User blog comment:James12708/Reapers VS. Forerunners VS. Angels from Darksiders VS. Humanity from Halo VS. Council and humanity from Mass Effect VS. Imperium of Man/@comment-4721065-20130112104250/@comment-4721065-20130113004539

Both Halo Nation AND Halopedia seem to disagree with you on this one:

''Halo doesn't kill Flood, it kills their food! Humans, Covenant, whatever - we're all equally edible! The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life!"

Furthermore, both wikis state that the pulse only destroys specific nervous cells, so the Orks's spores would be fine. Not sure about Tyranids now, but if the the Flood are completely immune, then the Tyranids might as well be the same (with the added advantage that they feed on everything organic, not just sentient life).