Are grasshoppers monogamous?
Male grasshoppers often mate several times with multiple female, and some females will mate again after, and perhaps even before they lay their eggs. So no, they are not monogamous. 

Rufous grasshoppers for example have a very special mating cycle, the male will attract other females, possibly more than one with "pursuit sounds". After copulation, he keeps her close to prevent other males from mating with her, and the females enter a defensive mode where they refuse intercourse from other males. After laying eggs however, she enters a "relaxed" mode where she is open to others for mating.

Sources:

jrank

Rufous grasshopper: inaturalist