The Scourge serve as kamikaze-attack flying units, capable of causing 110 damage to aerial targets. Like Zerglings, Scourge spawn in pairs from a single Larva and can be produced in mass numbers in a very short period of time.

To get the most of the Scourge, use them in combination with Mutalisks, Guardians or Devourers. The Scourge can quickly even the odds in the favor of an already established Zerg air force and with the Mutalisks/Guardians/Devourers there to absorb most of the damage from the enemy, the Scourge have a much better chance of a successful attack run.

Scourge acquire their targets very quickly and if used to patrol key areas, can be useful for putting an end to an enemy drop tactic. However, in the battlefield, all available Scourge may lock onto a single target and may simultaneously attack, resulting in a huge waste of resources for only one kill. As a result, you should either keep a watchful eye and tight control on your Scourge groups or separate them into pairs in case of auto acquiring as well as for easier management during a heated battle.

Determining the number of Scourge for a Kill
One thing to keep in mind when using Scourge is knowing how many are needed to destroy any given target. Using too few Scourge will only damage a target and with the Terran ability to repair, Protoss ability to recharge shields and the Zerg ability to regenerate, only damaging a target can be a waste of time and resources. Using too many Scourge is a waste of precious Vespene Gas and should be avoided whenever possible. Listed below is the minimum number of Scourge needed to destroy each unit (but more may be necessary if these units are in groups):

Wraith - 2 Scourge
Dropship - 2 Scourge
Valkyrie - 2 Scourge
Science Vessel - 2 Scourge
Battlecruiser - 6 Scourge (1 will always be killed en route)

Observer - 1 Scourge
Shuttle - 2 Scourge
Scout - 3 Scourge
Corsair - 2 Scourge
Carrier - 5 Scourge
Arbiter - 4 Scourge

Overlord - 2 Scourge
Mutalisk - 2 Scourge
Guardian - 2 Scourge
Devourer - 3 Scourge

Upgrades.

Flyer Carapace - Each upgrade adds +1 to the Flyer Carapace for a total of +3 when fully upgraded.