They pretty much disappeared for some years before this current outbreak. It's just something to be endured until they vanish again, perhaps due to the parasitic flies killing them off - look for their white eggs on the heads of the caterpillars.

I remember them being particularly bad in thick, young stands of Red Alder, and the occasional apple tree of course. I just usually cut off the branch they're on.
It's been a few years since I've seen any nests.
mike...what a nice day!

Over here in Kitsap, they appear every year to some degree or another but they are much more frequent this season. Unfortunately they seem to infest a lot of the natives (alders in particular) and most anything in the Rose family and are often well beyond the reach of any possible control. Cutting off and destroying the branch tips with their nests is always a good choice if they appear in your garden, as is Bt when the cats are young enough to respond to that bacterium. At this point in the season, the cats have left the nests/tents and are grazing their way through the landscape - they were nibbling away on Viburnum davidii in a client's garden this weekend and I've found a couple on my Japanese maples that I pick off and smush.

Birds like 'em :-)

City of Seattle tent caterpillar fact sheet

Tent caterpillars turn into moths. Among the butterflies, in our region, only Painted Ladies and Pine Whites are as prolific and their cycle tends to be longer than the tenters.