When I checked it out, I found a mass of eastern tent caterpillars that were quite big already. Fortunately, they were still inside the webbing that protects the egg mass, so I was able to grab the entire nest (wearing gloves, of course) and stomp on the critters with my garden boots, (which was satisfying).
So I had another look at all my crab apples trees, and sure enough, I found more caterpillars and was able to get rid of them just as easily. In the weeks before, I had already done the rounds of the trees, and removed at least a half dozen tent caterpillar egg masses.
It's a good thing I noticed the problem early - before the caterpillars were all over the tree branches eating up the leaves. By then it would have been too late to do anything about them. For example, Btk (a natural insecticide), is not effective once the caterpillars are longer than an inch. In fact, the caterpillars I found last week were already quite large and ready to leave their nest. Manual control was the quick and easy solution.
The moral of the story: Noticing things while it's still easy to fix them is the key. (When stuff gets out of hand, fixing it gets harder.) Same thing with weeds, there's nothing more depressing for than a weedy takeover of a flower bed. Maybe this is the reason I'm never relaxed in my own garden: I'm always noticing too many things.