This is the second sawfly cocoon I found, also on a dogwood. But this was opened sometime last summer, or spring, and because it is so tough, it has remained on the stem for even longer than the one in the next photo which is still intact. It amazes me that the emerging sawfly can so neatly cut away the top of the cocoon. Sometimes, the flap is left on as in this image: http://www.pbase.com/laroseforest/image/157224281