Yellow sac spider bites in Colorado
Colorado has 181 verified yellow sac spider records in the GBIF biodiversity database — one of the highest counts in the country. Sightings concentrate in May, June, July, September. The bite itself appears stinging pain at the time of the bite and lasts about a week.
| Verified records | 181 |
|---|---|
| Peak sightings | May, June, July, September |
| State bite season | May to September |
| Active | night |
| Habitat | ceiling corners, curtain folds and wall junctions indoors; this is a common household spider nationwide |
| Bite appears | stinging pain at the time of the bite |
Identifying it in Colorado
Small, pale yellow-green, and the spider most likely to bite a sleeping person indoors. The bite stings at the time, unlike a bed bug, and appears singly rather than in a line — the two are otherwise easy to confuse because both happen overnight on uncovered skin.
Locally, the species most often confused with it are honey bee sting, paper wasp sting, bald-faced hornet sting — all of which are also recorded in Colorado.
Seek medical care if
- an enlarging ulcer
- spreading redness with fever
What else it could be in Colorado
These are recorded in Colorado too and are the ones most often confused with it here. The differences below are the ones that actually separate them.
| Yellow sac spider bite | Bed bug bite | Brown recluse spider bite | |
|---|---|---|---|
| Pattern | single, sometimes two nearby | lines or zigzags of three or more — the classic "breakfast, lunch and dinner" | single site |
| Where on body | neck, arms, torso — often bites received in bed | arms, shoulders, neck | torso under clothing, upper arms, thighs — often where clothing was put on with the spider inside |
| Appears | stinging pain at the time of the bite | hours to several days — many people notice nothing on the first night | often painless at first |
| Records here | 181 | 35 | 18 |
Season in Colorado specifically
Records in Colorado concentrate in May, June, July, September, which is what local sighting data shows rather than a national average — seasonality shifts substantially with latitude. The general bite season in Colorado runs May to September.
Because ceiling corners, curtain folds and wall junctions indoors, exposure in Colorado tends to be concentrated in specific places rather than spread evenly — which is why a bite history ("where were you in the 24 hours before?") narrows the answer faster than the appearance of the mark.
Care
- wash, cold pack, antihistamine if itchy
- keep the area clean while it heals
Avoiding them in Colorado
- vacuum ceiling corners and behind curtains
- seal gaps around window frames
Questions
When are yellow sac spiders active in Colorado?
Colorado records peak in May, June, July, September. The general bite season in Colorado runs May to September.
Are they common in Colorado?
Colorado accounts for 181 verified records in GBIF, ranking it 2nd among states for this species. Records reflect reporting effort as well as abundance.
What does the bite look like?
a red, tender bump, occasionally with a small blister, red, sometimes with a pale centre. Single, sometimes two nearby.
Get a photo assessment in the app
Everything above is free to read. The app adds what a page cannot: point your camera at the bite and get an assessment of the most likely cause, a severity read, and a day-by-day care plan — including the specific signs that mean you should be seen.
iPhone and iPad. Free to download.