Grassi was the first to demonstrate the life cycle of human dwarf tapeworm" Taenia nana", and that this tapeworm does not require an intermediate host, contrary to popular belief. He was the first to demonstrate the direct life cycle of the roundworm "Ascaris lumbricoides" by self-experimentation. He described canine filarial worm "Dipetalonema reconditum", and demonstrated the parasite life cycle in fleas, "Pulex irritans". He invented the genus of threadworms "Strongyloides". He named the spider "Koenenia mirabilis" in 1885 after his wife, Maria Koenen. He pioneered the foundation of pest control for phylloxera of grapes.