Archive for July 15th, 2010

Jeepers! Creepers!

A week or so ago, I noticed that a nest is under construction in the bur oak tree just outside the house. No, not a bird’s nest. The builders are Fall Webworms (Hyphantria cunea). It seems a little early for fall insects, since it is just the middle of July, but the common name is a bit deceiving. Webworms can be found hard at work before the autumn arrives.

Superficially, webworm nests look like those of the better-known Tent Caterpillars (Malacosoma americanum). Tent caterpillars aren’t closely related to webworms in spite of the common web nest-building strategy. The webs can be differentiated quite easily. Tent caterpillar webs are mostly seen early in the year, in spring, while webworms start up more or less when the tent caterpillars are just concluding their web season.

Further, while tent caterpillars tend to construct their web nest at the fork of a tree, webworms begin building at the tip of a branch. Most of the work on the web nest is completed at night. As the nest increases in size, it may grow to three feet long and encompass several branches.

While tent caterpillars leave their nest to feed on leaves, webworms enclose the leaves they feed on right inside the web, and you rarely see webworms outside of their nest until they are about to leave and pupate.

Inside the nest, you can see the droppings and shed skins of the caterpillars. They go through six molts as they grow. This amounts to over a quarter of a caterpillar’s life that is spent in the process of shedding its skin or waiting for its new skin to harden. After molting five times, the mature caterpillar leaves the nest and constructs a cocoon. You might think the web was a safe place for a caterpillar to live, but webworms are enjoyed by birds, especially yellow warblers, as well as several species of hornets.

Caterpillars who make it to adulthood have a bright future ahead of them. Bright white, that is. Fall webworms are the larvae of a type of tiger moth. The stout-bodied adults have a wingspan of about 2 inches. Webworm moths are difficult to differentiate from the closely-related Agreeable Tiger Moth (Spilosoma congrua), pictured below. An expert may differentiate the two species by the pattern of spots. Thank you to Birdgirl for the moth photograph. For more on webworms, you can read her post at The Marvelous in Nature.

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