Culling

Culling was a term used in the Pegasus Galaxy for when the Wraith travel to a planet and take great numbers of the inhabitants to feed upon.

Their strategy in a culling includes the arrival of Wraith Hive Ships over the planets. The Wraith would later send darts to the surface and "scoop" up anybody they can find until only a handful of people are left. Then legions of Wraith Warriors are sent down to find any large number of humans hiding in the shadows often feeding on them as they go. To prevent the inhabitants from escaping the planet by Stargate, the Wraith would dial another Stargate to the planet. Apparently after a certain amount of time, the Wraith stop dialing in, presumably as by that point there's pretty much no one left to use it. Major John Sheppard and Teyla Emmagan witnessed a culling from a cloaked Puddle Jumper, where there was nothing they could do except watch. The two were able to rescue a small group of survivors and eventually were able to dial out once the Stargate shut down and the Wraith apparently stopped dialing in. Teyla, who had apparently seen many cullings before, said she'd never seen a culling so bad or one that had taken as many people. (SGA: "Letters from Pegasus")

The Wraith purposefully leave some of the planet's population alive so they can repopulate in time for the next culling.

If a planet's technology becomes advanced enough to potentially threaten the Wraith’s dominance over the galaxy then that planet will immediately be culled down to as few remaining as can escape the wraith, then the planet will be bombarded with Wraith energy weapons from orbit, destroying all traces of the civilization and ensuring none of them survive. Some worlds have hidden their advancements from the Wraith in order to prevent this, most notably the Genii. According to Dr. Elizabeth Weir, there's more advanced societies out there then anyone realizes. (SGA: "The Siege, Part 2", "Ghost in the Machine")

Part of the Wraith's standard procedure is to block a planet's Stargate in order to prevent escape, usually by dialing in from another planet, but when they tried to cull Earth, they simply brought a Pegasus Stargate into orbit which overrode Earth's Gate.

In 2009, the Wraith underling betrayed Todd, seizing the ZPM powered Hive Ship, and tried to use it to cull Earth. During the event, Colonel Samantha Carter said that their usual procedure is to first scan the surface for military installations, then target them with weapons fired from orbit, then continue with the power grid to send the population into a panic, and finally to launch the Darts to cull the population. During this attempted culling, Earth launched everything they had at the improved hive ship (the only one that knew of Earth's location), including Atlantis, but the attack failed. Atlantis was nearly destroyed and with it gone, the Wraith would have been able to cull Earth unopposed, but the ZPM powered hive ship was destroyed from the inside with a nuclear warhead planted onboard by Lt. Colonel John Sheppard. He and his team were able to use the hive ship's Stargate to escape to the Milky Way Alpha Site while the bomb destroyed the hive ship. Afterwards, Atlantis landed near the Golden Gate Bridge, cloaked to maintain its secrecy. (SGA: "Enemy at the Gate")