A false black widow egg sac is a small, white, spherical ball of silk that looks like a loose cotton ball. Unlike the smooth, tightly woven egg sacs of true black widows, false black widow egg sacs have a fuzzy appearance with indistinct margins, making them relatively easy to distinguish once you know what to look for.
What the Egg Sac Looks Like
The egg sac is roughly pea-sized, white to off-white, and round. Its most distinguishing feature is its texture: the outer silk layer is loosely woven, giving it that cottony, slightly messy look. There’s no clean edge or defined surface. If you picture a small ball of cotton pulled from a medicine bottle, you’re close. Inside, each sac holds 200 or more cream-colored eggs packed tightly together.
This soft, irregular appearance sets it apart from the egg sacs of related spiders. A true western black widow produces a smooth, yellow egg sac that looks almost papery, sometimes with a pointed top. A brown widow’s egg sac is also yellow but covered in small silk spikes, giving it a spiny appearance often compared to a sea mine. The false black widow’s cottony white sac lacks both the smooth finish and the spikes.
Where You’ll Find Them
False black widows strongly prefer indoor environments, which is one reason their egg sacs turn up in homes more often than those of true black widows. Common spots include washrooms, the insides of cupboards, underneath appliances, behind furniture, and inside cabinets. The spider typically suspends its egg sac within or near its tangled cobweb, often in a dark, undisturbed corner. Garages, sheds, and window frames are also frequent locations.
If you’re finding egg sacs outdoors, they’ll usually be in sheltered spots like the undersides of outdoor furniture, inside meter boxes, or tucked beneath siding. The spider tends to stay close to its egg sac, so you’ll often spot the adult female nearby.
Timing and Reproduction
False black widows mate in the spring. Females begin producing egg sacs from around May, and egg-laying can continue through the summer and into autumn depending on the species and climate. A single female can produce three or more egg sacs in one season, with output partly dependent on how much food she’s catching. That means a single spider in your home could be responsible for 600 or more eggs over the course of a few months.
Once laid, the eggs take two to four months to hatch. Spiderlings emerge tiny and pale, dispersing from the sac over a period of days. In warm indoor environments, development may be faster than in cooler outdoor conditions. Because egg sacs are produced at intervals rather than all at once, you might find sacs at different stages of development in the same web, some fresh and white, others older and slightly discolored.
How to Tell It Apart From a Black Widow Egg Sac
The easiest way to distinguish false black widow egg sacs from true black widow egg sacs is by color and texture together. Here’s a quick comparison:
- False black widow: White, round, cottony with fuzzy edges, no defined surface
- Western black widow: Yellow, round or slightly pointed at the top, smooth and papery
- Brown widow: Yellow, round, covered in small silk spikes
Location also helps. True black widows tend to build webs outdoors or in garages and woodpiles, while false black widows are far more comfortable living fully indoors. If you find a cottony white egg sac inside a cupboard or behind a bathroom cabinet, it’s very likely from a false black widow.
What to Do if You Find One
False black widows are not dangerous in the way true black widows are. Their bite can cause localized pain and swelling, but serious medical reactions are rare. Still, finding egg sacs means an active breeding population, and each sac can release hundreds of spiderlings into your home.
Removing egg sacs is straightforward. You can vacuum them up or collect them with a tissue and dispose of them outside. Clearing away the surrounding cobweb discourages the spider from rebuilding in the same spot. Because false black widows favor dark, undisturbed spaces, regularly cleaning behind furniture, inside cupboards, and around appliances reduces the chances of them settling in. Sealing gaps around windows and doors also helps limit entry points.
If you’re finding multiple egg sacs across different rooms, that suggests several adult females are established in the home. In that case, a thorough cleaning of all sheltered spaces, combined with removing visible webs and sacs, is usually enough to bring the population under control without professional pest treatment.