Spiders in the Treasure Valley: A Practical Guide for Homes in Boise, Meridian & Nampa
Room-by-Room Risk Map
- Garage & Sheds: dark storage, undisturbed corners, and stacked boxes = prime black widow habitat; check around equipment, shelving, and door tracks. University of Idaho Library
- Basements/Crawl spaces & Window Wells: cool, low-traffic areas favored by hobo/funnel-weaving spiders and common house spiders; look for sheet/funnel webs along sill plates and vents. Utah State University Extension
- Eaves/Porches/Outdoor Lights: orb-weavers build large evening webs near lights where insects gather; harmless but conspicuous. CSU Engagement & Extension
- Living Areas: late summer/early fall wandering male spiders (various species) show up inside while searching for mates—often mistaken for “infestations.” Utah State University Extension
Myth vs. Fact
- Myth: “Brown recluse is common in Idaho homes.”
- Fact: Brown recluse is not established in Idaho; most reports are misidentifications or hitchhikers. Extension and regional references note no established populations. Ask Extension
- Myth: “Hobo spiders cause dangerous necrotic bites.”
- Fact: Current evidence does not support hobo spiders as medically important; the PNW handbook and UC IPM both note they’ve been exonerated as a cause of necrotic wounds. Black widow remains the primary medically significant spider in our region. PNW Pest Handbooks+1
- Myth: “Seeing more spiders in fall means they invaded the house.”
- Fact: Fall peaks are largely seasonal behavior—males wander looking for mates, and big outdoor webs are more visible. Utah State University Extension+1
Quick ID Cards
- Black widow (Latrodectus spp.): glossy black female with red hourglass on the underside of the abdomen; messy tangle webs in dark, undisturbed spots (garages, crawl spaces, meter boxes). Treat with caution; bites can be medically significant. University of Idaho Library
- Hobo/funnel-weavers (Eratigena/Tegenaria spp.): brown/tan, fast runners; sheet + funnel web near ground or in window wells/crawl spaces. Medical risk is low based on current evidence. UC IPM
- House/cobweb & cellar spiders: thin-legged web builders indoors; nuisance only, and they reduce flying insects. CSU Engagement & Extension
- Orb-weavers: large circular webs on eaves/landscaping near lights; striking but beneficial outdoor predators. CSU Engagement & Extension
Season Calendar for the Valley
- Late Spring–Summer: webs increase outdoors as insect prey rises; orb-weavers expand around lights. CSU Engagement & Extension
- Late Summer–Early Fall: peak sightings indoors as males wander; outdoor webs look biggest now. Utah State University Extension
- Winter (indoors only): activity persists in warm, sheltered spots (garages, crawl spaces) for species like black widows. University of Idaho Library
Action Steps
1) Reduce the food = fewer spiders.
- Replace/maintain door sweeps and window screens; manage exterior lighting (use warmer bulbs or reduce overnight uptime) to cut flying-insect attraction; address other insect issues first. CSU Engagement & Extension
2) Exclusion & habitat cleanup.
- Seal gaps at utilities and foundation cracks; store items off the floor; shake out seldom-used items; declutter corners; keep firewood off the ground and away from structures. CSU Engagement & Extension
3) Safe removal.
- Indoors: use a vacuum for spiders/webs/egg sacs (dispose of the bag).
- Outdoors: knock down inactive webs and reduce harborage; leave harmless orb-weavers if you can—they’re doing free pest control. CSU Engagement & Extension
4) When to call a professional.
- Confirmed black widow in living spaces or areas with kids/pets; persistent sightings despite sanitation/exclusion; sensitive settings (elderly care, daycares); or if you’re unsure about ID. An IPM-based plan targets underlying insect prey, applies precise treatments where warranted, and pairs it with web/egg-sac removal and follow-up. Utah State University Extension
Sources
- Pacific Northwest Pest Management Handbooks — Public Health: Spiders (hobo spider exonerated; medically important species context): https://pnwhandbooks.org/insect/structural-health/public-health/public-health-spider PNW Pest Handbooks
- UC ANR Statewide IPM — Hobo Spider (Pest Notes) (not considered medically important; biology/identification): https://ipm.ucanr.edu/PMG/PESTNOTES/pn7488.html UC IPM
- Colorado State University Extension — Spiders in the Home (seasonal fall wandering; common indoor species; benefits): https://extension.colostate.edu/resource/spiders-in-the-home/ CSU Engagement & Extension
- Utah State University Extension — Spider Encounters Most Likely in the Fall (male wandering, seasonal peak): https://extension.usu.edu/archive/spider-encounters-most-likely-in-the-fall Utah State University Extension
- University of Idaho Extension — Spiders (regional identification & widow context) (PDF): https://objects.lib.uidaho.edu/uiext/uiext28630.pdf University of