I found these buggers growing on a peach tree in my yard. This is located near Pittsburgh, Pennsylvania, United States.
- Are they harmful?
- If harmful, effective control measure.
Tent caterpillars. They are usually bad because they have huge appetites as this is their growth stage, so they can strip all or most of the leaves off a tree. We used to light a firecracker in their tent and blow it up. You might want to treat with Bt, Baccillus thurengiensis powder on the leaves. They have to eat Bt for it to be effective.
Those look like tent caterpillars to me. Not sure of which specifically, but they are all pests. They are harmful.
I don't have any good advice regarding removal; I can only tell you how not to do it: my dad got rid of them by dousing the tent in gasoline one night (when all the little buggars are in there) and lighting it (and the tree, and the tree next to it, and all the grass around it, and one of the 4x4 supports for our deck...) on fire. Don't do it that way.
Eastern Tent caterpillar I'd say - if they're a major problem on your tree/s every year, treatment might be necessary, otherwise, not too big a deal, easier to remove the egg masses when they're on the twigs really. They're laid in clusters on the twigs and overwinter there, so removing them in fall or in winter will solve the problem for next year - more info in the link below