Notable concerns
- Western black widow
- Brown widow
- Desert recluse (south)
- Chilean recluse (isolated)
Medically significant species in California
| Species | Scientific | Significance | Range note |
|---|---|---|---|
| Western Black Widow | Latrodectus hesperus | Medically significant | Western North America from southern British Columbia south to Mexico, east to the Great Pl |
| Brown Widow | Latrodectus geometricus | Moderate | First documented in Florida in 1935; major range expansion across the U.S. South began in the 2000s. Continues to displace L. mactans in southern California urban habitats. |
| Yellow Sac Spider | Cheiracanthium inclusum | Moderate | Throughout the continental United States and southern Canada. |
| Chilean Recluse | Loxosceles laeta | Medically significant | Established L. laeta populations in the U.S. are limited to specific buildings in the Los Angeles area, first documented in the 1960s. |
All documented species in this index
Species below are present in California according to extension publications and the rangeStates records in our species index. Click through for full profiles.
- Western Black Widow - Latrodectus hesperus (Medically significant)
- Brown Widow - Latrodectus geometricus (Moderate)
- Yellow Sac Spider - Cheiracanthium inclusum (Moderate)
- Common House Spider - Parasteatoda tepidariorum (Low)
- Wolf Spider - Lycosidae (family overview) (Low)
- Cellar Spider - Pholcus phalangioides (Not medically significant)
- Bold Jumping Spider - Phidippus audax (Low)
- Yellow Garden Spider - Argiope aurantia (Not medically significant)
- Grass Spider - Agelenopsis spp. (Not medically significant)
- Southern House Spider - Kukulcania hibernalis (Low)
- Giant House Spider - Eratigena atrica (Not medically significant)
- Chilean Recluse - Loxosceles laeta (Medically significant)
Seasonal activity in California
Spider activity in California follows the documented peak season of March–November. Indoor sightings often spike in late summer and early fall as males wander to find mates and as cooling outdoor temperatures push synanthropic species toward heated structures.
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Highlighted months reflect the state extension peak-activity window. Individual species (especially indoor recluse populations) may be active outside this range.
Prevention checklist for California homes
- Seal exterior gaps wider than 1/8 inch around utility penetrations, dryer vents, and weep holes. Install door sweeps on garage and basement entries.
- Reduce harborage by storing firewood, lumber, and seldom-used boxes off the ground and at least 18 inches from foundation walls.
- In recluse-active areas, inspect stored clothing, bedding, and shoes before use. Sticky monitor traps placed along baseboards quantify indoor density.
- For widow-active areas, wear gloves when handling outdoor electrical boxes, irrigation valves, and stacked materials. Widows favor undisturbed voids.
- Vacuum webs, egg sacs, and spiders rather than spraying. Mechanical removal outperforms most over-the-counter residual sprays for synanthropic species.
If you are bitten
Nearby states in the Pacific region
- Spiders of Oregon - peak activity may–september
- Spiders of Washington - peak activity may–september
Sources
- [1]UC IPM: Spiders - UC IPM Cooperative Extension. Accessed 2026-06-14.
- [2]Spiders and scorpions submitted as suspected brown recluse - Vetter RS, 2008. Accessed 2026-06-14.