Greenfly was a human-designed terraforming machine programmed to break apart terrestrial planets and transform them into self-sufficient habitats filled with vegetation. Composed of swarms of Von Neumann robots, it was fully capable of self-replication and autonomous action -- at least, actions in keeping with its primary function. First seen aboard the lighthugger Hirondelle in 2303, it was taken by the pirate Run Seven in 2309, and unleashed some time after in the Ross 128 system.
Greenfly was programmed to adapt to the external challenges it encountered. Because of this, potential threats were -- in a sense -- assimilated, and used to improve and evolve greenfly. By 2931, the Nestbuilders, or rather their Slug overlords, were convinced that greenfly had been deliberately exposed to the Melding Plague to circumvent any limitations placed on its self-replication, and that they had assimilated a sample of Inhibitor machinery, as well. They were concerned that even their advanced military technology would be unable to counteract them.
Sometime after they encountered active resistance, greenfly began to categorize any intelligent life as a potential threat to its endless terraforming, and made destroying that life a priority.
By 40,000, greenfly had transformed vast swathes of the Milky Way galaxy.