Cocoons, Egg Cases, and Egg Sacs


Evergreen bagworm

Some insects, as part of their development, encase themselves in chrysalises or cocoons. This is typical of butterflies and moths. The evergreen bagworm caterpillar decorates its cocoon with material from its host plant. These caterpillars are serious pests of ornamental trees like arborvitae, cedar, juniper, cypress, and many others, and can kill trees with their feeding damage. After they’ve chewed on their host tree and grown big enough, the caterpillars secure their camouflaged cocoons to branches and begin to pupate and develop into their adult form. These cocoons are easily spotted on evergreen trees, since the plant material of the cocoon turns brown shortly after being added to the caterpillar’s case.

Adult males are fuzzy, brown, bumble-bee shaped moths. Adult females, on the other hand, retain juvenile traits with pale caterpillar-like bodies. The female has no eyes, no wings, no legs, and cannot eat. She calls for males to come to her by emitting a strong pheromone. She never leaves her cocoon, even mating from the cocoon through an opening in the case. Her eggs are retained inside her body, emerging from her only after she has died.

Praying mantis egg cases

The Chinese praying mantis is the largest mantis in North America. Its enormous size allows it to feed on anything it desires, including hummingbirds and small snakes. Their large size results in large egg cases which are often seen glued to vegetation or the sides of buildings. Each species produces a characteristic egg case, or ootheca (literally meaning “egg container”), which varies in color, texture, shape, and size. Oothecae contain many eggs and are protected by an outer covering. For praying mantises, the coating is foamy and produced by the mother mantis during the egg-laying process. Within several days, the coating dries hard like concrete, protecting the developing nymphs from predators, parasites, microorganisms, and water loss. Oothecae are laid in the fall. These cases endure the winter and hatch in the spring.

Smaller egg cases probably belong to the Carolina mantis, a smaller, native species in our area.

Spider egg sacs

The black and yellow garden spider, Argiope aurantia, is black and yellow with a white-silver sheen on its head region (its scientific name translates to “gilded silver-face”). These large and conspicuous spiders are well-known by homeowners for their bright colors, substantial size, and zipper-like silken pattern in the center of their web. These non-aggressive spiders are sometimes fed by intrigued homeowners who enjoy the presence of these spiders in their yard.

The females lay their eggs on several layers of silken sheets, with a leathery, brown silk on the outermost layer. The egg-sheets are then rolled into a ball with an upturned neck, looking something like a fat “water drop” shape when fully constructed. The egg sac is usually hung near the center of the web where the female rests and protects the eggs from predators. The mother spider will die with the first frost, but the egg sac will survive the winter and hatch in the spring.

There may be up to a thousand spiderlings in each sac. Upon emerging, they are as small as dust particles. The spiderlings disperse by “ballooning” or releasing a long strand of silk which carries them on the wind to new places.