Q: On my citrus tree I found one leaf strangely “sewn” into a pocket. As there was nothing inside the pocket it has me stumped.
A: My guess is a leafroller moth. There are several hundred moths of this kind. They lay eggs on leaves of apple, pear, citrus, canna and many other plants. The eggs hatch into grub-like larvae, which feed on the leaf and then sew the leaf edges together to provide protection.
The ones that survive chew their way through the leaf and drop to the ground to pupate and become a moth once again.
The easiest way to control them on an ornamental tree is to watch for sewn leaves and crush the larvae when seen.