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Integrated Pest Management Manual

Spiders and Scorpions

spiders and scorpions
This module is intended to serve as a source of basic information needed to implement an integrated pest management program for spiders and scorpions. Any pest management plan or activity must be formulated within the framework of the management zones where it will be implemented. Full consideration must be given to threatened and endangered species, natural and cultural resources, human health and safety, and the legal mandates of the individual parks. Recommendations in this module must be evaluated and applied in relation to these broader considerations.

Most people are familiar with the general appearance of both spiders and scorpions. Spiders and scorpions are both arachnids, which is a group of animals that also includes mites, ticks, and harvestmen (daddy longlegs). The arachnids are closely related to insects. Both spiders and scorpions, like insects, have a hard external body, but spiders and scorpions have four pairs of legs while insects have three pairs.

Many people may fear spiders and scorpions because of misunderstandings concerning their dangerous nature. In reality there are only a few species of spiders and scorpions that warrant caution. Both spiders and scorpions are a normal and desirable part of the ecosystems in which they occur. They feed on other insects, including species which are pests of plants and nuisance species such as biting flies, as well as other spiders and scorpions. Therefore, spiders and scorpions are generally considered to be beneficial organisms.

Despite the generally benign nature of most scorpions and spiders, bites or stings by some species can be life-threatening to small children, the elderly, or people who are hypersensitive. There are three spider groups of medical importance: the widow spiders (including the black widow, Latrodectus mactans ), the brown recluse spider, Loxosceles reclusa (and the related Loxosceles laeta), and the aggressive house spiders (genus Tegenaria). There is only one genus of scorpion found in the United States that is of medical importance (Centruroides), commonly known as bark scorpions. None of these dangerous species of scorpions and spiders bite or sting humans without provocation. The majority of bites or stings occur because the spider or scorpion has been sat, rolled, or stepped on, or because attempts were made to pick up the spider or scorpion.

In this module, the biology and habits of the major species of spiders and scorpions that are potential health hazards will be discussed in detail. Control measures and education programs to increase public awareness of the potential hazard of these species are outlined. The geographic regions in which specific dangerous species of spiders and scorpions occur are identified and the most common places in which humans are likely to be exposed are discussed. This report does not include emergency medical procedures for care of persons bitten or stung by these dangerous species. However, some information is provided on how to determine if medical treatment should be sought. THIS INFORMATION IS PROVIDED ONLY AS A GUIDE. If in doubt, always consult medical professionals.


Scorpions are predaceous on various small invertebrates (insects, arachnids and related arthropods) and vertebrates (small lizards and rodents). Scorpions can withstand long periods of starvation (up to five months) without any noticeable effect. In areas where there are occasional subfreezing temperatures scorpions will hide in warmer rock and bark crevices (Ebeling 1975). Scorpions become inactive at temperatures of 40-50oF (4-10o C) and at extremely high temperatures (Polis 1990).

Scorpions give birth to live young. Immediately following birth, the young scorpions crawl onto the back of their mother where they remain for 5-15 days. The young scorpions are white and soft during this time. Following pigment development (tanning), the young scorpions will leave their mother's back and begin to forage for food. Scorpions may live for several years depending on the availability of food and water.

Geographic Distribution

In the United States, scorpions are most abundant in the arid and semi-arid regions of the Southwest. No scorpion species occur in the Rocky Mountains, North- Central,or Northeastern United States Only one species which occurs throughout most of the Southeastern United States, and one species occurs in northern Florida, while three species occur in southern Florida. Southern Nevada, southeastern California, and central Arizona have the highest diversity of scorpion species in the United States (Polis 1990).

The bark scorpion, C. exilicauda (=sculpturatus), which is the only American species of scorpion whose sting may be life-threatening, occurs primarily in the southeastern part of Arizona, but may also be found in southern New Mexico and southern California. However, a related but less venomous species, C. vittatus, occurs in Texas, especially in Big Bend National Park, and in the southeastern United States from South Carolina south to Florida and west to New Mexico. ( C. vittatus was formerly divided into three species, C. vittatus, C. partherienois, and C. chisosarius).


