Importance
The occurrence of Mermis nematodes is unquestionably related to the occurrence of favorable hosts, but due to the protracted development period of the nematode the relationship can best be described as delayed densitydependent. Various studies have reported high levels of parasitism in grasshoppers: up to 76% of Melanoplus bivittatus in Vermont, 71% of Melanoplus femurrubrum in
Michigan, 70% in Hesperotettix viridis in North Dakota, 22% of Melanoplus nymphs in Indiana, 36% in Melanoplus sanguinipes in Manitoba, and up to 43% of mixed populations
in Quebec. Although often considered to be absent from arid areas such as the western Great Plains of the United States, they do occur where water is available. In Colorado, up to 50% of Melanoplus femurrubrum was found to be infected from irrigated cropland, whereas the same species was unparasitized in nearby arid rangeland (Capinera 1987). Although claims sometimes are made about the suppressive effects on grasshopper populations by M. nigrescens, their long life cycle and need for high moisture conditions reduce their reliability as a biological
control agent.