It's random based on personality.
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A Wurmple will evolve into either a Silcoon or a Cascoon at level 7. Then at level 10, it will evolve into a Beautifly (Silcoon) or Dustox (Cascoon).A wurmple will evolve in either silcoon or cascoon depending on its personality value.
It has to do with Personality Value, which you can't see, but for you to get a silcoon the Value would be a <= and to get a cascoon it would be a >=
Wurmple evolves according to a hidden value in the game's coding that is called a Personality Value, there is no way to see this value in-game so Wurmple evovling into Silcoon or Cascoon is just basically a matter of luck on the trainer's part but it will still evolve at Level 7 no matter what. Silcoon can only evolve into Beautifly whereas Cascoon can only evolve into Dustox.
A Wurmple will evolve into either a Cascoon or a Silcoon at level 7. Then at level 10, it will evolve into a Dustox (Cascoon) or Beautifly (Silcoon). A wurmple will evolve in either cascoon or silcoon depending on its personality value. Once you catch a wurmple what it will evolve into has already been set in stone, nothing you can do will change it. Personality values are also not accessible by normal means and are unchangeable, no other stats affect the evolution and really the only way to know what you get is by evolving it. Luckily level 7 isn't a very difficult level to reach and wurmple's are very common and easy to catch, so if you evolve one (or two or three or four) and don't get what you want just keep catching them and trying again
From Bulbapedia:Wurmple (Japanese: ケムッソ Kemusso) is a Bug-type Pokémon. Wurmple will evolve into either Silcoon or Cascoon starting at level 7 depending on its personality value. As personality values cannot be influenced by the player, what a given Wurmple will evolve into is essentially random.