Outdoor performance of Metarhizium acridum against grasshoppers and Mormon cricket in eastern Montana

Metarhizium acridum, particularly the commercial IMI330189 (Green Muscle, “GM”) and FI985 (Green Guard, “GG”), presents an important potential biological tool to manage grasshopper and Mormon cricket populations in the U.S. There is question about how well either strain would perform on the U.S. Northern Plains. Because M. acridum has not yet been isolated in the U.S., regulatory authorities consider it “nonindigenous” and therefore the fungus cannot yet be “released into the environment.” Therefore, highly contained, outdoor, cage trials were conducted targeting the Differential Grasshopper, Melanoplus differentialis, and the Mormon cricket, Anabrus simplex under permit from USDA APHIS PPQ. Both strains were compared to the U.S. commercial Beauveria bassiana GHA. Individual adult insects, topically inoculated in the lab with an LD₉₅ (7-10 d) for each fungus, were caged outdoors in thermally transparent cages and observed for subsequent mortality. Against M. differentialis both M. acridum strains were superior to GHA, but efficacy was not evident until after 14 d post inoculation; maximum efficacy was 88% (GM) and 72% (GG) at 21 d post inoculation. Against Mormon cricket, GG was superior to GM, both in overall efficacy and speed of action, and both were far superior to GHA and an M. robertsii, DWR346, even though they were not the most pathogenic in a parallel cohort of insects held in the lab at 28° C. Maximum efficacy (36% for GM, 76% for GG) was not manifest until 45 d post inoculation however. Host body temperatures may have been a major inhibitory factor.