As far as I can tell the initial damage from the BroodLord benefits from +flyer attack, while the damage the broodling does benefits from +ground attack. Since the broodlings attack a lot faster than the BroodLord, it seems boosting ground attack will increase the effective damage the BroodLord does?
BroodLord gets +2 damage / 2.5s from flying upgrades, while the broodling gets +1 /0.6455s from ground attacks.
Assuming the broodling attacks once as soon as it lands, and then lives for 0.64s, it will have broken even. If it lives any longer than melee upgrades would be better?
Should I be upgrading ground attack in preference to flyer attack? (IE, ground attack AND flyer carapace) if I am using brood lords?