Those few sunny warm days we had last week must have queued the hatching of these tent caterpillars. I spotted a few groups just emerging from their eggs on an apple tree I was pruning in the Skagit Valley, near Mount Vernon. I’m sure they’ve emerged in Seattle as well, as it’s almost a climate zone warmer there.
Tent caterpillars (Malacosoma sp.) feed on Rosaceae family plants (apple, plum, cherry, crabapple, etc.) as well as several other trees such as alder, ash, and willow. They are most noticeable in the late spring and early summer. By this time they’re full grown at about 2 inches and have spun extensive silken webs for shelter and protection from predators. They mainly feed at night and rest in their webs during the day. Large outbreaks can sometimes defoliate entire trees, or in extreme cases sections of forest, but usually don’t cause lasting damage or die-back unless the tree is stressed for other reasons or infested for multiple seasons.
For control it’s most effective to pick out the gray foamy looking egg masses starting late summer to late spring, although this can be unpractical on large trees. I’ve only seen them on pencil to finger width diameter branches, but they can also be laid on the trunk and in bark crevices. Later in the spring when the silk nests are more obvious they can be stripped out or pruned out and destroyed. There are a number of predator insects that will feed on or parasitize tent caterpillars, so any broad spectrum insecticides should not be used.