Finding a strange, foam-like brown object on a fence post or tree branch is often a surprising experience, but it is almost certainly a praying mantis egg case, scientifically known as an ootheca. Usually measuring about 1 to 2 inches long with a tan or light brown color, this hardened foam structure is created by a female mantis to protect her offspring from winter’s cold, birds, and other insects until the weather warms up.

Praying mantises act as beneficial garden predators that hunt down flies, crickets, moths, and caterpillars, effectively serving as a “natural pesticide” to keep pest populations under control. During late summer or autumn, the female produces a frothy substance and whips it into a foam while laying eggs, creating a tough, spongy shell with ridges and hundreds of tiny compartments that insulate the eggs against freezing temperatures and rain.

These egg cases can be found tucked inside thick shrubs, attached to tall weed stems, or on the underside of garden furniture and wooden railings. It is best to leave them alone because they are completely harmless to humans, pets, and property, whereas removing them often kills the hundreds of tiny mantises waiting inside.

Inside a single brown structure, anywhere from 100 to 400 eggs rest through the winter until microscopic nymphs emerge all at once in mid-to-late spring to hunt pests. If an egg case must be relocated due to painting or moving furniture, it can be safely removed with a knife or by snipping a twig, then tied to a branch in a bushy area with the original “up” side facing upward.