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When the wielder is at 1/3 HP or less, all of its Bug-type moves will have 50% more power.
Outside of battle, it can be used to increase the wild encounter rate.
There isn't much competitive use to this ability, since most Bug-types suck or have better abilities. The best one to look forward to might well be Volcarona, since it takes 50% damage from Stealth Rock, so is worn down quickly, and with Quiver Dance it may well be a potent threat, but unfortunately Swarm is illegal on it at the time of writing, until it is released in the Dream World. Escavalier is also a Pokemon that can hit hard and use strong Bug-type moves, so the boost is welcome there too.
Found in Dream World