The effects of temperature on the pathogenicity of heat-sensitive mutants of the entomopathogenic fungus, Beauveria bassiana, toward the migratory grasshopper, Melanoplus sanguinipes

Heat-sensitive (HS) mutants of Beauveria bassiana were shown to kill grasshoppers at the growth-per missive temperature (PT) of 20 degrees C but not at 30 degrees C or more, the nonpermissive temperatures (NPT). The migratory grasshopper, Melanoplus sanguinipes, exhibited near complete poikilothermy in the bioassay system employed for B. bassiana strains. Temperature shift, from 20 to 30 degrees C, during the bioassays was used to halt the mutant cell development postinfection. The resultant mortality data were analyzed using median effect plots, Timing of the infection was correlated with the required time at PT for grasshopper death. An incubation period of 4.6 to 6.4 days, postinfection, at PT was needed in order for the values for onset of infection, mortality rates, and the LT(50) for the three HS mutants to reach those insects infected with control strain, GK2016, at the PT. The time of PT exposure required to ultimately kill half of the test insect population, termed LE(50), ranged from 3.9 to 5.1 days. A minimum time for the insect infection at PT, defined by the LE(50) value, is required for infection to lead to mortality, This time, the initial 4.4 days of infection, may reflect certain critical event(s) after which, whether or not additional fungal growth at NPT occurs, grasshopper death is unavoidable, These observations should help the development of models for predicting the outcome of infection processes and elucidation of critical events. (C) 1996 Academic Press, Inc.