Elderberry stems are semi-woody, the interior is filled with pith. The late John Fairey, renowned botany professor at Clemson, told students that this pith was used to pack delicate scientific instruments and still used by repairmen to hold tiny parts of jewelry and such. Mason bees and other insects, however, have long used the older hollow stems as places to construct egg-laying or brood chambers. So if you have elderberries in your yard, cut a few stems half way down every year to expose that pith-filled interior to cavity nesting bees. Another option is to put stems and other small branches or rotting wood in a mulch pile. Then you can order a Pollinator Friendly Habitat sign from the Xerces Society. Read their page about building a better mulch pile for more ideas about making your yard pollinator friendly.