"Is this Jim Anderson who writes for the Source?" the caller will ask.
"Yes, it is," I'll reply, "what can I do for you?"
"I want to know what those ugly (sometimes, "repulsive" will be used), twitching, hairy things are all over my bushes!" And that's the subject of the phone calls I'll be getting at least once a day over the next couple of months when the weather warms up.
Those squirmy, fuzzy things in the photo - and the obvious ones you'll see in silken tents really soon - are western tent caterpillars (which will eventually become moths) chomping the leaves of Antelope Bitterbrush, Purshia tridentata. Like most long-term residents of Central Oregon, you sort of grow up with tent caterpillars every spring. We notice them out of the corner of our eye as a causal part of the landscape, but once in a while, they seem to be "more-than-casual" and show up, "all over the place."
In answer to the next question as to whether tent caterpillars do any harm, that depends on your perspective. Yes, they can almost defoliate their host plant, which, from the human perspective, may be "harmful." From the perspective of nature, however, what's going on is between moths, plants, parasites, predators, a whole lot of biological interaction, and chemical and energy is exchanged. So, the question about "harm" is relative to the human perspective, not nature's. My answer is that what's happening just "is."
The life history of tent caterpillars, however, is fascinating. The eggs hatch as soon as the cold, frosty nights are gone in spring. ("Spring" in Central Oregon means almost nothing regarding air temperature. Over the millennia, I think tent caterpillars - like so many other organisms in our neck-of-the-woods, including you and I - have adapted to the extreme changes we experience in our so-called, "spring.")
Be that as it may, tent caterpillars must get a head start on spring and emerge at the same time the new buds on bitter brush pop open. They're in a race with their host plant and they have to eat the young tender leaves before they're too tough to chew on. If you watch tent caterpillars over their short lifetimes, you'll see they grow very rapidly, almost at the same rate as their food plants.
Often, due to the cold nights, the plants outgrow the caterpillars and consequently, they starve to death and never become moths. Sometimes, parasites of all kinds lay their eggs in and on the caterpillars and when the eggs hatch, the grubs eat the caterpillars before they can metamorphose into moths.
The twitching business (which is repulsive to some people) is a group protection strategy to being disturbed. A parasitic wasp attempting to lay its eggs on or in the caterpillar will have difficulty doing so with the wild twitching going on. A bird that may be able to eat that "hairy" body will be confused by all the movement and perhaps fly away without snapping up even one - or conversely, some other bird may grab the one nearest it, but then find it can't keep the squirming beast in its mouth.
All in all, tent caterpillars have a lot of problems growing up, but also a lot going for them. Their group protection usually works, and on cold nights they'll get under the silken tent and huddle together to keep warm. Like the honey bees that maintain their home at 90 degrees in winter and summer, tent caterpillars elevate the temperature within their huddled masses more than 50 degrees Fahrenheit warmer than the surrounding temperature on cold, sunny mornings.
For control, some people cut out the caterpillars and their tents on cold mornings and burn them in a metal bucket of combustibles away from buildings and anything else that can catch fire. If you feel it's necessary to remove them from the bitterbrush, this is the best form of "control."
Please stay away from chemicals. There is a host of parasites, predators and mere bystanders that depend on tent caterpillars to make a living ("natural control" if you will). Plus, there are other things going on we don't understand. But, what we do know is that chemicals are bad news for the good guys as well as the bad guys in your garden. Who knows, maybe all that frass (caterpillar poop) dropping into the soil beneath the bitterbrush is fertilizer and therefore "good" for the plant.
As the "Bard of the North," Robert Service says in his delightful work, "The World's All Right":
There's much that's mighty strange, no doubt;
But Nature knows what she's about;
And in a million years or so
We'll know more than to-day we know...
And by watching and learning more about tent caterpillars, we'll be following the advice of our libraries that urge us to "know more!"