Some of my Maypops (P. incarnata) were stripped too but too many vines climbing up trees to escape/hide from the Fritillary caterpillars.
Now if I knew what was eating birch leaves. Oh well, I am not going to worry about birch trees.
Why do we kill caterpillars? You can see here how many Fritillaries wins would be missing if we were bent on killing caterpillars. So many caterpillars are not harmful and the trees will recoup.
I live in Austin, Texas and got a HUGE passion vine....over 6 ft. high and 10 ft. wide in full bloom. NOTHING smells as great as this. The name is perfect. Very aggressive however.