Mycosis Inhibits Cannibalism by Melanoplus sanguinipes, M. differentialis, Schistocerca americana, and Anabrus simplex

Cannibalism is common among the Acrididae and the Mormon cricket, Anabrus simplex Haldeman (Orthoptera: Tettigoniidae). This behavior has been proposed as a mechanism for the horizontal transmission of Microsporida and entomopathogenic fungi. Aanecdotal observations suggested that the migratory grasshopper, Melanoplus sanguinipes Fabricius (Acrididae), and A. simplex did not eat cadavers that had been killed by insect pathogenic fungi. The hypothesis tested was that A. simplex or M. sanguinipes would not cannibalize individuals freshly killed by the entomopathogenic fungi, Beauveria bassiana Bals.-Criv. (Vuill.) (Hypocreales: Clavicipitaceae), or Metarhizium acridum (Driver and Milner) Bischoff, Rehner, and Humber. Cannibalism was examined in a series of no-choice tests with individual insects. Test insects included healthy adults of M. sanguinipes; the differential grasshopper, M. differentialis (Thomas); the American grasshopper, Schistocerca americana (Drury) (Acrididae); and A. simplex. Individual, starved Acrididae or A. simplex were confined in small cages with either a fungus-killed (but unsporulated) or uninfected cadaver. The insects were then observed periodically for the first 4 hr. After 24 hr, the cadavers were scored for the degree to which they had been consumed. Very few mycotic cadavers were fed upon by the healthy insects, and, at most only the tarsi were eaten. All four species generally refused to eat fungus-infected cadavers. In contrast, freeze-killed cadavers were partly or entirely consumed by most of the test insects, often within a few hours. Transmission of infection through contact in these tests was between 0-18.9%, depending upon the fungus and insect species, and was lower than the prevalence of cannibalism in all cases.