Scorpions are nocturnal and hide in crevices and shaded areas during daylight hours, in part to reduce loss of moisture. Even during evening hours, scorpions spend most of their time in burrows or hiding places (Polis 1990). Despite occurring in arid regions, scorpions need to drink water in addition to obtaining it from their food. Because of this need for moisture, scorpions may be more common near natural and artificial sources of water at night. During seasons when scorpions are active, most desert scorpion species are most active during the early hours of the evening (prior to midnight) (Polis 1990). During daylight hours, scorpions hide in areas where protection from daytime heat and sun, and water are available. Unfortunately, areas occupied by humans in arid regions are also associated with natural and artificial water sources, increasing the likelihood of human-scorpion encounters.

The bark scorpion is almost always found associated with trees, especially in riparian habitats. This species is commonly found in mesquite, cottonwood, and sycamore groves. It is a climbing species and is almost never found hiding in burrows, except during periods of hibernation. Its climbing habit distinguishes it from all other scorpions in its geographic range (Smith 1982).

Scorpions are attracted to water sources in buildings. Bathroom and kitchen areas are the most frequent places scorpions can be found at night in buildings. During the day, scorpions will seek out hiding places such as cracks and crevices in the floor, cabinets, attics, wall voids, and crawl spaces. Scorpions are most often a problem in buildings in newly developed areas (within three years). This is attributable to the disturbance or destruction of the scorpion's territories (Smith 1982). Additionally, buildings near arroyos or dry riverbeds may experience an influx of scorpions during periods of rain as the scorpions seek out higher ground. Scorpions in buildings are not likely to meet their normal requirements of temperature and prey density (Smith 1982).


The presence of scorpions in an area can be detected by trapping and visual scouting. Pitfall traps (a small hole dug into the ground and covered with a board, rock, etc.), although not scorpion specific, may help to identify the presence of scorpions. For best results a trap should be set near water sources and checked during daylight hours while wearing leather gloves. This is when scorpions may be hiding in the trap, except for the bark scorpion which hides under bark on trees during daylight hours. Visual scouting for scorpions can be done during both daylight and dusk (or early evening) hours. During daylight hours a visual search under rocks, loose bark, and other debris (while wearing leather gloves) can confirm the presence of scorpions. At night, the inside of buildings and outside areas may be searched using a ultra-violet fluorescent light fixture. Scorpions glow brightly under black light and are extremely conspicuous even from yards away (Smith 1982).


Programs to educate the public should be implemented in areas where scorpions are known to occur. These should include identification of scorpions, especially recognition of the dangerous species occurring in the region. People should be encouraged to avoid risky activity in areas where scorpions have been observed. The program should also educate the public about the beneficial role that scorpions play in the ecosystem, and the importance of scorpions as natural enemies of other arachnids and insects. Lastly, preventative behaviors should be outlined and the groups of people most at risk identified.

Additional precautionary methods that should be included in the education program are: 1) wearing leather gloves when moving objects and collecting firewood at campsites or in outdoor areas, 2) when camping, invert and shake out clothes, sleeping bags and other items that have been in contact with the ground or trees, and shake out shoes before putting them on, and 3) always wear shoes when walking at dusk or at night.

The best methods for controlling scorpions are 1) those that alter the habitat where human contact is likely in order to make it less suitable for scorpions and 2) the creation of barriers that restrict the movement of scorpions into buildings and areas where contact is likely. Cultural methods such as sanitation and elimination of harborages have been found to be effective in reducing scorpion numbers. Barriers to movement of scorpions into dwellings can also be effective in reducing exposure to scorpions. Barriers for scorpion exclusion include caulking windows and holes around plumbing.

Cultural Control

Sanitation and removal of debris are the primary methods recommended for the control of scorpions. Firewood should be stored away from the sides of buildings and off the ground. Other debris such as loose boards, rock piles, and trash should also be moved away from buildings. Shrubs should be pruned so that they do not make contact with the exterior of buildings.

Elimination of sources of open water may also reduce the occurrence of scorpions. Proper maintenance of toilets and plumbing to reduce leaks and coating the inside lip of toilets with petroleum jelly will reduce access of scorpions to water. Drains should be screened or plugged when not in use to prevent access from the outside.

