How to Manage Pests
Pests of Homes, Structures, People, and Pets
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 because of its potentially deadly relative.
BLACK WIDOW SPIDERS
Several 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 spider that is able to survive very hot, dry conditions. However, black widows also can be found in mountainous terrains above the 5,000-foot elevation in Southern California 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 structures attract the western black widow, these spiders are often very common around homes, barns, outbuildings, and rock walls. In supportive habitats, a mature female can be found every few feet.
The mature female western black widow spider is about 1/2 inch long, not including the legs, and has a rounded abdomen and very characteristic coloration. 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 hourglass. 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 triangles 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 frequently mistake it for a black widow.
As easy as it is to identify an adult female black widow, the immatures look nothing like the mother. When baby black widow spiderlings emerge from their egg sac, 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 abdomen and three diagonal stripes on the flanks with a small black dot at the uppermost portion of each diagonal stripe.
Like all spiders, as the spiderlings grow larger, they molt in order to shed their restrictive exoskeleton. With successive molts in females, the white stripes become thinner, the olive gray darkens toward black, and eventually the spider acquires its well-known black coloration. Some mature females retain one or two conspicuous, indented white lines on the front surface of the abdomen 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 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 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 although sometimes their legs are almost as long as the adult female.
One more variation involves the longitudinal stripe that runs up the middle of the top surface of the abdomen in immature 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 familiar 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.
The 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 defined, 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 typically 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.
The western black widow spider is found almost everywhere in California where people live. Although they can be found inside homes, black widows typically are outside, around the home and in clutter. In the garage, they usually make webs by doors, near vents, and in other places where they are guaranteed lots of insect traffic for food. Because most people don’t tolerate large numbers of insects in their living spaces, widow spiders usually won’t find sufficient prey to survive inside 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 often 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 recognizable pattern although it does have vertical support threads above and below the central areas where the spider sits while it waits for prey at night. The lower support 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 connects 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 existing 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 spider 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 results in an audible ripping sound. During World War II, black widow silk was used to make crosshairs for gun sights.
The 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. Within 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 appendicitis; sweating, sometimes of just the bitten body part, such as a bite to the hand that results in only the arm sweating profusely; pain that can be local, radiating, or regional; urine retention; and—less commonly—numbness, agitation, fever, and patchy paralysis. Another symptom is bite victims will move or rock back and forth incessantly to try to lessen the pain from the venom injection 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.
If bitten, seek medical attention immediately. 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 minutes. The antivenom is based on horse serum, so physicians need to monitor for anaphylactic shock. American physicians 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 SPIDERS
The brown widow spider, Latrodectus 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 throughout the Gulf Coast states and up the Atlantic Coast into South Carolina.
While it was expanding in the Southeastern United States, it was being collected in great numbers in Southern California. Initial news reports exaggerated the impact of the brown widow. However, unlike the black widow, this spider isn’t much more toxic to humans than the typical spider.
The brown widow spider is a mottled collection of tan, brown, and gray. 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 diagonal 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.
The egg sac of the brown widow is so characteristic, the spiders themselves 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.
The 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 woodpiles 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 displacing 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 eventually spread up the coast toward Santa Barbara or into the Central Valley.
The 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 verified 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 hospitalization.
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, making 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 reduction in spider bite risk compared to previous decades.
The false black widow, Steatoda grossa, is not a true widow spider; however, it is in the same family, Theridiidae, 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 that looks like a cotton ball with indistinct margins. Unlike black widow spiders, baby false black widow spiderlings 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 appeared to eliminate the venom effects.
One 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 other 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 coming indoors, seal cracks in the foundation 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, firewood, and boxes. Stack woodpiles away from your house, and never pick up pieces of wood unless you are wearing 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 materials. Removing ivy and other heavy vegetation growing around foundations 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 possible, keep lighting fixtures off structures and away from windows and doorways. Sweep, mop, hose, or vacuum webs and spiders off buildings regularly. 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 flashlight 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 opening 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 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 throughout the year.
Typically pesticide control of spiders is difficult. Various insecticides are registered for control of spiders including pyrethrins, resmethrin, allethrin, or combinations of these products; however, they usually aren’t very effective. 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 affected. Sprays won’t affect egg sacs, and if you apply them to the outside perimeter 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 newspaper or your shoe or to vacuum it up. Removing harborage sites such as clutter, woodpiles, or heavy ground cover is essential for reducing widow spider populations. Sticky traps offer a noninsecticidal 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.
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Pest Notes: Spiders. Nov. 2007. R. S. Vetter. Oakland: Univ. Calif. Div. Agric. Nat. Res. Publ. 7442.
Author: R. S. Vetter, Entomology, UC Riverside.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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