What is this growing on my arborvitae? Is it good or bad? My trees have lots of them.
This is Bagworm. Some seasons they really infest arborvitae. I don't know where you live, but I saw them quite a lot when I lived in NJ. Get rid of them as soon as possible, before they start to hatch (May/ June).
You can just cut off the nests that you can reach, directly into a plastic bag (a double layer of recycled grocery bags would do). Make sure that the bags are really sealed off before disposal, so that the moths don't hatch and spread. You may want to look into spraying, or other options for the ones in the trees that you can't reach.
Here's a link to start you on your research- https://uconnladybug.wordpress.com/tag/arborvitae/