In order to create barriers to scorpion movement into dwellings, window frames and screens should be periodically checked for holes large enough for scorpions to enter through. Screens should be repaired and window frames caulked to fill all gaps. Baby cribs and cots can be protected by placing the legs into clean widemouth jars (scorpions cannot climb clean glass surfaces). Holes associated with wiring and plumbing should also be caulked to fill gaps.


There is little evidence that chemical control tactics are effective against scorpions so they should only be considered as a last resort. However, chemical control methods have been used to control scorpions in areas in which infestations are already identified. There are chemicals registered for the control of scorpions, but proper application is essential for adequate control.

Application of insecticides during daylight hours is largely ineffective against scorpions since scorpions are only active at dusk and at night. Some residual insecticides are registered for use; these may be the method of choice if scorpions are a persistent problem inside buildings (after attempts have been made to exclude them through cultural methods). If scorpions are found in buildings or are frequently found in outside areas where visitors are likely to be active after dark, the use of pesticides may be necessary. If pesticide use is considered, they should be applied to all potential hiding places and points of entry (including, but not exclusively, wall voids, cracks and crevices, attics, and window sills). Dusts are preferred because they can be blown into wall voids, etc.

Consult your regional Integrated Pest Management coordinator to determine which pesticide, if any, is best suited to your integrated pest management program.


The Brown Recluse Spider

The brown recluse spider, Loxosceles reclusa, and a related species, L. laeta, are also commonly referred to as violin or fiddleback spiders because they have a dark fiddle shaped pattern on their upper body. They vary in color from tan to dark brown. A second identifying characteristic of the brown recluse spider is the presence of only three pairs of eyes. (Most spiders have eight.) (Akre and Catts 1990.)

The brown recluse spider is sedentary and builds an irregular web that is often not recognized as a spider web. Females lay eggs in flattened egg sacs that are frequently attached to the underside of objects. Mating in this species occurs from February to September. Up to 40 spiderlings may hatch from a single egg sac. A single female may produce up to five egg sacs in a summer. Females can live up to four years, males less.

Indoors, the brown recluse can usually be found in infrequently disturbed areas away from light sources, such as behind pictures, beneath or behind furniture, in boxes, in clothing, among stored papers, between the corrugation of boxes, and under food sacks (Hite et al. 1966).

The natural habitat of the brown recluse includes the underside of rocks, loose bark, and crevices in decaying logs (Hite et al. 1966). However, many outdoor refugia provided by the activities of man are frequently inhabited by the brown recluse spider. For example, a survey of piles of junk in Kansas, piles of old tires and inner tubes, furniture, old boards, and trash were found to be inhabited by the brown recluse. Once the debris was removed and the natural vegetation returned to the area, the colony was eliminated.

The brown recluse spider occurs in a region roughly delineated to the north by the northern boundary of Illinois, to the west by the western boundary of Kansas and Oklahoma, and to the east by Tennessee (Akre and Catts 1990). Additionally, single specimens of the brown recluse, presumably artificially transported to these areas, have been reported from Washington D.C., Arizona, California, Florida, New Jersey, North Carolina, Pennsylvania, and Wyoming (Gladney 1972). L. laeta , native to South America, has also been reported in the United States in Massachusetts (Gertsch 1967) and California (Keh 1970). Both of these reports of L. laeta indicate that it has been transported into the United States by people. It is unlikely that L. laeta has become established in the United States, but it may be an occasional problem in areas where products are frequently shipped from South America.

The Widow Spiders

There are five species of widow spiders (Latrodectus) in the United States. The combined geographic range of these spiders encompasses the entire United States. Three of these species can generally be considered to be "black widows." Females of all of these species are metallic black with reddish marks commonly forming an hourglass shape on the underside of their thorax. The most well-known species, the common black widow spider, Latrodectus mactans occurs from southern New England to the southern United States The Northern widow, L. variolus , occurs from the mid- Atlantic states north to Canada. The western widow, L. hesperus, occurs west of the Rocky Mountains. Two additional species, the brown widow, L. geometricus, and the red widow, L. bishopi, are tropical species whose United States distribution is restricted to southern Florida. Latrodectus geometricus is another introduced species that primarily occurs in domestic situations, but its distribution is sporadic (Gertsch 1979).

