Those small, round, silky bundles you’ve spotted on your window frame or sill are almost certainly spider egg sacs. They’re one of the most common places spiders choose to lay eggs because windows offer shelter, warmth, and a steady supply of insects drawn to light. The good news: most egg sacs on windows belong to harmless species, and removing them is straightforward.
What Spider Egg Sacs Look Like
Spider egg sacs are typically white, off-white, or yellowish and wrapped in silk. They range from the size of a pea to roughly the size of a marble, depending on the species. Most look like small cotton balls or papery spheres tucked into a corner of the window frame, along the track of a sliding window, or wedged between the glass and the exterior trim.
A few features help you narrow down what made it. Common house spider sacs are usually tan or brownish, loosely woven, and slightly papery in texture. Cellar spiders (the long-legged spiders you see hanging upside down in corners) produce thin, almost translucent sacs that look like a loose bundle of eggs held together by a few strands of silk. If you see a round, yellow sac covered in small spiky projections, almost like a sea urchin or old naval mine, that’s characteristic of a brown widow. A western black widow sac is also round and yellow but smooth, sometimes with a slight point at the top.
How Many Spiderlings Are Inside
A single egg sac holds far more eggs than most people expect. A common house spider sac contains between 140 and 380 eggs. Black widows pack 200 to 800 eggs into one sac. Brown widows average around 250. Even cellar spiders, which produce the smallest sacs, carry 13 to 60 eggs each. If a spider has been busy on your window, you may find several sacs clustered together, multiplying those numbers significantly.
Incubation times vary. House spider eggs can hatch in about a week under warm conditions. Black widow eggs hatch in roughly 8 days, though the spiderlings stay inside the sac for another 9 or so days while they molt before emerging. Brown widow eggs take 14 to 21 days to hatch, with spiderlings remaining inside for anywhere from 4 days to a full month. Brown recluse eggs, if you’re in their range, take 24 to 36 days from laying to emergence. The practical takeaway: if you’ve found a sac and aren’t sure how old it is, remove it sooner rather than later.
Why Spiders Choose Windows
Windows create a microhabitat spiders love. Exterior lights or interior lamps attract flies, gnats, moths, and mosquitoes, giving a spider a reliable food source right next to her egg sac. Window frames also offer crevices and corners that anchor silk well and protect sacs from wind and rain. In cooler months, the warmth radiating through glass can speed up egg development, making window frames especially appealing in fall and early winter.
Identifying a Potentially Dangerous Egg Sac
The vast majority of egg sacs on windows belong to species that pose no risk to you. But in parts of the southern and western United States, brown widows and black widows do nest around windows, particularly on the exterior side. Brown widow sacs are the easiest to identify: round, yellow, and covered in unmistakable spiky silk projections. Black widow sacs are smooth and yellow, sometimes with a slight teardrop shape. If you see a sac matching either description, look for the adult spider nearby. Brown widows have an orange or yellowish hourglass on the underside of the abdomen. Black widows have the well-known red hourglass.
If you confirm a widow species, it’s worth calling a pest control professional rather than handling it yourself. The adult spider is more likely to bite when she’s guarding an egg sac, and disturbing the area without proper precautions increases the chance of an encounter.
How to Remove Spider Egg Sacs Safely
For common, non-dangerous species, removal takes only a few minutes. The most effective method is a vacuum with a hose attachment. Position the nozzle directly over the sac and vacuum it up along with any surrounding webbing. If your vacuum uses a bag, remove the bag immediately afterward and place it in an outdoor trash bin. For bagless models, empty and clean the canister outside so any eggs or newly hatched spiderlings don’t end up back in your home.
If you don’t have a vacuum handy, a broom or any long-handled tool works. Gently dislodge the sac, collect it along with any debris, and dispose of it outdoors. After removing the sac, wipe down the area to clear away remaining silk and any pheromone traces that might attract spiders back to the same spot. A solution of water and household bleach works well for this cleanup step.
Keeping Spiders From Returning
Removing one egg sac solves the immediate problem, but spiders return to the same spots if conditions stay favorable. A few changes make your windows less attractive. Switch exterior lights to yellow or warm-toned bulbs, which draw fewer insects and reduce the food supply spiders depend on. Seal gaps around window frames with caulk, paying attention to the corners where frames meet siding or brick. Clean window tracks and sills regularly, since even a thin layer of undisturbed dust signals to a spider that the spot is safe for nesting.
On the exterior, clear any vegetation or debris that touches the window frame. Overgrown shrubs and vines act as bridges, giving spiders easy access. Knocking down webs as soon as they appear also discourages spiders from settling in. Most species will relocate after rebuilding a web two or three times in the same spot without success.