Extra Damage to Zombies is a projectile behavior characterized by dealing more damage than that tower's base damage to Zombie Bloon properties per hit. It is part of the attack interactions family.
Extra Damage to Zombies is represented on Bloons Wiki as Template:Extra Damage to Zombies, the image for the Mace Stake in Bloons Adventure Time TD, as the icon for the Silver Dagger 
was already used by Extra Damage to Regrows, and the Mace Stake is the only other object in the game besides Clarence to make an attack deal bonus Zombie Bloon damage.
