Spooky Spiders No Cause for Alarm
Spiders are commonly encountered in the fall as they scurry to find mates and protected winter hideouts where they can produce egg sacs to provide for next year’s brood. Most people don’t like spiders, but most don’t realize how beneficial they can be. All spiders are predators, usually feeding on small insects and sometimes other spiders. They provide a free service in keeping down the number of pest insects in our gardens, landscapes and homes. Spiders are masters of capturing prey with innovative methods such as ambushing, pouncing and webs. They can produce up to eight types of silk to ensnare their prey and make draglines and different parts of the web as well as egg sacs. All spiders use venom to paralyze their prey, but most have weak venom and mouthparts too small to bite a person.
In North America, the spiders that can be most dangerous to humans are the black widow and brown recluse. The black widow group includes some of the most venomous spiders worldwide and causes the most medical concern in Utah. The brown recluse lives in the 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. Necrosis at the bite site is caused by secondary infection with bacteria more so than venom toxicity. 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 more than 600 spider species from 13 family groups. The most frequently submitted spider to the Utah Plant Pest Diagnostic Laboratory on the Utah State University campus is the hobo spider. Hobos migrate indoors from August to October to find mates, and are therefore 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; 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.
- 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 places indoors.
- Crab spiders (Thomisidae) – these small spiders walk sideways and are ambush hunters; many are brightly colored to match their environment (flowers, leaves, rocks).
Because spiders provide beneficial biological control, avoid harming them unless it is poisonous. Use 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: place sticky traps 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 become necessary, target spider hot spots, such as near exterior doorways, and time them with spider activity — spring for the 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 a spider bites you, try to collect it for identification. Clean and disinfect the bite using a topical antiseptic covered with a sterile bandage. Keep the wound clean to prevent bacterial infection. Monitor, and if symptoms progress, see a doctor.
For more information on spiders, read these USU Extension facts sheets: http://extension.usu.edu/files-ou/publications/factsheet/spidersn-2012pr.pdf and http://extension.usu.edu/files-ou/publications/factsheet/hobo-spiders08.pdf.
By: Diane Alston - Oct. 28, 2013