Answer: The caterpillar you described on the parsley is the larvae of a butterfly. You might want to leave it in place; it will eat the parsley for a short time and then move on. Your parsley should survive and you will be able to enjoy the butterfly in your garden, too. Keep your parsley well watered and possibly topdress with a little compost to encourage it to regrow.
At this time of year you are probably seeing the Eastern tent caterpillar in your tree. This is not going to kill your tree. To remove it, you can wind the tent web onto a stick and put it in the trash. Next spring, if you see the caterpillars early before they are encapsulated in the tent, you can spray the tree with a product containing Bt. It is too late to spray with it once they are inside the tent.
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