I haven't checked this ability yet but I intend to do something similar. I'm guessing you should find the effect that summon infested marines in the effect tab, and replace it by a search area effect linking a damage effect !
I'll keep your informed.
How can I modify the infested terran ability to have the egg explode after a short period of time?
@ajilejay: Go
Change the amount in behavior "Infested Terran Egg - Timed Life"