It is a quest item for the 1st part of Bleed Me Dry. Red Lucy of The Thorn will give the Courier 200 caps for them, 300 with a Barter check of 50. It spawns once the quest has started, it is interacted with not taken and cannot be dropped.
The pile will spawn on the third level (Food Production) of Vault 22, as part of the quest Bleed Me Dry. The eggs will spawn at a dead end in the cave tunnels, guarded by many giant mantises as well as one giant mantis female. The Vault 22 cave door keycard is needed to access the area. The marker points out the exact location.
Alternatively, if twelve mantis eggs are collected, the quest will convert them into the proper quest item and the objective will complete.