What Is the Tan Foam Lump on a Plant Stem? Praying Mantis Egg Case

You are cutting back perennials, clearing brush, or trimming a shrub in fall when you notice something odd firmly attached to a woody stem: a tan, hardened, foam-like lump, roughly an inch or two long, wrapped around the twig or stuck to a fence rail or wall. It looks manufactured — like a dab of dried spray-foam insulation, a wad of hardened papier-mâché, or a strange spongy cocoon — and it clearly did not grow there. This is an ootheca, the egg case of a praying mantis. Female mantises lay these cases in late summer and fall, and each one shelters dozens to hundreds of eggs through the winter before they hatch into tiny mantises in spring. Finding them is a classic fall discovery, because the leaves are dropping and the cases suddenly become visible on bare stems. Here is how to identify a mantis egg case and what to do with one.

How to Identify a Mantis Egg Case

An ootheca is unlike anything else you will find on a stem, and a few features make it easy to recognize:

• The texture: it looks and feels like hardened foam, spongy Styrofoam, or papier-mâché — the female produces a frothy liquid that hardens around the eggs into a tough, insulating shell
• The color: tan, light brown, or grayish; freshly laid cases are paler, sometimes cream or white, and darken as they harden and age
• The size and shape: usually about 1 to 1.5 inches long, rounded or teardrop-shaped, often with a subtle ridged or banded seam down one side where the young will emerge
• The placement: firmly cemented to a woody stem, twig, tall weed, fence, or wall, typically a foot or two off the ground, and staying put all winter
• The season: laid in late summer and fall and most noticeable once leaves drop, remaining attached and dormant until the young hatch in spring

A hard, foam-textured tan lump cemented firmly around a stem in fall is a praying mantis egg case. Once you have seen one, the papier-mâché look is unmistakable, and you will start spotting them on winter walks through brushy fields and old gardens.

Which Mantis Made It? Reading the Shape

The shape of the ootheca is often the best clue to which mantis laid it, since the three mantises common across much of North America each build a distinctive case. The Chinese mantis — a large introduced species — makes a rounded, puffy, roughly walnut-sized case that looks like a tan lump of aerated foam. The European mantis, also introduced, builds a more elongated, narrower case. The native Carolina mantis produces a smaller, flatter, more streamlined case, often described as looking like a tiny fossil trilobite pressed against a flat surface. Because two of the three most common mantises in the United States are introduced, a chunky, foamy egg case on a garden shrub is frequently the work of the widespread Chinese mantis rather than a native species.

Found One? Here’s What to Do

The best thing to do with a mantis egg case is usually to leave it exactly where it is. Mantises are generally welcome garden insects — voracious predators of many pest bugs — and the case is built to survive winter outdoors, so it needs no help from you. If you find one on a stem you are about to prune or on brush you are clearing, simply set that section aside outdoors, or gently tie or wedge the case onto a nearby shrub a foot or two off the ground, and the eggs should still hatch in spring. It is best not to bring a case indoors: the warmth can trick the eggs into hatching early, and you will suddenly have dozens or hundreds of tiny mantises loose in the house with nothing to eat. It is also worth knowing that the store-bought mantis egg cases sometimes sold as garden pest control are usually the non-native Chinese mantis, which can eat beneficial insects — and even the occasional hummingbird — right alongside pests, so encouraging the mantises already present is generally the wiser path.

Found a mysterious foam-like lump on a stem and want to confirm it is a mantis egg case — or curious which mantis built it? Snap a clear photo of the case and its shape and let Spotter help you identify it as a starting point for understanding what is overwintering in your garden.