a jar over them and slipping a piece of paper underneath to seal off the open-ing when you life the jar up. Release the spider about 100 feet from your home into a natural area.

One aspect that makes controlling widow spiders difficult is that they, like many spiders, exhibit a behavior called ballooning. When the spiderlings are very small, on warm days when there is an updraft they climb to the top of a fence post or piece of vegetation, raise their abdomens into the air, and release a small filament of silk.

When the updraft currents overtake the forces of gravity, the spiderling is carried into the air to another location. This may only be a few feet away, or it could be miles. Ballooning spiderlings

Figure 7. Mature false black widow female.

Figure 8. False black widow egg sac (left) and black widow egg sac (right).

October 2009 Black Widow and Other Widow Spiders

◆ 5 of 5 ◆

have been captured at 10,000 feet from the ground and 200 miles offshore. Because spiderlings will be dropping down on your property continually, eliminating them will be a task that needs to be done repetitively through-out the year.

Chemical ControlTypically pesticide control of spiders is difficult. Various insecticides are registered for control of spiders includ-ing pyrethrins, resmethrin, allethrin, or combinations of these products; however, they usually aren’t very ef-fective. Sprays work only if you apply them directly to the spider or their web, since the spray residual does not have a long-lasting effect. This means a spider can walk over a sprayed surface a few days—and in many cases, a few hours—after treatment and not be af-fected. Sprays won’t affect egg sacs, and if you apply them to the outside pe-rimeter of a structure, they won’t keep spiders from moving in.

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 news-paper or your shoe or to vacuum it up. Removing harborage sites such as clut-ter, woodpiles, or heavy ground cover is essential for reducing widow spider populations. Sticky traps offer a nonin-secticidal way to remove spiders from your home as long as you can place the traps where pets and curious children can’t tamper with them.

REFERENCESClark, R. F., S. Wethern-Kestner, M. V. Vance, and R. Gerkin. 1992. Clinical presentation and treatment of black widow spider envenomation: a review of 163 cases. Annals Emergency Medicine. 21:782-787.

Goddard, J., S. Upshaw, D. Held, and K. Johnnson. 2008. Severe reaction from envenomation by the brown widow spider, Latrodectus geometricus (Araneae: Theridiidae). Southern Medical Journal. 101:1269-1270.

Kaston, B. J. 1970. Comparative biol-

AUTHOR: R. S. Vetter, Entomology, UC Riverside.

TECHNICAL EDITOR: M. L. Flint

EDITOR: M. L. Fayard

ILLUSTRATIONS: Figs. 1 and 4-8, R. S. Vetter; and Figs. 2-3, J. K. Clark.

This and other Pest Notes are available atwww.ipm.ucdavis.edu.

For more information, contact the University ofCalifornia Cooperative Extension office in your county. See your telephone directory for addresses and phone numbers, or visit http://ucanr.org/ce.cfm.

University of CaliforniaAgriculture and Natural Resources Program

Produced by UC StatewideIntegrated Pest Management ProgramUniversity of California, Davis, CA 95616

University of California scientists and other qualified professionals have anonymously peer reviewed this publication for technical accuracy. The ANR Associate Editor for Urban Pest Management managed this review process. To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products that are not mentioned. This material is partially based upon work supported by the Extension Service, U.S. Department of Agriculture, under special project Section 3(d), Integrated Pest Management.

WARNING ON THE USE OF CHEMICALS Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original, labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock. Pesticides applied in your home and landscape can move and contaminate creeks, rivers, and oceans. Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens containing fruits or vegetables ready to be picked. Do not place containers containing pesticide in the trash or pour pesticides down the sink or toilet. Either use the pesticide according to the label, or take unwanted pesticides to a Household Hazardous Waste Collection site. Contact your county agricultural commissioner for additional information on safe container disposal and for the location of the Household Hazardous Waste Collection site nearest you. Dispose of empty containers by following label directions. Never reuse or burn the containers or dispose of them in such a manner that they may contaminate water supplies or natural waterways.

NONDISCRIMINATION STATEMENT The University of California prohibits discrimination or harassment of any person on the basis of race, color, national origin, religion, sex, gender identity, pregnancy (including childbirth and medical conditions related to pregnancy or childbirth), physical or mental disability, medical condition (cancer-related or genetic characteristics), ancestry, marital status, age, sexual orientation, citizenship, or service in the uniformed services (as defined by the Uniformed Services Employment and Reemployment Rights Act of 1994: service in the uniformed services includes membership, application for membership, performance of service, application for service, or obligation for service in the uniformed services) in any of its programs or activities. University policy also prohibits reprisal or retaliation against any person in any of its programs or activities for making a complaint of discrimination or sexual harassment or for using or participating in the investigation or resolution process of any such complaint. University policy is intended to be consistent with the provisions of applicable State and Federal laws. Inquiries regarding the University’s nondiscrimination policies may be directed to the Affirmative Action/Equal Opportunity Director, University of California, Agriculture and Natural Resources, 1111 Franklin Street, 6th Floor, Oakland, CA 94607, (510) 987-0096.

ogy of American black widow spiders. Transactions San Diego Society Natural History. 16:33-82.

McCrone, J. D. 1964. Comparative le-thality of several Latrodectus venoms. Toxicon. 69:201-203.

Muller, G. J. 1993. Black and brown widow spider bites in South Africa: A series of 45 cases. South African Medical Journal. 83:399-405.

v

Vetter, R. S. and G. K. Isbister. 2008. Medical aspects of spider bites. Annual Review of Entomology. 53:409-429.

Vincent, L. S., R. S. Vetter, W. R. Wrenn, J. K. Kempf, and J. E. Berrian. 2008. The brown widow spider, Latrodectus geomet-ricus C. L. Koch, 1841, in Southern Cali-fornia. Pan-Pac. Entomol. 84:344-349.