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Ischoolpestmanager.Org › Docs › 180.0.Pdf PEST NOTES WIDOW SPIDERS AND THEIR RELATIVES Integrated Pest Management for Homes, Gardens, and Landscapes There are two species of widow spiders structures provide ideal habitat for the in California, the western black widow western black widow, these spiders are and the brown widow. Both are in the often very common around homes, genus Latrodectus and are characterized barns, outbuildings, and rock walls. In by a similar body shape, reclusive habit, such supportive habitats, mature and irregular cobwebs. females can be found every few feet and sometimes within inches of each The western black widow, a native spe- other. cies, is widespread, and is the spider posing the greatest potential envenom- Identification ation threat to humans in the western Figure 1. Mature female western black United States. It is well known in many A mature female western black widow widow spider. localities, and nonprofessionals can spider (Figure 1) is about ½ inch (13 identify it easily. mm) in body length, and has a rounded abdomen and very characteristic color- In the first decade of the 21st century, ation. She is shiny jet black all over her the non-native brown widow became body and legs, except for a red pattern established in southern California. on the underside of the abdomen, Although it isn’t nearly as dangerous as which looks, in perfect specimens, like the black widow, it causes alarm an hourglass. Some specimens have a because of the reputation of its relative. brownish or plum-colored tinge, but usually these are females that are so BLACK WIDOW SPIDERS well fed that the black-pigmented abdo- Figure 2. Immature female western black men has been stretched until it looks widow spider. Several species of black widow are com- brown instead of black. mon in North America, but in the west- ern United States the only species The red hourglass varies in appearance; present is the western black widow, Lat- it can be two perfect triangles with rodectus hesperus. Its habitat ranges from points merged to make a perfect hour- British Columbia and Alberta to Mexico glass, two triangles separated by a and throughout the Rocky Mountains space, a triangle and a small bar, mini- to the western portions of the Great mal or almost imperceptible red color- Plains. ation, or, on rare occasions, completely absent. The false black widow, which is In California, it is a common desert spi- discussed below, is chocolate brown der that can survive very hot, dry con- and never has red coloration, although ditions. However, black widows also many people frequently mistake it for a can be found in mountainous terrain black widow. above 5,000-foot elevation in Southern Figure 3. Western black widow spider- California, where snow covers the As easy as it is to identify an adult lings and egg sac. ground every winter. Outside Califor- female black widow, the immatures, nia, western black widows are common looking nothing like the mother (Figure abdomen is mostly white with a few in urban Colorado, in central and east- 2), can be difficult to recognize. When black spots. ern Washington state, and in southern black widow spiderlings emerge from British Columbia. their egg sac (Figure 3), they have tan legs and a tan cephalothorax (the body As the spider grows, the background coloration of the abdomen becomes Because the holes, cracks, crevices, trash, part to which the legs attach), while the olive-gray or tan; and there is a and clutter associated with human P EST NOTES Publication 74149 Statewide Integrated Pest Management Program March 2017 March 2017 WIDOW SPIDERS AND THEIR RELATIVES longitudinal white stripe on the top of fluffy cotton balls, making it difficult to the abdomen and diagonal stripes on determine exactly where the egg sac the flanks, with a small black dot at the starts). The egg sac covering is very uppermost portion of each diagonal tough and difficult to rip apart. stripe. A female western black widow typically As the spiderlings continue to grow lays about 300 eggs per sac. Because larger, they molt, like all spiders, in females can store sperm from their only order to shed their restrictive exoskele- mating, they can produce more than 10 tons. With successive molts in females, egg sacs without subsequent matings the white stripes become thinner, the and without a decrease in the number olive-gray or tan coloration darkens of eggs or a reduction in the percentage toward black, and eventually the spider of eggs that will successfully hatch into acquires its well-known black color- spiderlings. ation. Some mature females retain one or two conspicuous, indented white Habitat lines on the front surface of the abdo- men that look like a corporal’s chevrons. The western black