Widow spiders are cobweb builders; a typical web of a widow spider is a small, tangled maze of coarse fibers that are made in dark corners or crevices. Frequently these webs are made near ground level. These webs may not even be recognizable as an active spider web. Eggs of the widow spiders are laid in sacs of silk within the female's web. A single egg sac may contain up to 400 eggs. The eggs of widow spiders hatch in three to four weeks. The hatchlings are highly cannibalistic and therefore most of the young will be consumed by their brothers and sisters. Web- spinning spiders such as the widow spiders are not active outside of their webs. This is especially true of the western widow spider which creates webs primarily in cracks and crevices.

Aggressive House Spiders

The aggressive house spiders are in the genus Tegenaria. Only one native species, Tegenaria chiricahuae, occurs in the United States, but at least six introduced species of Tegenaria now occur in the United States. These spiders as a group are often referred to as funnel-web spiders. They build funnel shaped webs in dark, moist areas such as basements and crawl spaces, and sit in these webs and wait for prey to walk by. Generally, these spiders are yellow to pale tan in color with long legs. These spiders occur in highest frequency in July through September and reproduce during this period. Females produce an egg sac that is placed near the opening of the funnel in their webs. Eggs hatch the following spring.

Although the bite of these species is not considered to be as dangerous as that of either the brown recluse or widow spiders, it can cause a similar ulceration of the skin as the brown recluse and may involve systemic reactions. The species that cause the worst bite reactions are found in the northwestern United States; Tegenaria agrestis occurs from Idaho to Vancouver and Winnipeg in Canada. It builds a web at or near ground level, and rarely climbs up vertical surfaces (Akre and Catts 1990). This spider is called an aggressive house spider because it will bite with little provocation if cornered or threatened. This may be related to their hunting strategy and may increase the likelihood that humans will be bitten by these spiders.


Sanitation and habitat modification are key tactics for control of spiders indoors. This includes vacuuming in corners, window sills, and attic areas, and keeping premises free of unneeded, unwanted items such as undisturbed clothing, papers, and other litter. Indoor habitat modification that creates a barrier to the movement of spiders into buildings is also a key tactic to effective spider control.

Cultural Control

Sanitation is one of the key methods of controlling spiders in buildings. The corners and crawl spaces of buildings should be kept free of spider webs. This may be accomplished by simply dusting these areas or by using a vacuum to remove existing webs. Vacuuming removes active spider webs, adult spiders, and spider egg sacs. Living spiders will desiccate quickly in the vacuum bag, but depending on the design of the vacuum, it may be useful to empty the bag immediately after use in order to prevent the spiders' escape (Akre and Catts 1990). Removing litter such as newspaper and wood from the interior and the sides of buildings is also crucial for effective elimination of spiders. In addition to sanitation, creating a physical barrier to movement of spiders into buildings is also an effective management technique. Pruning shrubbery and other plants away from buildings will also limit the access of spiders to buildings.

Barriers also limit access of buildings to spiders. Caulking, repairing screens, and filling cracks and crevices around windows, doors, and foundations with materials such as expanding polyurethane foam will exclude many spiders from buildings. Common areas to inspect for holes and gaps include entry holes for plumbing and electrical lines, and window and door casings. Window and door screens should be repaired to fill in holes large enough for entry of spiders. Gaps in the wall boards and ceiling-wall interfaces should be closed, and door and window casings should be filled with caulking or a foam insulation material. Foam insulation material can also be used to fill wall voids and crawl spaces if spiders come in through these areas. Spiders can easily gain access to buildings through gaps beneath doors. Placing a piece of weather stripping under a door so that there is no gap between the bottom of the door and the floor when the door is closed will alleviate this problem.

If crawl spaces are a breeding area for spiders, the reason is usually excess moisture. By eliminating moisture from crawl spaces, spiders can be eliminated. Placing plastic over bare soil can eliminate moisture in some areas, such as beneath cabins. The key to many moisture problems is to increase venting. Therefore, opening up ducts under a foundation may eliminate moisture from a crawl space, without allowing increased access of the building for spiders.

