The egg masses are visible in winter, are grayish-brown and about one inch long. As soon as the eggs hatch in April or May, the caterpillars begin eating leaves and create “tents” – white, silky shelters that cover the forks and tips of tree branches and shrubs. They feed during daylight and return to their tents at night.… Picture by Orion Tippens. Note: Do not copy or use any of the images here or herein without written consent (contact Orion at travelingorion.wordpress.com). © Copyright 2014 Orion Tippens. All Rights Reserved
The caterpillars are easy to recognize. They have a yellow checkered pattern with a blue dotted line down their backs, and can be three inches long when mature. The caterpillars molt four times over a period of five or six weeks, then stop eating. They choose a place to spin a cocoon and lay their eggs, and moths emerge about two weeks later. The adult moths mate immediately, lay eggs, and die a few days later; the eggs winter over until spring, when the whole process begins again.
Seattle.gov site, their numbers are increasing throughout the Northwest US. Here is a little more information from the same site, on their life cycle and silk process; as shown in the picture: