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Spiders

Spiders



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Ask a Specialist: Spooky Spiders No Cause for Alarm Diane Alston, Utah State University Extension entomologist, diane.alston@usu.edu or 435-797-2516

Spiders conjure up images of Halloween, creepy-crawlies, and the fear of being bitten. Spiders can be commonly encountered in the fall while they scurry to find mates and protected winter hideouts, and produce egg sacs to provide for next year's brood. Most people don't like spiders, but most don't realize how beneficial they are to humankind. All spiders are predators, usually feeding on small insects and sometimes other spiders. They provide a free service in keeping down the numbers of pest insects in our gardens, landscapes, and homes. Spiders are masters of capturing prey with innovative methods using ambush, pouncing, and webs. Spiders can produce up to eight different types of silk to make draglines, different parts of the web, egg sacs, and ensnare their prey. All spiders use venom to paralyze their prey, but most have weak venom and mouthparts too small to bite a person.

There are only a few species of spiders in North America that can be dangerous to humans: primarily black widow and brown recluse. The black widow group contains some of the most venomous spiders worldwide, and is the primary spider of medical concern in Utah. The brown recluse resides in southern and Midwestern U.S., but not in Utah. The hobo spider, formerly called the aggressive house spider, has been implicated in necrotic bites in Utah. The necrosis is most likely caused by secondary infection with bacteria and not by the toxicity of the venom. In the hobo spider's native range in Europe, dangerous spider bites are uncommon. The yellow sac spider is probably responsible for most spider bites in Utah; however, its venom is not generally harmful to humans and the primary effect is a painful and itchy red bump, much like a mosquito bite.

In Utah, there are over 600 spider species from 13 family groups. The most frequently submitted spider to the Utah Plant Pest Diagnostic Laboratory on the campus of Utah State University in Logan is the hobo spider. Hobos migrate indoors from August to October to find mates, and so are commonly encountered by people. The most common spiders in Utah include:

• Funnel-web spiders (Agelenidae) - hobo and grass spiders; common outdoors in vegetation and rock and wood piles, and come indoors to seek shelter and mates; webs are finely woven flat sheets and funnels.

• Comb-footed spiders (Therididae) - widow spiders; comb-like hairs on their legs allow them to walk on messy cobwebs; black widows usually build their webs in dark corners and undisturbed places; only the widow spiders in this family are venomous.

• Orb-weavers (Araneidae) - orb weavers build the classic circular, spiral web of Charlotte's Web fame; in Utah, the cat-face and banded garden spiders are the most common; their webs reduce the number of flying insects around the home; in the fall, they lay large egg sacs in protected places, such as under the eaves of buildings.

• Wolf spiders (Lycosidae) - moderate to large sized ground-hunting spiders that can be mistaken for small tarantulas; common outdoors and in garages and outbuildings; distinctive eye pattern with two rows of eight eyes (small below, large above); a female will carry her spiderlings on her back.

• Sac spiders (Clubionidae) - the yellow sac spider is a common household spider that can cause a painful bite, but the venom is not harmful to most people; they can climb walls and ceilings, and build a white silken retreat where the ceiling and wall meet.

• Jumping spiders (Salticidae) - these small "furry" spiders are very common and pounce on their prey; they congregate on window sills and other well-lit spots indoors.

• Crab spiders (Thomisidae) - these small spiders walk sideways and are ambush hunters; many are brightly colored to match their environment (flowers, leaves, rocks).

• Woodlouse spiders (Dysderidae) - these large red and brown spiders look scary with their large fangs, but they are harmless; they eat isopods, or pillbugs; common in gardens and landscapes, and will come indoors.

• Crevice-weaving spiders (Fillstatidae) - common in southern Utah, these spiders build webs in cracks on the outside of buildings; they are not harmful, but are sometimes mistaken for the brown recluse in appearance. Because spiders provide beneficial biological control, avoid harming them unless the spider is poisonous. Emphasize exclusion, trapping, cleaning, and habitat modification, and only use chemical control if there is a serious infestation.

• Exclusion: seal cracks and crevices and install weather stripping around doors and windows in exterior walls of buildings.

• Trapping: use commercial sticky traps available in home and garden stores; place near entryways along baseboards and behind furniture.

• Cleaning and habitat modification: indoors, vacuum regularly, especially behind doors and furniture and reduce clutter where spiders can hide; outdoors, remove dense vegetation and piles of firewood and rocks near buildings and clean undisturbed areas of outbuildings to eliminate black widow refuges; use sodium vapor bulbs (yellow) in exterior lighting to attract fewer insects that serve as spider food.

• Insecticides: if chemicals are necessary, target spider hot-spots, such as near exterior doorways, and time them with spider activity, such as spring for hatch of hobo spider eggs and late summer and early fall when adults are active; spiders walk on tippy-toe on their claws, so treatments that contact the spiders themselves work best; dust formulations can be applied to voids and secluded places; outdoors, microencapsulated formulations are most effective; common insecticide active ingredients include deltamethrin, permethrin, pyrethrin (organic formulations available), and lambda-cyhalothrin. Always read the product label for registered uses, application and safety information. Never use a product indoors that is only registered for outdoor use.

If you are bitten by a spider, try to collect it for identification. Clean and disinfect the bite using a topical antiseptic and cover with a sterile bandage. Keep the wound clean as bacterial infection is the most common cause of problematic spider bites. Monitor the bite and if symptoms progress, visit your doctor. Treatment with an antibiotic may be recommended.

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Diane Alston

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