Theory: Waluigi is a Botanist

Abstract
Waluigi is a botanist and bred some Piranha Plants in secret to defeat the Mario Bros.

Arguments

 * Argument 1: Waluigi's Vine Whip in Mario Strikers Charged and "Wall-Luigi". These are Waluigi's special abilities in the game, both involve plants.


 * Argument 2: Waluigi's amiibo description says that he's been silently working in the shadows against the Mario Bros. What better way than to create carnivorous plants that will be prohibiting them in their adventures? They don't seem to be a part of Bowser's army, so where did they come from? And if Waluigi knew the Mario Bros were plumbers, that would explain why he would breed enemies commonly found in pipes.


 * Argument 3: Waluigi Island in Mario Party 3 is decorated with Piranha Plants that seem to like Waluigi, plus his MP3 render shows him holding a plant.