What natures make Wurmple evolve into Silcoon What natures make Wurmple evolve into Cascoon
The Wurmple factor is not determined by Nature, but by the Personality Value.
Wurmple's evolution does not depend on time of day, despite what many guides say. In fact, the evolution is determined by taking pw % 10. If the remainder is less than 5, Wurmple will become a Silcoon, and if it is greater than or equal to 5, it will become a Cascoon.
http://bulbapedia.bulbagarden.net/wiki/Personality_value