Split evolutions are an odd duck in Pokémon Go because they sometimes function differently than they would in a regular Pokémon game. Wurmple thankfully has the same evolution method as always, though there is still one annoying twist to work around.
How to Evolve WURMPLE into BEAUTIFLY (Pokemon GO Gen 3 Evolution)
Wurmple has two evolution paths you can follow in Pokémon Go. One will give you the Butterfly Pokémon Beautifly, while the other will result in the Poison Moth Pokémon Dustox. Getting either of these Pokémon will require a little luck once you've evolved your Wurmple. Here's a full guide to this sometimes messy Bug-like situation.
In order to fully evolve Wurmple into Beautifly or Dustox, you first need to make sure that your Wurmple evolves into the correct Cocoon Pokémon. Unfortunately, that means you're at the mercy of chance.
First, you'll use 12 Wurmple Candy to evolve your Wurmple into either Cascoon or Silcoon. This isn't something you can choose: you have a 50 percent chance of getting a Cascoon or Silcoon whenever you hit the Evolve button. Depending on which Cacoon Pokémon you get, you'll know which final evolution you'll get.