What Is in That White Ball? Spider Egg Sacs

An egg sac is often easier to identify than the spider, because it is stationary, it stays for weeks, and the construction differs sharply between families.

It is also the only thing you will find in some cases — the female may be dead or hidden — and it tells you what is living where you found it.

The one worth recognising

Brown widow — the sac is covered in silk spikes or tufts, like a small sea urchin, tan to grey, 8–12 mm. Nothing else in North America makes anything like it.

Brown widows are spreading through the southern United States and southern California, and the sac is a more reliable identification than the spider, whose colouring is variable enough to be unhelpful.

Black widow — smooth, pale cream to grey, pear-shaped or spherical, 8–12 mm, with a papery surface. Suspended in an untidy tangle web, usually in a dark void — under a pot rim, in a meter box, under outdoor furniture. Often several in one web.

Finding either means widows are established there rather than passing through, and the places to check are listed in are there black widows where you live.

The common harmless ones

The cellar spider one is striking if you see it — a female with a visible cluster of eggs held in her chelicerae, essentially unwrapped. She carries them until they hatch.

The wolf spider carries hers attached to her spinnerets, and after hatching the young ride on her abdomen, which is worth knowing before you find a spider that appears to be covered in moving specks — see wolf spider or nursery web spider.

When it will hatch

Timing varies by species and temperature. Broadly:

• Sacs laid in late spring or summer hatch in two to four weeks.
• Sacs laid in autumn usually overwinter and hatch in spring.
• Indoors, with central heating, autumn sacs may hatch out of season.

Spiderlings often stay in a cluster near the sac for a few days after emerging, then disperse — many by ballooning, releasing silk threads and lifting off on air currents. That is where the gossamer on autumn grass comes from.

Removing one

Indoors, if you would rather not have a few hundred spiderlings:

• Vacuum it. Simplest, and it takes the sac and its contents in one go. Empty the vacuum outside afterwards.
• Scrape it into a container and put it outside, if you would rather it survived.
• Do not crush it in place. Sacs are tough and crushing tends to release the contents rather than destroy them.

Outdoors, leaving them is generally the better choice. Orb weaver sacs under eaves produce the spiders that spend next summer eating flies around your house.

What is not an egg sac

• A cocooned prey item. Spiders wrap prey in silk and store it in the web. It is usually a different shape — elongated rather than spherical — and often has legs or wings showing.
• A moth or butterfly cocoon, which is denser and usually attached differently.
• A silk retreat with no eggs in it, which is where the spider sleeps. Jumping spiders and yellow sac spiders both build them, and they are flat rather than rounded.
• Mealybugs and woolly aphids on plants, which are white and fluffy and are insects.
• Spider mite webbing, which is fine, sheet-like and covers leaf surfaces rather than forming a discrete ball.

The one to check first

If you are in the southern United States and find a spiky tan sac, look for the spider before removing it — brown widows are frequently in numbers, and the sac tells you they are breeding rather than visiting. Elsewhere, the overwhelming majority of egg sacs found in and around a house belong to house spiders and orb weavers, and neither is worth acting on.