Tents among the willows

In a willow tree where redwing blackbirds stand guard ...

Redwing and swallow, Centennial Beach Park

 and sing to their mates in the reeds below ...

two families of tent caterpillars have set up camp.

Basking in the sunshine. When it sets, or the rain returns, they will hide under their warm blanket.

Five caterpillars, and oodles of caterpillar poop.

These are the western tent caterpillars, Malacosoma californicum, recognizable by their location, their tents, and the orange and blue spots along their sides and backs. They feed on willows, like this one, cottonwoods, aspen, apple, plum, and other local deciduous trees and shrubs (such as roses), sometimes covering the whole tree with one enormous web.

Overnight, or in cool weather, they huddle in their web, but when the weather is warm they venture out to feed on new leaves and buds, stripping the branches down to the bare twigs. Back in the warmth of the nest, they digest the edible part of the leaves and excrete the rest in little tube-shaped pellets, which eventually rain down to fertilize the ground underneath the tree.

I always shudder to see a tree completely encased in caterpillar tents, so I was reassured by a half-dozen web pages saying that their depredations rarely kill a healthy tree; in a month or so, after they have eaten their fill and gone off to build cocoons instead, the tree puts out new leaves, none the worse for wear.

In the case of a few infested branches, it is easy enough to cut them down on a rainy day, and destroy them. We used to burn them, but drowning or squashing will do.

Some birds may eat them; I don't know if the redwing blackbirds will.