The waterproof scales on Butterfree’s wings are loosely attached, and if the Butterfree flaps them hard enough, they will be released into the air. This resulting “dust” is toxic, and can be irritating if inhaled. In some cases, additional powders will be mixed in, some causing sleep and others paralysis.
In addition to the usual powers possessed by Bug-type Pokémon, Butterfree also learns a variety of powerful Psychic-type moves -Bulbapedia