widow is found in populated areas almost everywhere in In the youngest spiders, the space California. Although they can be found where the hourglass develops starts off in homes, black widows typically nest being a whitish shield. As the spider outside, around the home in holes and grows and molts several times, the Figure 4. Mature male western black widow crevices, and within clutter. In garages, color of this shield turns from white- spider. they usually make webs by doors, near yellow to orange-red and changes from vents, and in other places where insects a shield with a thick middle portion to may pass by. Because most people don’t an hourglass with a thin, tapered mid- tolerate large numbers of insects in their dle. living spaces, widow spiders usually won’t find sufficient prey to survive In contrast to the female, the male black inside homes. widow (Figure 4) retains the coloration of the juvenile. After it matures, it stops Black widows are shy spiders that seek eating and its abdomen shrinks (its only retreats, such as holes between bricks or task at this point is to mate). The male spaces around pipe penetrations in still retains one longitudinal abdominal walls, where they can hide during the stripe and a set of diagonal flank stripes day and then come out at night. In nat- on each side of the abdomen. The males ural settings, you can often find them in are much smaller than the females in rodent burrows and crevices in rock body length, although sometimes their faces. The spider makes a web of tan- legs are almost as long as those of the gled silk extending from this retreat adult female. hole. One more color variation involves the The web doesn’t have a very recogniz- longitudinal stripe that runs up the able pattern, although it does have ver- middle of the top surface of the abdo- tical support threads above and below men in immature black widows; some- the central areas where the spider sits times it has a vivid red stripe within the while it waits for prey at night. The confines of the white border. This color- Figure 5. Black widow spider and egg case. lower support threads also alert the ation can cause anxiety for anyone who widow to the presence of a prey item isn’t familiar with widow immatures, Egg Sac blundering into the web. because they might incorrectly identify it as the Australian redback widow spi- The egg sac of the western black widow In most cases, a widow spider will seek der, which has red markings on the top is yellowish and teardrop-shaped, a retreat near the ground as the home but otherwise is uniformly black rather tapered at the top and bulbous at the base for her web, which connects to the than mottled as an adult. Redback bottom (Figure 5). The margins of the retreat, allowing the spider to emerge to widow spiders are not found in North sac are well defined (as opposed to catch both flying and crawling prey at America. some spider egg sacs, which look like night, usually within three feet of the ◆ 2 of 7 ◆ March 2017 WIDOW SPIDERS AND THEIR RELATIVES ground. However, some spiders will Another sign of envenomation is that make a retreat well above ground level, bite victims will sometimes move or BROWN WIDOW SPIDERS such as in the eave of a house, and then rock back and forth incessantly to try to drop down 10 or more feet before build- lessen the pain from the venom injec- The brown widow spider, Latrodectus ing their web. tion process. geometricus, is found worldwide in sub- tropical habitats. It probably originated Widow spiders come out at dusk. After Although these are the most severe from Africa, or possibly from South making improvements to their existing manifestations of black widow bites, America. web, they take up a position in the mid- symptoms most often merely resemble dle, their underside facing upward, to those of the flu. Black widow bites In North America, the brown widow wait for prey. Any large disturbance of don’t cause conspicuous swelling, was found only in Florida for many the web that indicates something larger necrosis, or deterioration of tissue decades, where it remains rather com- than a prey item causes the spider to around the bite. mon. However, in the first decade of the quickly move toward the safety of its 21st century, the spider began appearing retreat. As a neurotoxin, the venom of a black elsewhere, from Texas throughout the widow affects the nerve-muscle junc- Gulf Coast states and up the Atlantic The silk of a mature black widow is tion in the body. Normally, the body’s coast into South Carolina. very strong. If you run your finger neurons work like a light switch: they through the web of a large spider, the make the muscle turn on and then off While they were expanding in the result is an audible ripping sound.
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