Pecans that do not fall but stay in trees into the winter are infested with….

Hickory shuck worms that tunnel around in the husks starting in late summer. They cut off supply lines to developing kernels, stopping their growth. The nuts stop developing and hang in the trees well into winter. When they do fall, they are hollow. Use a general-purpose fruit and pecan insecticide in early August. Repeat the spray in late August.