Additional precautionary measures which may reduce the risk of being bitten by spiders include wearing shoes at all times, using leather gloves when moving rocks, wood or other debris, and shaking out sleeping bags and clothing before using them.


Chemical control of spiders inside of buildings is not recommended and should be considered only as a last resort. Residual sprays are not recommended for use in buildings that are occupied, or are to be occupied in the near future. If residual materials are used in buildings not currently occupied or in areas where other methods fail, applications are recommended only along baseboards, door casements, and corners, and only where spiders are present (Akre and Catts 1990).

Chemical control using a long-lasting residual pesticide can be effective in controlling populations of spiders outdoors. Many residual materials are registered for control of spiders. Consult your regional Integrated Pest Management coordinator to determine which registered material is best for your integrated pest management program. Problem outdoor areas usually needing treatment include porches, garages, eaves of the roof, crawl spaces, and other areas beneath buildings (Bennett and Williams 1989).

First Aid

For any bite or sting it is important to reduce stress and help the inflicted person to relax. There is evidence that this will reduce the toxic effects of some bites and stings (Ebeling 1975). An ice cube may be applied for a short time to reduce the pain at the site of the bite or sting; this does not reduce the effect of the bite, but may make the afflicted person more comfortable. (DO NOT IMMERSE THE WHOLE LIMB IN WATER.) If in doubt about the seriousness of a bite or sting, or if a person is bitten or stung by any of the medically important species discussed in this module, contact your local poison control center or a physician immediately. Also, collect the scorpion or spider in question if possible to assist in the treatment of the sting. For further discussion of medical treatment and the progress of envenomation by scorpions, or spiders, see Smith 1982, Ebeling 1975, or Polis 1990 for treatment of scorpion envenomation.

The sting of most scorpion species and the bite of most spider species are not considered to be dangerous. However, if a person is stung by a scorpion in an area in which the bark scorpion occurs medical attention should always be sought since its sting may be life-threatening (Smith 1982). Additionally, any bite or sting may elicit an unusual allergic reaction by persons who are hypersensitive to the bite of a specific species. For this reason all bites must be examined to ensure the safety of those involved. A hyperallergic reaction can lead to anaphylactic shock and in very severe cases, respiratory distress may develop. It is not unusual for a person to have some pain and numbness in the same region as the site of the bite. However, if, for instance, a person is bitten on their hand and their legs begin to swell, this is indicative of a systemic reaction, and this person should receive medical attention as soon as possible. People who are known to be hypersensitive to other stinging insects such as bees and wasps are not necessarily hypersensitive to spider bites or scorpion stings. Likewise, each spider or scorpion has a very specific type of venom and a person may be sensitive to the venom of one species and not sensitive to the venom of a closely- related species. Lastly, some anti-venoms are available for treatment of some bites and stings, but their availability is variable. Contact your local poison control center for information regarding anti-venoms if dangerous spiders or scorpions are a problem in your region.


1. Akre, R.D., and E.P. Catts. 1990. Spiders. Wash. St. Coop. Ext. Ser.

2. Bennett, G.W., and R.E. Williams. 1989. Spiders. Extension Bulletin E-72, Purdue Univ. Coop. Ext. Ser. West Lafayette, IN.

3. Ebeling, W. 1975. Urban Entomology. Univ. Calif., Div. Agric. Sci., Los Angeles, CA.

4. Gertsch, W.J. 1979. American Spiders, 2nd ed. Von Nostrand Rheinhold Company, New York, NY.

5. Gladney, W.J. 1972. Controlling the brown recluse spider. USDA Leaflet no. 556, USDA, ARS, Washington, D.C.

6. Hite, J.M., W.J.Gladney, J.L. Lancaster, Jr., and W.H. Whitcomb. 1966. Biology of the brown recluse spider. Div. Agric., Univ. Ark. Agr. Expt. Stat. Bull. 711.

7. Keh, B. 1970. Loxosceles spiders in California. California Vector Views 17(5):30- 34.

8. Polis, G.A., Ed. 1990. The Biology of Scorpions. Stanford Univ. Press, Stanford, CA.

9. Smith, R.L. 1982. Venomous Animals of Arizona. Univ. Ariz. Coop Ext. Ser. Bull. 8245.

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