Venomoth are typically a solitary pokemon, meeting up with others of its species only briefly to mate or when lured together by streetlights. But when a city or forest is about to burn, even before the smoke rises a swarm of Venomoth will descend upon the area and wait for the blaze.
This is not, as commonly believed, a form of mass suicide. Venomoth are weak to fire, and some of the more careless ones are burnt to a crisp, but the reason they do this is a far more common one in nature; they need to eat. Many small insects are flushed out of houses and trees by flames, and it is these which make up the majority of a Venomoth's diet; to them, a fire is a feast.
Tragically, this attraction to flames has drawbacks which kill even more Venomoth than the fire itself. Believing them to be a cause of the fire, and not merely a scavenger, superstition has labeled this pokemon an enemy. When a forest burns, they are safe, but when a city burns more Venomoth are killed by trainers and trained pokemon than by the fire itself.
