Different assemblages of primary and secondary pest grasshopper species were found in 5 different rangeland plant communities in western North Dakota. The action window for their control (i.e., the interval when control is likely to be efficacious) with short-lived insecticides can be visualized in terms of time, grasshopper phenology, or plant phenology. Action windows ranged from 14 d duration in habitats dominated by western wheatgrass to 38 d duration in habitats dominated by needle-and-thread or by crested wheatgrass. Action windows opened 11–15 d later in rhizominous grass habitats than in bunchgrass habitats because of increased presence of late-hatching secondary pest species. In all habitat types, action windows closed primarily in response to imminent oviposition by a ubiquitous primary pest species, Melanoplus sanguinipes (F.). In all habitat types, action windows tended to open during seed development of 2 common and conspicuous grasses, needle-and-thread and green needlegrass. No such consistent indicator for closure of action windows was detected.