From The Stargate Omnipedia
Term used to describe the Wraith's
systematic capture of humans for later feeding.
When a population reaches a critical mass or must simply be terrorized to be kept in line, the Wraith arrive, often in Dart ships, to pick from the human "herd" with culling beams for the species' nourishment.
The captured are returned to the hive ship, where some victims are fed upon while others are put into cocoons in semi-conscious states to preserve them for later feeding.
Wraith Darts use a culling beam to transport victims aboard. Foot soldiers use stun rifles to immobilize their prey without damaging them.
Rising, Part 1 - Teyla takes Sheppard into a protected cave and translates the depictions of past Wraith cullings from the walls.
Rising, Part 2 - Three Wraith Darts arrive on Athos for a culling, abducting Teyla and Colonel Sumner, along with others.
Poisoning the Well - The team helps the Hoffans develop a drug that will protect them from future cullings, preventing the Wraith from feeding on them.
Underground - The Genii are shocked to learn that the next Wraith culling could come at any time, as the humans from Earth awoke the Wraith prematurely.