The pokemon that your wurmple will evolve into depends on it's personality value. The personality value is set in stone once you catch a pokemon. If your Wurmple has a personality value of 4 or less, it will be a Silcoon, and then a Beautifly. If it is 5 or more, it will evolve into a Cascoon, and then a Dustox. Personality values are super obscure, but hopefully this helps!