From Halopedia, the Halo wiki
The Seeder Infector is a type of Flood infection form. Seeder infectors are employed by the Flood as lighter-than-air forms that swim through the sky and attack aircraft. Seeder infectors can infect aircraft and convert them to Flood use, but are also capable of spitting bone fragments of their victims at enemy targets.
Seeders are the aerial equivalent of the land-bound Flood infester form as they are both primarily meant to infect enemy vehicles. Seeders, however, differ slightly with the abovementioned ability to also attack ground infantry with bone fragments. Nevertheless, Seeders are incredibly vulnerable and must rely on numbers and speed to remain effective.