The reusable End Caps are the third part of the Knox Cellars  'Bee Nester System' tube assembly. System include 1) water resistant Cardboard Tube (reusable), 2 ) Liner and 3) reusable End Cap that all together make it so easy to harvest cocoons. End Caps are sold as bags of 50 in case some get lost or you would like to assemble the Bee Nester System or they can be used to cap the back of the 48 or 96 Wood Nesting Blocks