Bush joined the faculty at the University of Texas, Austin, in 1966.[5] Many of his summers were spent doing field work in Door County, Wisconsin, where he and his students studied the fruit flies (family Tephritidae) that live in the localorchards. He moved to MSU in 1981, retiring from there in 2001.[1] In 1987, Bush became the inaugural director of the graduate program in Ecology and Evolutionary Biology at MSU.[1]
While at Harvard, Bush described several species in the genus of fruit flies, Rhagoletis.[6] He began to focus on the process of speciation, rather than the description of species, when he met Ernst Mayr, who was then the leading authority on speciation, during Bush's doctoral studies at Harvard. While Mayr was skeptical of the possibility of sympatric speciation, Bush’s research on the apple maggot fruit fly is considered to provide a particularly strong case for that process, one that continues to be built upon by later workers.[7] Specifically, Bush reasoned that phytophagous insects that both consume and reproduce on specific host plants provide opportunities for speciation without strict geographical isolation (as in allopatric speciation) because, as the insects adapt to feeding on a new host plant, they are also more likely to mate with other members of their species that prefer that new host.
On his passing in 2023, Kay Holekamp, a behavioral ecologist and later director of the EEB program at MSU, said of Bush: “He was a real prince of a human being, super smart, and inspirational in his development of the Rhagoletis system to study sympatric speciation.”[1] Bush authored over 100 scientific papers and, as of August 2024, has an h-index of 52.[8]